@article{youssef_liu_chescheir_skaggs_negm_2021, title={DRAINMOD modeling framework for simulating controlled drainage effect on lateral seepage from artificially drained fields}, volume={254}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2021.106944}, abstractNote={We demonstrated a DRAINMOD modeling framework to predict controlled drainage (CD) effect on the fate of water in artificially drained agricultural fields, which is key for determining the water quality benefits of the practice. To demonstrate this modeling framework, DRAINMOD simulated the hydrology of a subsurface drained grass field in Eastern North Carolina, U.S. under both free drainage (FD) and CD. Three scenarios were simulated for each water management: no lateral seepage (LS) and LS with constant and dynamic hydraulic head (Hr). For each scenario, predicted water table depth (WTD) and subsurface drainage were compared to observed values using Mean Absolute Error, Nash-Sutcliffe Efficiency, and Normalized Percent Error. Predicted water balance components for different scenarios were also investigated. Results clearly showed that LS was a significant component of the water balance for CD. Model predictions showed that 96% of the reduction in subsurface drainage due to CD could be attributed to LS (33.5 cm yr−1). The large values of LS predicted by the model were attributed to the presence of a permeable sandy layer in the soil profile, the shallow management depth of the drain outlet, and the small size of the experimental field plots. Agreement between predicted and observed WTD and subsurface drainage ranged from acceptable to excellent for FD with and without considering LS. In contrast, DRAINMOD simulations for CD yielded acceptable predictions only for the scenario considering LS with dynamic Hr. This study demonstrated the power of process-based simulation models, such as DRAINMOD, for interpreting and explaining data of experimental studies and underscored the importance of using a proper model calibration strategy for yielding reliable predictions. This study highlights the need for well-coordinated experimental and modeling research to further investigate how seepage affect CD performance for reducing drainage flow and nitrogen losses from artificially drained agricultural fields.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Youssef, Mohamed A. and Liu, Yu and Chescheir, George M. and Skaggs, R. Wayne and Negm, Lamyaa M.}, year={2021}, month={Aug} }
 @article{askar_youssef_vadas_hesterberg_amoozegar_chescheir_skaggs_2021, title={DRAINMOD-P: A MODEL FOR SIMULATING PHOSPHORUS DYNAMICS AND TRANSPORT IN DRAINED AGRICULTURAL LANDS: I. MODEL DEVELOPMENT}, volume={64}, ISSN={["2151-0040"]}, DOI={10.13031/trans.14509}, abstractNote={Highlights DRAINMOD-P has been developed to simulate phosphorus (P) dynamics in drained croplands. Key hydrological and biochemical processes affecting P cycling are represented in the model. The model predicts surface and subsurface P losses as affected by weather, soil, and management factors. Abstract . High phosphorus (P) loads to streams and lakes can promote harmful algae blooms and cause water quality deterioration. Recent research has identified subsurface drainage as an important pathway for the transport of dissolved P from drained croplands to receiving surface water bodies, particularly when macropore flow contributes a considerable portion of the subsurface drainage outflow. Currently, a few models are capable of simulating P dynamics in poorly drained soils with artificial drainage systems. The objective of this study was to develop DRAINMOD-P, a field-scale, process-based model that simulates P cycling and transport in drained croplands. Processes represented in the model include atmospheric deposition, organic and inorganic fertilizer applications, plant uptake, sediment-bound and dissolved P losses in both surface runoff and subsurface drainage, tillage practices, and P mineralization and immobilization. The model predicts P losses under different management practices, climatic conditions, drainage systems, and crop rotations. The model is an extension to the nitrogen model DRAINMOD-NII, with full integration of the nitrogen and P model components. DRAINMOD-P uses the recently modified hydrology component that simulates macropore flow. A soil erosion component, based on the RUSLE approach, has been incorporated into the model to estimate sediment loss and associated particulate P loss. Sediment deposition in tile drains is considered to quantify particulate P settling in the drainage system. In this article, we review the approaches used in DRAINMOD-P for simulating P-related processes. Model testing against field-measured data from a subsurface-drained field in northwest Ohio is presented in a companion article. Keywords: Best management practices, Phosphorus model, Phosphorus processes, Soil erosion, Water quality modeling.}, number={6}, journal={TRANSACTIONS OF THE ASABE}, author={Askar, Manal H. and Youssef, Mohamed A. and Vadas, Peter A. and Hesterberg, Dean L. and Amoozegar, Aziz and Chescheir, George M. and Skaggs, R. Wayne}, year={2021}, pages={1835–1847} }
 @article{askar_youssef_hesterberg_king_amoozegar_skaggs_chescheir_ghane_2021, title={DRAINMOD-P: A MODEL FOR SIMULATING PHOSPHORUS DYNAMICS AND TRANSPORT IN DRAINED AGRICULTURAL LANDS: II. MODEL TESTING}, volume={64}, ISSN={["2151-0040"]}, DOI={10.13031/trans.14510}, abstractNote={Highlights DRAINMOD-P was tested using a dataset from a drained field with desiccation cracks. Surface and subsurface phosphorus losses were mainly in the particulate form. Surface runoff was a major pathway for phosphorus loss in this field. The model performance in predicting edge-of-field phosphorus loss is promising. Abstract . The recently developed phosphorus (P) model DRAINMOD-P was tested using a four-year dataset from a subsurface-drained field in northwest Ohio with significant potential for desiccation cracking or preferential flow. The model satisfactorily predicted subsurface drainage discharge, with a monthly Nash-Sutcliffe efficiency (NSE) of 0.59 and index of agreement (IOA) of 0.89. Lack of annual water budget closure was reported and was likely caused by uncertainty in measured surface runoff and/or modeling approaches representing macropore flow. More than 80% of predicted surface and subsurface P losses were in the particulate form. Surface runoff was the major pathway for P loss, contributing 78% of predicted total P (TP) load. On average, predicted macropore flow represented about 15% of drainage discharge and contributed 21% of DRP loss via subsurface drains. The performance of DRAINMOD-P in predicting monthly dissolved reactive P and TP losses through subsurface drains can be rated as poor (NSE = 0.33 and IOA = 0.60) and very good (NSE = 0.81 and IOA = 0.95), respectively. DRAINMOD-P demonstrated potential for simulating P fate and transport in drained cropland. More testing is needed to further examine newly incorporated hydrological and biogeochemical components of the model. Keywords: Agricultural drainage, Edge-of-field phosphorus load, Macropore flow, Phosphorus model, Sediment yield, Water quality modeling.}, number={6}, journal={TRANSACTIONS OF THE ASABE}, author={Askar, Manal H. and Youssef, Mohamed A. and Hesterberg, Dean L. and King, Kevin W. and Amoozegar, Aziz and Skaggs, R. Wayne and Chescheir, George M. and Ghane, Ehsan}, year={2021}, pages={1849–1866} }
 @article{ghane_askar_skaggs_2021, title={Design drainage rates to optimize crop production for subsurface-drained fields}, volume={257}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2021.107045}, abstractNote={Agricultural subsurface drainage is critical for crop production in temperate humid regions. With the heightened concern of its water-quality implications, we need a method to design drainage systems for both crop production and environmental protection. The objective of this research was to develop an empirical equation that estimates the design drainage rate (DDR) for local soil and weather conditions. This DDR can then be used in the Hooghoudt equation to estimate the optimum drain spacing that maximizes profit. We conducted DRAINMOD simulations for each combination of four factors: three drain depths, three effective radii, seven locations, and five soils. For each combination of factors, simulations were repeated for a range of drain spacing from 5 to 100 m using 30 years (1990–2019) of weather data planted with continuous corn (Zea mays L.). Simulations provided a 30-year average relative corn yield and drainage discharge for each combination of factors. The drain spacings that maximized annual economic return on investment were identified as optimum spacings and used to calculate the DDR for each combination of factors. Results were then used in a multiple linear regression to develop two empirical equations for northeast and southeast USA with DDR as the dependent variable. The independent variables were the long-term average growing-season precipitation, drain depth, equivalent saturated hydraulic conductivity, and depth to restrictive layer. The environmental value of the empirical equations is that they help avoid too narrow of a drain spacing, thereby preventing more drainage than is needed. In conclusion, application of these empirical equations is a means for estimating the site-specific optimum drain spacing that maximizes economic return on investment.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Ghane, Ehsan and Askar, Manal H. and Skaggs, R. Wayne}, year={2021}, month={Nov} }
 @article{askar_youssef_chescheir_negm_king_hesterberg_amoozegar_skaggs_2020, title={DRAINMOD Simulation of macropore flow at subsurface drained agricultural fields: Model modification and field testing}, volume={242}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2020.106401}, abstractNote={Macropores are critical pathways through which water and pollutants can bypass the soil matrix and be rapidly transported to subsurface drains and freshwater bodies. We modified the DRAINMOD model to simulate macropore flow using a simple approach as part of developing the DRAINMOD-P model to simulate phosphorus dynamics in artificially drained agricultural lands. The Hagen-Poiseuille’s law was used to estimate the flow capacity of macropores. When ponding depths on the soil surface are greater than Kirkham’s depth, water is assumed to flow through macropores directly to tile drains without interaction with the soil matrix. In the modified model, macropore size is adjusted based on wet or dry conditions while connectivity is altered by tillage. The model was tested using a 4-year data set from a subsurface drained field in northwest Ohio. The soils at the field are classified as very poorly drained and are prone to desiccation cracking. The modified model predicted the daily and monthly subsurface drainage with average Nash-Sutcliffe efficiency (NSE) values of 0.48 and 0.59, respectively. The cumulative drainage over the 4-year simulation period was under-predicted by 8%. The new macropore component was able to capture about 75% of 60 peak drainage flow events. However, surface runoff was over-predicted for the entire study period. Annual water budgets using measured data (precipitation, subsurface drainage, and surface runoff) and model predictions (evapotranspiration, vertical seepage, and change in storage) were not balanced with an average annual imbalance of 6.4 cm. The lack of closure in the water balance suggests that errors may have occurred in field measurements, particularly, surface runoff. Overall, incorporating macropore flow into DRAINMOD improved predictions of daily drainage peaks and enabled the model to predict subsurface drainage flux contributed by macropore flow, which is critical for expanding DRAINMOD to simulate phosphorus transport in subsurface drained agricultural land.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Askar, Manal H. and Youssef, Mohamed A. and Chescheir, George M. and Negm, Lamyaa M. and King, Kevin W. and Hesterberg, Dean L. and Amoozegar, Aziz and Skaggs, R. Wayne}, year={2020}, month={Dec} }
 @article{skaggs_amatya_chescheir_2020, title={Effects of Drainage for Silviculture on Wetland Hydrology}, volume={40}, ISSN={["1943-6246"]}, DOI={10.1007/s13157-019-01202-6}, number={1}, journal={WETLANDS}, author={Skaggs, R. Wayne and Amatya, Devendra M. and Chescheir, George M.}, year={2020}, month={Feb}, pages={47–64} }
 @article{amatya_chescheir_williams_skaggs_tian_2020, title={Long-Term Water Table Dynamics of Forested Wetlands: Drivers and their Effects on Wetland Hydrology in The Southeastern Atlantic Coastal Plain}, volume={40}, ISSN={["1943-6246"]}, DOI={10.1007/s13157-019-01153-y}, number={1}, journal={WETLANDS}, author={Amatya, D. M. and Chescheir, M. and Williams, M. and Skaggs, W. and Tian, S.}, year={2020}, month={Feb}, pages={65–79} }
 @article{vepraskas_skaggs_caldwell_2020, title={Method to Assess Climate Change Impacts on Hydrologic Boundaries of Individual Wetlands}, volume={40}, ISSN={["1943-6246"]}, url={https://doi.org/10.1007/s13157-019-01183-6}, DOI={10.1007/s13157-019-01183-6}, number={2}, journal={WETLANDS}, publisher={Springer Science and Business Media LLC}, author={Vepraskas, M. J. and Skaggs, R. W. and Caldwell, P.}, year={2020}, month={Apr}, pages={365–376} }
 @article{amatya_williams_nettles_skaggs_trettin_2019, title={COMPARISON OF HYDROLOGY OF TWO ATLANTIC COASTAL PLAIN FORESTS}, volume={62}, ISSN={["2151-0040"]}, DOI={10.13031/trans.13387}, abstractNote={Abstract. This article compares the short-term and long-term hydrology of two typical forests in the humid Atlantic Coastal Plain, including a relatively undisturbed forest with natural drainage in South Carolina (SC) and a drained pine plantation in North Carolina (NC), using monitoring and modeling approaches. Highly dynamic outflow (O) from both of these systems is driven by the water table (WT) position, as influenced by rainfall (R) and evapotranspiration (ET). The annual runoff coefficient (ROC) varied from 5% in dry years to 56% in wet years, depending on the soil water storage (SWS), with a significantly higher average value for the NC site despite its deeper WT, on average, than the SC site. Although both sites behaved similarly in extreme climate conditions, the change in SWS above the WT influenced the annual RO, ROC, and ET. The 17-year average annual ET of 1114 mm (R – O, assuming annual balanced SWS) for the SC site was significantly higher (p = 0.014) than the ET of the drained NC site (997 mm) despite the SC site’s lower mean annual R of 1370 mm, compared to 1520 mm for the NC site. This may be due to both the higher potential ET (PET) and soil water-holding capacity of the SC site. The SC site had higher frequency and duration of WT near the surface during winter, deeper summer WT, and higher correlation of annual ET to annual R (r 2 = 0.90 vs. 0.15), suggesting that the SC site was often moisture-limited, particularly during the growing season. Most of the streamflow in these systems occurred during winter, with low ET demands. However, summer periods with tropical storms also resulted in large RO events, generally with higher frequency and longer durations at the drained NC site. These results are similar to an earlier short-term comparison with an unstable behavior period at the SC site after Hurricane Hugo (1989). This study highlighted (1) the differences in hydrology between coastal forests drained for silvicultural production and undrained natural forests managed only for restoration, (2) the importance of long-term monitoring and the effects of regeneration as well as vegetation management on flow regime, and (3) the application and limitations of two widely used models (MIKESHE and DRAINMOD) in describing the hydrology of these forests. Long-term studies can be a basis for testing new hypotheses on water yield, stormwater management, wetland hydrology, vegetation restoration, bioenergy production, and climate change, in addition to applications of proper models for assessing the eco-hydrologic impacts of land use and climate change on freshwater coastal forests linked with downstream riparian rivers and estuaries affected by tidal fluxes and sea level rise. Highlights Outflow, driven by water table position on these forest systems, is highly variable, depending on its soil water storage. The hydrologic responses of both forest sites were similar during extreme climatic events or disturbances. Effect of forestry drainage on runoff was obscured by its large interannual differences. Long-term monitoring provides better insights on climate and vegetation management effects on flow regime and model validation Keywords: Drainage, Evapotranspiration, Hydrologic models, Pine forest, Poorly drained soils, Runoff coefficient, Water table.}, number={6}, journal={TRANSACTIONS OF THE ASABE}, author={Amatya, D. M. and Williams, T. M. and Nettles, J. E. and Skaggs, R. W. and Trettin, C. C.}, year={2019}, pages={1509–1529} }
 @article{negm_youssef_chescheir_skaggs_2019, title={DRAINMOD-based tools for quantifying reductions in annual drainage flow and nitrate losses resulting from drainage water management on croplands in eastern North Carolina (vol 166, pg 86, 2016)}, volume={226}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2019.105811}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Negm, L. M. and Youssef, M. A. and Chescheir, G. M. and Skaggs, R. W.}, year={2019}, month={Dec} }
 @article{liu_youssef_chescheir_appelboom_poole_arellano_skaggs_2019, title={Effect of controlled drainage on nitrogen fate and transport for a subsurface drained grass field receiving liquid swine lagoon effluent}, volume={217}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2019.02.018}, abstractNote={Abstract Application of livestock manure has become a principal nutrient source in groundwater and surface water. The goal of this research was to investigate the effect of controlled drainage (CD) on nitrogen (N) fate and transport for a subsurface drained grass field receiving liquid swine lagoon effluent (SLE). A four-year field experiment was conducted on a naturally poorly drained pasture in eastern North Carolina. The 1.25 ha experimental field was artificially drained by subsurface drains installed at 1.0 m depth and 12.5 m spacing. Two treatments, replicated twice were implemented: conventional drainage (FD) and CD. The CD management protocol was more intensive compared to previous studies. The drain outlets of CD plot were set at 36 cm below soil surface all year round except several days before irrigation application when water table depth was shallower than 65 cm below surface. Controlled drainage significantly reduced drainage flow and TN loading via subsurface drain lines by an average of 397 mm yr−1 (93%) and 34.5 kg N ha−1 yr−1 (94%), respectively. DRAINMOD hydrologic simulations indicated that 96% of the reduction in predicted drain flow was attributed to increased lateral seepage. The nitrogen that did not drain from the field in response to CD was lost via enhanced denitrification (67%) and lateral seepage to adjacent fields (33%). This study clearly demonstrated how CD management affects the N fate and transport through seepage and denitrification process.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Liu, Yu and Youssef, Mohamed A. and Chescheir, George M. and Appelboom, Timothy W. and Poole, Chad A. and Arellano, Consuelo and Skaggs, R. Wayne}, year={2019}, month={May}, pages={440–451} }
 @article{cacho_youssef_shi_chescheir_skaggs_tian_leggett_sucre_nettles_arellano_2019, title={Impacts on soil nitrogen availability of converting managed pine plantation into switchgrass monoculture for bioenergy}, volume={654}, ISSN={["1879-1026"]}, DOI={10.1016/j.scitotenv.2018.11.133}, abstractNote={Biofuels derived from lignocellulosic materials is one of the options in addressing issues on climate change and energy independence. One of the most promising bioenergy crops is switchgrass (Panicum virgatum L.), particularly in North America. Future advancement in large-scale conversion of lignocellulosic feedstocks and relatively more competitive price for biomass and other economic advantages could lead to landowners opting to venture on switchgrass monoculture (SWITCH) in lieu of loblolly pine monoculture (PINE). Therefore, we investigated the conversion of previously managed loblolly pine stand into SWITCH in eastern North Carolina, U.S.A. on soil N availability. Treatments included PINE, SWTICH, and mature loblolly pine stand (REF). Each treatment was replicated three times on 0.8 ha plots drained by open ditches dug 1.0–1.2 m deep and spaced at 100 m. Rates of net N mineralization (Nm) and nitrification (Nn) at the top 20 cm were measured using sequential in-situ techniques in 2011 and 2012 (the 3rd and 4th years of establishment, respectively) along with a one-time laboratory incubation. On average, PINE, SWITCH, and REF can have field net Nm rates up to 0.40, 0.34 and 0.44 mg N·kg soil−1·d−1, respectively, and net Nn rates up to 0.14, 0.08 and 0.10 mg N·kg soil−1·d−1, respectively. Annually, net Nm rates ranged from 136.98 to 167.21, 62.00 to 142.61, and 63.57 to 127.95 kg N·ha−1, and net Nn rates were 56.31–62.98, 16.45–30.45, 31.99–32.94 kg N·ha−1 in PINE, SWITCH, and REF, respectively. Treatment effect was not significant on field Nm rate (p = 0.091). However, SWITCH significantly reduced nitrate-N production (p < 0.01). Overall, results indicated that establishment of SWITCH on poorly drained lands previously under PINE is less likely to significantly impact total soil N availability and potentially has minimum N leaching losses since soil mineral N under this system will be dominated by ammonium-N.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Cacho, Julian F. and Youssef, Mohamed A. and Shi, Wei and Chescheir, George M. and Skaggs, R. Wayne and Tian, Shiying and Leggett, Zakiya H. and Sucre, Eric B. and Nettles, Jami E. and Arellano, Consuelo}, year={2019}, month={Mar}, pages={1326–1336} }
 @article{strock_hay_helmers_nelson_sands_skaggs_douglas-mankin_2018, title={ADVANCES IN DRAINAGE: SELECTED WORKS FROM THE TENTH INTERNATIONAL DRAINAGE SYMPOSIUM}, volume={61}, ISSN={["2151-0040"]}, DOI={10.13031/trans.12668}, abstractNote={Abstract. This article introduces a special collection of fourteen articles accepted from among the 140 technical presentations, posters, and meeting papers presented at the 10th International ASABE Drainage Symposium. The symposium continued in the tradition of previous symposia that began in 1965 as a forum for presenting and assessing the progress of drainage research and implementation throughout the world. The articles in this collection address a wide range of topics grouped into five broad categories: (1) crop response, (2) design and management, (3) hydrology and scale, (4) modeling, and (5) water quality. The collection provides valuable information for scientists, engineers, planners, and others working on crop production, water quality, and water quantity issues affected by agricultural drainage. The collection also provides perspectives on the challenges of increasing agricultural production in a changing climate, with ever-greater attention to water quality and quantity concerns that will require integrated technical, economic, and social solutions. Keywords: ASABE Drainage Symposium, crop response, design and management, hydrology and scale, modeling, water quality.}, number={1}, journal={TRANSACTIONS OF THE ASABE}, author={Strock, J. S. and Hay, C. H. and Helmers, M. J. and Nelson, K. A. and Sands, G. R. and Skaggs, R. W. and Douglas-Mankin, K. R.}, year={2018}, pages={161–168} }
 @article{youssef_abdelbaki_negm_skaggs_thorp_jaynes_2018, title={DRAINMOD-simulated performance of controlled drainage across the US Midwest}, volume={197}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2017.11.012}, abstractNote={Abstract Controlled drainage (CD) has recently been proposed as a best management practice for reducing nutrient export from drained cropland in the U.S. Midwest to the Mississippi River and the Gulf of Mexico. We conducted a 25-year simulation study using the hydrological model, DRAINMOD, and the carbon and nitrogen (N) model, DRAINMOD-NII, to evaluate the performance of CD at 48 locations across the U.S. Midwest. Hydrological and Nitrogen predictions of this simulation study were compared to RZWQM-DSSAT predictions by Thorp et al. (2008) . Simulation results showed that CD reduced annual subsurface drainage by 86 mm (30%) and annual N drainage losses by 10.9 kg N ha−1 (32%), on average over the 48 sites. DRAINMOD predicted highest reductions in drain flow at the south and southeast locations and lowest reductions at the northwest locations. The large reductions in drain flow in the south and southeast locations resulted in a large increase in surface runoff, which could increase soil erosion and sediment transport to surface water. In the north and northwest locations, the smaller amount of water that did not pass through the drainage system because of CD was primarily lost as evapotranspiration. DRAINMOD-NII predictions of annual reductions in N drainage loss followed the same trend of annual reductions in drainage flow. DRAINMOD-NII predictions show that reductions in N drainage loss under CD were mainly attributed to increase in denitrification. The declining trend in predicted annual denitrification from the southern to the northern locations of the Midwest region is most likely attributed to the lower temperature and less precipitation at the northern locations. RZWQM-DSSAT predicted reductions in annual drainage and N loss under CD conditions showed a similar trend to DRAINMOD/DRAINMOD-NII predictions. RZWQM-DSSAT, however, predicted substantially higher reductions in both drain flow (regional average of 151 mm yr−1, 53%) and N drainage losses (regional average of 18.9 kg N ha−1 yr−1, 51%). The discrepancies between DRAINMOD/DRAINMOD-NII and RZWQM-DSSAT predictions of annual reductions in drain flow and N loss under CD conditions were caused by differences in model predictions of individual components of the water and nitrogen balances under both free drainage and controlled drainage scenarios. Overall, this simulation study showed that climate variation across the region has a substantial impact on CD efficacy for reducing N drainage loss.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Youssef, Mohamed A. and Abdelbaki, Ahmed M. and Negm, Lamyaa M. and Skaggs, R. Wayne and Thorp, Kelly R. and Jaynes, Dan B.}, year={2018}, month={Jan}, pages={54–66} }
 @article{poole_skaggs_youssef_chescheir_crozier_2018, title={EFFECT OF DRAINAGE WATER MANAGEMENT ON NITRATE NITROGEN LOSS TO TILE DRAINS IN NORTH CAROLINA}, volume={61}, ISSN={["2151-0040"]}, DOI={10.13031/trans.12296}, abstractNote={Abstract. Short-term studies have demonstrated that drainage water management (DWM), or controlled drainage (CD), can be used to substantially reduce the loss of nitrogen (N) from drained lands for a wide range of soils, crops, locations, and climates. Long-term studies on the effects of the practice are limited. This article presents results on the effects of CD on nitrate-N (NO 3 -N) losses for three crops, corn ( L.), wheat ( L.), and soybean ( [L.] Merr.), in a two-year rotation in North Carolina. Nitrate losses were measured on replicated plots under CD and conventional, or free drainage (FD), treatments for nine years between 1992 and 2012 on a tile-drained site near Plymouth, North Carolina. The site is on a Portsmouth sandy loam soil with parallel drains 22.9 m apart and 1.15 m deep. The subsurface drainage characteristics under FD were drainage intensity (DI) = 8 mm d -1 , drainage coefficient (DC) = 14 mm d -1 , and Kirkham coefficient (KC) = 18 mm d -1 . Compared to FD, CD reduced annual drainage outflow by 33% and NO 3 -N export by 30%, with an average annual reduction of 6.3 kg ha -1 year -1 . CD increased average NO 3 -N concentrations by 0.9 mg L -1 , but the difference was not significant. The reduction in NO 3 -N export observed in the CD treatment was equal to the increase in N removed by the harvested grain. The results document the effects of CD on NO 3 -N export over a wide range of weather conditions during the nine-year study. While the average 30% reduction in NO 3 -N losses in drainage water is in the midrange of that reported by previous studies for different soils and climates, this is believed to be the first time such a reduction has been attributed to the effect of CD on increasing yields and N removed in the harvested grain. Keywords: Controlled drainage (CD), Corn, Drainage water, Drainage water management (DWM), Nitrate, Nitrogen, Soybean, Water quality, Wheat.}, number={1}, journal={TRANSACTIONS OF THE ASABE}, author={Poole, C. A. and Skaggs, R. W. and Youssef, M. A. and Chescheir, G. M. and Crozier, C. R.}, year={2018}, pages={233–244} }
 @article{skaggs_2017, title={COEFFICIENTS FOR QUANTIFYING SUBSURFACE DRAINAGE RATES}, volume={33}, ISSN={["1943-7838"]}, DOI={10.13031/aea.12302}, abstractNote={Abstract. It is proposed that technical papers on drainage research studies and engineered design projects should report standard coefficients or parameters that characterize the hydraulics of the system. The following coefficients define key subsurface drainage rates that can be used to quantify and compare the hydraulics of drainage systems across sites, soils and geographic locations. (1) The steady subsurface drainage rate (cm/d) corresponding to a saturated profile with a ponded surface. This subsurface drainage rate defines the length of time that water remains ponded on the soil surface following large rainfall events. It is proposed that this rate be called the Kirkham Coefficient (KC) in honor of Professor Don Kirkham who derived analytical solutions for saturated drained profiles for most soil and boundary conditions of interest. (2) Drainage intensity (DI), which represents the drainage rate (cm/d) when the water table midway between parallel drains is coincident with the surface. The DI can be estimated by the Hooghoudt equation and is dependent on the effective saturated hydraulic conductivity of the profile, drain depth, spacing, and depth of the soil profile or restrictive layer. (3) The drainage coefficient (DC), which quantifies the hydraulic capacity of the system. This value is the rate (cm/d) that the outlet works can remove water from the site. It is dependent on the size, slope, and hydraulic roughness of the laterals, submains, mains, and, in cases where pumped outlets are used, the pumping capacity. Routine inclusion of these three coefficients in the documentation of research and design projects would facilitate comparison of results from different soils and drainage systems, and generally, the meta-analysis of data pertaining to drainage studies. Keywords: Drainage, Drainage intensity, Drainage coefficient, Drainage nomenclature, Kirkham Coefficient.}, number={6}, journal={APPLIED ENGINEERING IN AGRICULTURE}, author={Skaggs, R. Wayne}, year={2017}, pages={793–799} }
 @article{ssegane_amatya_muwamba_chescheir_appelboom_tollner_nettles_youssef_birgand_skaggs_et al._2017, title={Calibration of paired watersheds: Utility of moving sums in presence of externalities}, volume={31}, ISSN={0885-6087}, url={http://dx.doi.org/10.1002/hyp.11248}, DOI={10.1002/hyp.11248}, abstractNote={Historically, paired watershed studies have been used to quantify the hydrological effects of land use and management practices by concurrently monitoring 2 similar watersheds during calibration (pretreatment) and post-treatment periods. This study characterizes seasonal water table and flow response to rainfall during the calibration period and tests a change detection technique of moving sums of recursive residuals (MOSUM) to select calibration periods for each control–treatment watershed pair when the regression coefficients for daily water table elevation were most stable to minimize regression model uncertainty. The control and treatment watersheds were 1 watershed of 3–4-year-old intensely managed loblolly pine (Pinus taeda L.) with natural understory, 1 watershed of 3–4-year-old loblolly pine intercropped with switchgrass (Panicum virgatum), 1 watershed of 14–15-year-old thinned loblolly pine with natural understory (control), and 1 watershed of switchgrass only. The study period spanned from 2009 to 2012. Silvicultural operational practices during this period acted as external factors, potentially shifting hydrologic calibration relationships between control and treatment watersheds. MOSUM results indicated significant changes in regression parameters due to silvicultural operations and were used to identify stable relationships for water table elevation. None of the calibration relationships developed using this method were significantly different from the classical calibration relationship based on published historical data. We attribute that to the similarity of historical and 2010–2012 leaf area index on control and treatment watersheds as moderated by the emergent vegetation. Although the MOSUM approach does not eliminate the need for true calibration data or replace the classic paired watershed approach, our results show that it may be an effective alternative approach when true data are unavailable, as it minimizes the impacts of external disturbances other than the treatment of interest.}, number={20}, journal={Hydrological Processes}, publisher={Wiley}, author={Ssegane, H. and Amatya, D. M. and Muwamba, A. and Chescheir, G. M. and Appelboom, T. and Tollner, E. W. and Nettles, J. E. and Youssef, M. A. and Birgand, François and Skaggs, R. W. and et al.}, year={2017}, month={Sep}, pages={3458–3471} }
 @article{muwamba_amatya_chescheir_nettles_appelboom_ssegane_tollner_youssef_birgand_skaggs_et al._2017, title={Water Quality Effects of Switchgrass Intercropping on Pine Forest in Coastal North Carolina}, volume={60}, ISSN={2151-0040}, url={http://dx.doi.org/10.13031/trans.12181}, DOI={10.13031/trans.12181}, abstractNote={Abstract. Interplanting a cellulosic bioenergy crop (switchgrass, L.) between loblolly pine ( L.) rows could potentially provide a sustainable source of bio-feedstock without competing for land currently in food production. The objectives of this study were to: (1) quantify the concentrations and loads of drainage water nitrogen (N) and phosphorus (phosphate) associated with establishment and growth of switchgrass treatments and compare them with those for a mid-rotation pine forest (control), and (2) quantify the treatment effects on drainage water N and phosphate (IC) and switchgrass only (SG). Thinned mid-rotation loblolly pine with natural understory (MP) was used as the control. Pretreatment calibration equations for nutrients were obtained using a paired watershed approach and bootstrap geometric regression with 2007-2008 data, when pine on all sites had reached canopy closure. Treatment effects were calculated as the difference between expected values from the pretreatment relationship and measured data for the treatment period. Precipitation, outflow, and N and phosphate concentrations in the outflow were measured during calibration (Jan. 2007 to Dec. 2008), site preparation for switchgrass establishment (Nov. 2009 to Mar. 2012), and switchgrass growth (Apr. 2012 to Apr. 2014). Mean NO 3 -N concentrations and loads were significantly (a = 0.05) greater for SG than for IC during the switchgrass growth period. Average treatment concentrations with standard errors and total load effects during switchgrass growth for NO 3 -N followed the trends SG (-0.002 ±0.01 mg L -1 ) > IC (-0.12 ±0.04 mg L -1 ) and SG (0.75kg ha -1 ) > IC (0.23kg ha -1 ), respectively. For phosphate average concentrations and loads, the treatment effects during switchgrass growth followed the trends SG (-0.004 ± mg L -1 ) >IC ( -0.02 ± mg L -1 ) and IC (-0.43 kg ha -1 ) > SG (-0.70 kg ha -1 ), respectively. Average concentration effects for NO 3 -N and phosphate and total load effects for phosphate significantly (a = 0.05) decreased for IC compared to the MP control. These results suggest that the intercropping treatment (IC) with loblolly pine and switchgrass improved water quality by reducing NO 3 -N and phosphate concentrations and phosphate loads. Keywords: Bioenergy crop, Bootstrap geometric regression, Loblolly pine, Nutrients, Paired watershed.}, number={5}, journal={Transactions of the ASABE}, publisher={American Society of Agricultural and Biological Engineers (ASABE)}, author={Muwamba, Augustine and Amatya, Devendra M. and Chescheir, George M. and Nettles, Jami E. and Appelboom, Timothy and Ssegane, Herbert and Tollner, Ernest E and Youssef, Mohamed A. and Birgand, Francois and Skaggs, R. Wayne and et al.}, year={2017}, pages={1607–1620} }
 @article{negm_youssef_chescheir_skaggs_2016, title={DRAINMOD-based tools for quantifying reductions in annual drainage flow and nitrate losses resulting from drainage water management on croplands in eastern North Carolina}, volume={166}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2015.12.014}, abstractNote={Nitrogen (N) leachate of drained agriculture has continued to be pervasive in the U.S. water resources. Nitrogen credit exchange program is a trading market to facilitate pollutant reductions and protect the environment. A simple tool suitable for eastern North Carolina (NC) was developed to quantify drainage flow and N mass reductions resulting from drainage water management (DWM); an efficient and common conservation practice for drained agricultural lands. The tool comprises a set of regression equations estimating the performance of DWM as a function of local site conditions. DRAINMOD and DRAINMOD-NII models simulations were conducted for a wide range of soil types, weather conditions, and management practices for different locations in eastern NC. Simulation results were used with SAS 9.3 software to develop a set of multi-linear regression equations to estimate DWM-caused reductions in annual drainage flow and corresponding nitrate-N (NO3-N) losses for continuous corn (CC) and corn–wheat–soybean (CWS) cropping systems. The regression model estimations of annual drainage flow were highly correlated with DRAINMOD simulated values with an adjusted coefficient of multiple determination (R2adj) equal to 0.91 or higher for different management scenarios. Similarly, the regression model estimations of annual nitrate losses achieved an R2adj of 0.88 or higher for all management scenarios. The developed regression models were further compared on a year-by-year basis to the calibrated DRAINMOD and DRAINMOD-NII models for local conditions of an experimental site in eastern NC over 25 years. Estimated annual drainage flow and NO3-N losses were in good agreement with corresponding values simulated by DRAINMOD-based models for CC and CWS under free and controlled drainage modes. In terms of DWM-induced annual reductions in drainage flow and N losses, noticeable differences occurred in several years between predictions of DRAINMOD-NII and the regression models. A comparison based on the 5-year moving average of DWM-induced reductions smoothed out the extreme year-to-year variations and indicated very similar reduction trends provided by both methods. The results presented in this case study indicated that the simple regression method provides an adequate alternative to the processes based DRAINMOD suite of models for estimating annual reductions in drainage rates and N mass losses resulting from implementation of DWM. Similar tools can be developed for other regions in the US and abroad that initiate nitrogen trading markets involving DWM.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Negm, L. M. and Youssef, M. A. and Chescheir, G. M. and Skaggs, R. W.}, year={2016}, month={Mar}, pages={86–100} }
 @article{skaggs_tian_chescheir_amatya_youssef_2016, title={Forest drainage}, DOI={10.1079/9781780646602.0124}, journal={Forest Hydrology: Processes, Management and Assessment}, author={Skaggs, R. W. and Tian, S. and Chescheir, G. M. and Amatya, D. M. and Youssef, M. A.}, year={2016}, pages={124–140} }
 @article{mcdaniel_skaggs_negm_2016, title={INJURY AND RECOVERY OF MAIZE ROOTS AFFECTED BY FLOODING}, volume={32}, ISSN={["1943-7838"]}, DOI={10.13031/aea.32.11633}, abstractNote={<abstract> Proper soil environment for adequate root growth is crucial to maintain crop yields. Excessively wet soil conditions cause poor root growth and restrict plant water and nutrient uptake. The purpose of this study was to investigate corn root response to flooding and the root system ability to recover after saturation. Corn plants were grown in 1.3 m tall acrylic cylinders under controlled conditions. Plants were subjected to flooding for 1, 3, or 5 days at growth stages V4, V8, V12, and R1. Data were collected throughout the growing season on: root growth, root depth, and root mass. Root mortality was noted after the flooding treatment. Roots responded quickly to saturation (flooding) and all growth ceased within 24 h. Recovery to normal growth rates occurred within five days unless the saturation caused severe plant damage. One day of flooding did not affect root mass at any stage of growth. Root mass was not affected by three days of flooding during the V4 and V8 stages. A significant reduction of root mass was noted for more than 35 days following 3-day inundation during V12 and R1. Flooding for five days during stage V4, V12, and R1 caused a significant reduction in root mass. Plants flooded during V8 suffered no significant reduction in root mass. Assessment of root response to saturated soil conditions is necessary to improve groundwater table management practices in high water table soils, as well as to enhance the performance of simulation model that predict plant growth and yield on drained lands.}, number={5}, journal={APPLIED ENGINEERING IN AGRICULTURE}, author={McDaniel, V. and Skaggs, R. W. and Negm, L. M.}, year={2016}, pages={627–638} }
 @article{muwamba_amatya_ssegane_chescheir_appelboom_tollner_nettles_youssef_birgand_skaggs_et al._2015, title={Effects of Site Preparation for Pine Forest/Switchgrass Intercropping on Water Quality}, volume={44}, ISSN={0047-2425}, url={http://dx.doi.org/10.2134/jeq2014.11.0505}, DOI={10.2134/jeq2014.11.0505}, abstractNote={A study was initiated to investigate the sustainability effects of intercropping switchgrass (Panicum virgatum L.) in a loblolly pine (Pinus taeda L.) plantation. This forest-based biofuel system could possibly provide biomass from the perennial energy grass while maintaining the economics and environmental benefits of a forest managed for sawtimber. Operations necessary for successful switchgrass establishment and growth, such as site preparation, planting, fertilizing, mowing and baling, may affect hydrology and nutrient runoff. The objectives of this study were (i) to characterize the temporal effects of management on nutrient concentrations and loadings and (ii) to use pretreatment data to predict those treatment effects. The study watersheds (∼25 ha each) in the North Carolina Atlantic Coastal Plain were a pine/switchgrass intercropped site (D1), a midrotation thinned pine site with natural understory (D2), and a switchgrass-only site (D3). Rainfall, drainage, water table elevation, nitrogen (total Kjedahl N, NH4–N, and NO3–N), and phosphate were monitored for the 2007–2008 pretreatment and the 2009–2012 treatment periods. From 2010 to 2011 in site D1, the average NO3–N concentration effects decreased from 0.18 to −0.09 mg L−1, and loads effects decreased from 0.86 to 0.49 kg ha−1. During the same period in site D3, the average NO3–N concentration effects increased from 0.03 to 0.09 mg L−1, and loads effects increased from −0.26 to 1.24 kg ha−1. This study shows the importance of considering water quality effects associated with intensive management operations required for switchgrass establishment or other novel forest-based biofuel systems.}, number={4}, journal={Journal of Environment Quality}, publisher={American Society of Agronomy}, author={Muwamba, A. and Amatya, D. M. and Ssegane, H. and Chescheir, G.M. and Appelboom, T. and Tollner, E.W. and Nettles, J. E. and Youssef, M. A. and Birgand, F. and Skaggs, R. W. and et al.}, year={2015}, pages={1263} }
 @article{arnold_youssef_yen_white_sheshukov_sadeghi_moriasi_steiner_amatya_skaggs_et al._2015, title={Hydrological processes and model representation: Impact of soft data on calibration}, volume={58}, DOI={10.13031/trans.58.10726}, abstractNote={Hydrologic and water quality models are increasingly used to determine the environmental impacts of climate variability and land management. Due to differing model objectives and differences in monitored data, there are currently no universally accepted procedures for model calibration and validation in the literature. In an effort to develop accepted model calibration and validation procedures or guidelines, a special collection of 22 research articles that present and discuss calibration strategies for 25 hydrologic and water quality models was previously assembled. The models vary in scale temporally as well as spatially from point source to the watershed level. One suggestion for future work was to synthesize relevant information from this special collection and to identify significant calibration and validation topics. The objective of this article is to discuss the importance of accurate representation of model processes and its impact on calibration and scenario analysis using the information from these 22 research articles and other relevant literature. Models are divided into three categories: (1) flow, heat, and solute transport, (2) field scale, and (3) watershed scale. Processes simulated by models in each category are reviewed and discussed. In this article, model case studies are used to illustrate situations in which a model can show excellent statistical agreement with measured stream gauge data, while misrepresented processes (water balance, nutrient balance, sediment source/sinks) within a field or watershed can cause errors when running management scenarios. These errors may be amplified at the watershed scale where additional sources and transport processes are simulated. To account for processes in calibration, a diagnostic approach is recommended using both hard and soft data. The diagnostic approach looks at signature patterns of behavior of model outputs to determine which processes, and thus parameters representing them, need further adjustment during calibration. This overcomes the weaknesses of traditional regression-based calibration by discriminating between multiple processes within a budget. Hard data are defined as long-term, measured time series, typically at a point within a watershed. Soft data are defined as information on individual processes within a budget that may not be directly measured within the study area, may be just an average annual estimate, and may entail considerable uncertainty. The advantage of developing soft data sets for calibration is that they require a basic understanding of processes (water, sediment, nutrient, and carbon budgets) within the spatial area being modeled and constrain the calibration.}, number={6}, journal={Transactions of the ASABE}, author={Arnold, J. G. and Youssef, M. A. and Yen, H. and White, M. J. and Sheshukov, A. Y. and Sadeghi, A. M. and Moriasi, D. N. and Steiner, J. L. and Amatya, D. M. and Skaggs, R. W. and et al.}, year={2015}, pages={1637–1660} }
 @article{cacho_youssef_chescheir_skaggs_leggett_sucre_nettles_arellano_2015, title={Impacts of switchgrass-loblolly pine intercropping on soil physical properties of a drained forest}, volume={58}, DOI={10.13031/trans.58.11238}, abstractNote={<abstract> Intercropping switchgrass ( L.) with managed loblolly pine ( L.) has been proposed as an alternative source of bioenergy feedstock that does not require conversion of agricultural cropland. Different management practices may alter soil physical properties (SPP), which could influence productivity, hydrologic and biogeochemical processes. Therefore, we investigated the effect of switchgrass-loblolly pine intercropping on the SPP of a poorly drained forest soil in eastern North Carolina using three management regimes: young loblolly pine stand (PINE), switchgrass-pine intercropping (PSWITCH), and a 38-year-old loblolly pine stand (REF). Measurements of SPP were conducted before and after the third annual harvesting operation using intact soil cores taken from three points within each of three replicated plots and at three depths: 0-15 cm, 15-30 cm, and 30-45 cm. Pre- and post-harvest values of SPP in PSWITCH were not significantly different. Compared to PINE, changes in bulk density and in both total porosity and saturated hydraulic conductivity in PSWITCH were significant only in the top 30 and 15 cm of soil, respectively. Volume drained and drainable porosity in PSWITCH decreased significantly at water table depths ≤45 cm. Cumulative effects of V-shearing for switchgrass seedbed preparation and the first and second harvest operations may have caused structural changes to the surface soil layer in PSWITCH that subsequently resulted in the measured differences in SPP between PSWITCH and PINE. We suggest that soil disturbance should be minimized during field operations to lessen the adverse effects on SPP, and models used to quantify impacts of management practices and land use change on the hydrology and biogeochemistry of managed forests should consider SPP changes caused by management regimes.}, number={6}, journal={Transactions of the ASABE}, author={Cacho, J. F. and Youssef, M. A. and Chescheir, G. M. and Skaggs, R. W. and Leggett, Zakiya H and Sucre, E. B. and Nettles, J. E. and Arellano, C.}, year={2015}, pages={1573–1583} }
 @article{tian_youssef_sun_chescheir_noormets_amatya_skaggs_king_mcnulty_gavazzi_et al._2015, title={Testing DRAINMOD-FOREST for predicting evapotranspiration in a mid-rotation pine plantation}, volume={355}, ISSN={["1872-7042"]}, DOI={10.1016/j.foreco.2015.03.028}, abstractNote={Evapotranspiration (ET) is a key component of the hydrologic cycle in terrestrial ecosystems and accurate description of ET processes is essential for developing reliable ecohydrological models. This study investigated the accuracy of ET prediction by the DRAINMOD-FOREST after its calibration/validation for predicting commonly measured hydrological variables. The model was tested by conducting an eight year simulation of drainage and shallow groundwater dynamics in a managed mid-rotation loblolly pine (Pinus taeda L.) plantation located in the coastal plain of North Carolina, USA. Modeled daily ET rates were compared to those measured in the field using the eddy covariance technique. In addition, the wavelet transform and coherence analysis were used to compare ET predictions and measurements on the time–frequency domain. Results showed that DRAINMOD-FOREST accurately predicted annual and monthly ET after a successful calibration and validation using measured drainage rates and water table depth. The model under predicted ET on an annual basis by 2%, while the Nash–Sutcliffe coefficient of model predictions on a monthly basis was 0.78. Results from wavelet transform and coherence analysis demonstrated that the model reasonably captured the high power spectra of ET at an annual scale with significantly high model-data coherency. These results suggested that the calibrated DRAINMOD-FOREST collectively captured key factors and mechanisms controlling ET dynamics in the drained pine plantation. However, the global power spectrum revealed that the model over predicted the power spectrum of ET at an annual scale, suggesting the model may have under predicted canopy conductance during non-growing seasons. In addition, this study also suggested that DRAINMOD-FOREST did not properly capture the seasonal dynamics of ET under extreme drought conditions with deeper water table depths. These results suggested further refinement to parameters, particularly vegetation related, and structures of DRAINMOD-FOREST to achieve better agreement between ET predictions and measurements in the time–frequency domain.}, journal={FOREST ECOLOGY AND MANAGEMENT}, author={Tian, Shiying and Youssef, Mohamed A. and Sun, Ge and Chescheir, George M. and Noormets, Asko and Amatya, Devendra M. and Skaggs, R. Wayne and King, John S. and McNulty, Steve and Gavazzi, Michael and et al.}, year={2015}, month={Nov}, pages={37–47} }
 @article{negm_youssef_skaggs_chescheir_kladivko_2014, title={DRAINMOD-DSSAT Simulation of the Hydrology, Nitrogen Dynamics, and Plant Growth of a Drained Corn Field in Indiana}, volume={140}, ISSN={["1943-4774"]}, DOI={10.1061/(asce)ir.1943-4774.0000738}, abstractNote={DRAINMOD-DSSAT is an integrated model recently developed to simulate the hydrology, water quality, and crop growth for artificially drained croplands. DRAINMOD-DSSAT is an advanced research tool that contributes to increasing productivity, reducing cost, and enhancing sustainability of crop production on high water table soils with artificial drainage. In this study, the performance of the model was evaluated using a six-year data set (1985–1990) collected from a subsurface drained agricultural research site in Indiana in the United States. Subsurface drains were installed at three different spacings (5, 10, and 20 m). During the simulation period, all treatments were planted to corn receiving high preplant N-fertilization rates. Rainfall patterns varied significantly among the years. DRAINMOD-DSSAT predictions of monthly and annual drainage flow, and nitrate losses, were in good agreement with measured values. Similarly, variations in corn yield patterns were well captured by the model across different treatments. Other crop growth-related variables and soil-N process rates were reasonably predicted compared with values reported in the literature. These results demonstrated the potential of DRAINMOD-DSSAT to simulate different components of an agricultural system involving different management practices and subjected to variable climatic conditions.}, number={8}, journal={JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING}, author={Negm, Lamyaa M. and Youssef, Mohamed A. and Skaggs, Richard W. and Chescheir, George M. and Kladivko, Eileen J.}, year={2014}, month={Aug} }
 @article{brown_skaggs_hunt_2013, title={Calibration and validation of DRAINMOD to model bioretention hydrology}, volume={486}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/J.JHYDROL.2013.02.017}, DOI={10.1016/j.jhydrol.2013.02.017}, abstractNote={Previous field studies have shown that the hydrologic performance of bioretention cells varies greatly because of factors such as underlying soil type, physiographic region, drainage configuration, surface storage volume, drainage area to bioretention surface area ratio, and media depth. To more accurately describe bioretention hydrologic response, a long-term hydrologic model that generates a water balance is needed. Some current bioretention models lack the ability to perform long-term simulations and others have never been calibrated from field monitored bioretention cells with underdrains. All peer-reviewed models lack the ability to simultaneously perform both of the following functions: (1) model an internal water storage (IWS) zone drainage configuration and (2) account for soil–water content using the soil–water characteristic curve. DRAINMOD, a widely-accepted agricultural drainage model, was used to simulate the hydrologic response of runoff entering a bioretention cell. The concepts of water movement in bioretention cells are very similar to those of agricultural fields with drainage pipes, so many bioretention design specifications corresponded directly to DRAINMOD inputs. Detailed hydrologic measurements were collected from two bioretention field sites in Nashville and Rocky Mount, North Carolina, to calibrate and test the model. Each field site had two sets of bioretention cells with varying media depths, media types, drainage configurations, underlying soil types, and surface storage volumes. After 12 months, one of these characteristics was altered – surface storage volume at Nashville and IWS zone depth at Rocky Mount. At Nashville, during the second year (post-repair period), the Nash–Sutcliffe coefficients for drainage and exfiltration/evapotranspiration (ET) both exceeded 0.8 during the calibration and validation periods. During the first year (pre-repair period), the Nash–Sutcliffe coefficients for drainage, overflow, and exfiltration/ET ranged from 0.6 to 0.9 during both the calibration and validation periods. The bioretention cells at Rocky Mount included an IWS zone. For both the calibration and validation periods, the modeled volume of exfiltration/ET was within 1% and 5% of the estimated volume for the cells with sand (Sand cell) and sandy clay loam (SCL cell) underlying soils, respectively. Nash–Sutcliffe coefficients for the SCL cell during both the calibration and validation periods were 0.92.}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Brown, R.A. and Skaggs, R.W. and Hunt, W.F., III}, year={2013}, month={Apr}, pages={430–442} }
 @article{poole_skaggs_cheschier_youssef_crozier_2013, title={Effects of drainage water management on crop yields in North Carolina}, volume={68}, ISSN={["1941-3300"]}, DOI={10.2489/jswc.68.6.429}, abstractNote={Research studies on a wide range of soils, crops, locations, and climates have shown that drainage water management (DWM), or controlled drainage (CD), can be used to substantially reduce the loss of nitrogen (N), and in some cases, phosphorus (P) from drained agricultural lands to surface waters. The adoption and widespread application of DWM depends on a variety of factors including its impact on crop yields. This paper presents results from a long term field study on the effect of DWM or CD on crop yields in a three-crop, two-year corn/wheat–soybean rotation. Yields were measured on replicated field scale plots under CD and conventional or free drainage (FD) treatments for a total of 18 crops on two experimental sites during the period from 1990 to 2011. Data were collected on 7 corn (<i>Zea mays</i> L.) crops, 5 wheat (<i>Triticum aestivum</i> L.) crops, and 6 soybean (<i>Glycine max</i> L.) crops. Controlled drainage had no significant effect on yields of winter wheat, which in North Carolina is grown in the wettest, coolest part of the year. Controlled drainage increased corn yields compared to FD in all seven years. The average yield increase for corn was 11%. Controlled drainage also increased soybean yield in all years with an average increase of 10% compared to FD. Such yield responses will promote the application of DWM, which will result in both economic and environmental benefits.}, number={6}, journal={JOURNAL OF SOIL AND WATER CONSERVATION}, author={Poole, C. A. and Skaggs, R. W. and Cheschier, G. M. and Youssef, M. A. and Crozier, C. R.}, year={2013}, pages={429–437} }
 @article{kim_amatya_chescheir_skaggs_nettles_2013, title={Hydrologic Effects of Size and Location of Fields Converted from Drained Pine Forest to Agricultural Cropland}, volume={18}, ISSN={["1943-5584"]}, DOI={10.1061/(asce)he.1943-5584.0000566}, abstractNote={Hydrological effects of land-use change are of great concern to ecohydrologists and watershed managers, especially in the Atlantic coastal plain of the southeastern United States. The concern is attributable to rapid population growth and the resulting pressure to develop forested lands. Many researchers have studied these effects in various scales, with varying results. An extended watershed-scale forest hydrologic model, calibrated with 1996–2000 data, was used to evaluate long-term hydrologic effects of conversion to agriculture (corn–wheat–soybean cropland) of a 29.5-km2 intensively managed pine-forested watershed in Washington County in eastern North Carolina. Fifty years of weather data (1951–2000) from a nearby weather station were used for simulating hydrology to evaluate effects on outflows, evapotranspiration, and water table depth compared with the baseline scenario. Other simulation scenarios were created for each of five different percentages (10, 25, 50, 75, and 100%) of land-use conversion occurring at upstream and downstream locations in the pine-forest watershed. Simulations revealed that increased mean annual outflow was significant (α=0.05) only for 100% conversion from forest (261 mm) to agricultural crop (326 mm), primarily attributed to a reduction in evapotranspiration. Although high flow rates >5 mm day−1 increased from 2.3 to 2.6% (downstream) and 2.6 to 4.2% (upstream) for 25 to 50% conversion, the frequency was higher for the upstream location than the downstream. These results were attributed to a substantial decrease in soil hydraulic conductivity of one of the dominant soils in the upstream location, which is expected after land-use conversion to agriculture. As a result, predicted subsurface drainage decreased, and surface runoff increased as soil hydraulic conductivity decreased for the soil upstream. These results indicate that soil hydraulic properties resulting from land-use conversion have a greater influence on hydrologic components than the location of land use conversion.}, number={5}, journal={JOURNAL OF HYDROLOGIC ENGINEERING}, author={Kim, Hyun Woo and Amatya, Devendra M. and Chescheir, George M. and Skaggs, Wayne R. and Nettles, Jami E.}, year={2013}, month={May}, pages={552–566} }
 @misc{tian_youssef_skaggs_chescheir_amatya_2013, title={Predicting dissolved organic nitrogen export from a drained loblolly pine plantation}, volume={49}, ISSN={["1944-7973"]}, DOI={10.1002/wrcr.20157}, abstractNote={[1] Dissolved organic nitrogen (DON) export from terrestrial ecosystems influences the ecology of receiving surface waters. The soil carbon (C) and nitrogen (N) model, DRAINMOD-N II, was modified to simulate key processes associated with DON transformations and transport in the soil profile. DON production is modeled by tracking dynamic C:N ratios of dissolved organic matter originating from various organic matter pools. The Langmuir isotherm was used to quantify the assumed instantaneous equilibrium between potentially soluble organic N in solid and aqueous phases. DON transport with soil water was simulated using a numerical solution to the advection-dispersion reaction equation. The modified model was used for simulating temporal variations of DON export from three loblolly pine (Pinus taeda L.) plantations located in eastern North Carolina. Results showed that the model can accurately predict DON export dynamics during storm events with Nash-Sutcliffe efficiency (E) of 0.5, seasonal DON losses with E above 0.6, and annual DON losses with E above 0.7. In addition to the well-recognized role of hydrological processes, reasonable quantifications of the seasonal changes in the potentially soluble soil organic matter, the DON sorption to soil particles, and the dynamic C:N ratios of dissolved organic matter were found to be essential for mechanistic representation of DON export dynamics. Specifically, adapting the dynamic C:N ratios enabled the model to reasonably describe the temporal variations of correlations between DON and dissolved organic carbon in drainage water.}, number={4}, journal={WATER RESOURCES RESEARCH}, author={Tian, Shiying and Youssef, Mohamed A. and Skaggs, R. Wayne and Chescheir, G. M. and Amatya, Devendra M.}, year={2013}, month={Apr}, pages={1952–1967} }
 @misc{amatya_rossi_saleh_dai_youssef_williams_bosch_chescheir_sun_skaggs_et al._2013, title={Review of nitrogen fate models applicable to forest landscapes in the southern US}, volume={56}, DOI={10.13031/trans.56.10096}, abstractNote={<abstract><title><italic>Abstract.</italic></title> Assessing the environmental impacts of fertilizer nitrogen (N) used to increase productivity in managed forests is complex due to a wide range of abiotic and biotic factors affecting its forms and movement. Models developed to predict fertilizer N fate (e.g., cycling processes) and water quality impacts vary widely in their design, scope, and potential application. We review the applicability of five commonly used eco-hydrologic models (APEX, MIKESHE-DNDC, DRAINMOD-FOREST, REMM, and SWAT) in assessing N fate and transport in southern forest landscapes (<50 km<sup>2</sup>) because of their comprehensiveness and multi-scale predictions. The field-scale models DRAINMOD-FOREST and REMM contain process-level components characterizing hydrology, forest growth, and N dynamics, but they have limited capability to describe transport processes at the landscape scale. APEX can describe hydrology, forest growth, N fate processes, and plant competition at the landscape and small watershed scales mostly for upland. SWAT is best suited to hydrologic simulations at watershed scale (>50 km<sup>2</sup>), although N routing below the subbasin level does not yet exist. Similarly, the distributed MIKESHE-DNDC model has been used to assess N cycles across different spatial scales, on both uplands and lowlands, but was not intended to model lateral N transport. However, MIKESHE alone is capable of describing the hydrology and N transport. The strengths of each of the models reflect their original design and scope intent. Based on this review, none of the five models that we considered is independently adequate to address the fate of N fertilizers applied to forest stands at both small and large scales, including uplands and lowlands. While efforts are underway to extend these tools’ capabilities and address their various limitations, the models must be validated using experimental data before using their outputs, together with uncertainty analysis, for developing forest fertilization guidelines and the fate and transport of N.}, number={5}, journal={Transactions of the ASABE}, author={Amatya, D. M. and Rossi, C. G. and Saleh, A. and Dai, Z. and Youssef, M. A. and Williams, R. G. and Bosch, D. D. and Chescheir, G. M. and Sun, G. and Skaggs, R. W. and et al.}, year={2013}, pages={1731–1757} }
 @article{skaggs_youssef_chescheir_2012, title={A Drainmod-based method to estimate effects of drainage water management on annual nitrogen loss to surface water}, volume={55}, DOI={10.13031/2013.41515}, abstractNote={Effects of drainage water management (DWM) on nitrogen (N) losses to surface waters can be estimated by multiplying the change in subsurface drainage and surface runoff, as predicted by DRAINMOD, by long-term mean nitrogen (N) concentration, which is a function of local site conditions (climate, soil, cropping system, farming practices, and drainage system). Consistent with experimental observations from several sources, this approximate method assumes that the effect of DWM on N losses is proportional to its effect on drainage and surface runoff volumes. The reliability of the method was evaluated by comparing estimated annual N losses for two sites (one in North Carolina and one in Illinois) to those predicted by the process-based nitrogen model DRAINMOD-NII. The long-term N concentrations used in the approximate method were based on DRAINMOD-NII predictions. DRAINMOD-NII simulations predicted that DWM would reduce long-term average N losses to surface waters by 35% for continuous corn (CC) and 33% for a corn-wheat-soybean (CWS) rotation in North Carolina and by 31% and 26% for CC and a corn-soybean (CS) rotation in Illinois. The approximate method estimated the annual effect of DWM on N losses within 3 kg ha-1 of that predicted by DRAINMOD-NII in over 68% of the years for the CWS and CS rotations at both sites and in over 51% of the years for CC. Overestimated effects in some years were balanced by underestimated effects in others. However, relatively large errors occurred in about 25% of the years when N concentrations in the drainage water varied significantly from the long-term average. These errors typically occurred following periods when crop yields deviated significantly from the average. When these outliers were excluded from the analyses, the goodness of fit between annual N losses predicted by DRAINMOD-NII and annual losses estimated by the approximate method was substantially improved (Nash-Sutcliffe EF values ranging from 0.54 to 0.79). Although more research is needed to improve the approximate method when concentrations vary significantly from the long-term average, the results presented herein indicate that the method provides a reliable means of assessing impacts of DWM under normal conditions.}, number={3}, journal={Transactions of the ASABE}, author={Skaggs, R. W. and Youssef, M. A. and Chescheir, G. M.}, year={2012}, pages={799–808} }
 @article{skaggs_fausey_evans_2012, title={Drainage water management}, volume={67}, ISSN={["1941-3300"]}, DOI={10.2489/jswc.67.6.167a}, abstractNote={This article introduces a series of papers that report results of field studies to determine the effectiveness of drainage water management (DWM) on conserving drainage water and reducing losses of nitrogen (N) to surface waters. The series is focused on the performance of the DWM (also called controlled drainage [CD]) practice in the US Midwest, where N leached from millions of acres of cropland contributes to surface water quality problems on both local and national scales. Results of these new studies are consistent with those from previous research reported in the literature that DWM can be used to reduce N losses (primarily in the nitrate nitrogen [NO3-N] form) from subsurface drained fields. The measured impact varied over a wide range (18% to more than 75% reduction in N loss to surface waters), depending on drainage system design, location, soil, and site conditions. Crop yields were increased by DWM on some sites and not on others, with the year-to-year impacts of DWM on yields dependent on weather conditions, as well as the above factors. Papers reporting advances in the development of datasets and models to predict the impact of drainage intensity and DWM on hydrology and water quality at watershed and…}, number={6}, journal={JOURNAL OF SOIL AND WATER CONSERVATION}, author={Skaggs, R. Wayne and Fausey, Norman R. and Evans, Robert O.}, year={2012}, pages={167A–172A} }
 @article{skaggs_youssef_chescheir_2012, title={Drainmod: model use, calibration, and validation}, volume={55}, DOI={10.13031/2013.42259}, abstractNote={DRAINMOD is a process-based, distributed, field-scale model developed to describe the hydrology of poorly drained and artificially drained soils. The model is based on water balances in the soil profile, on the field surface, and, in some cases, in the drainage system. This article briefly describes the model and the algorithms that are used to quantify the various hydrologic components. Inputs for soil properties, site parameters, weather data, and crop characteristics required in the application of the model are presented and discussed with respect to their role in calibration. Methods for determining field effective values of key inputs to the model, either independently or as a part of the calibration process, are demonstrated in a case study. The case study involved calibrating DRAINMOD with two years of field data for a subsurface drained agricultural field in eastern North Carolina, followed by testing or validation of the model with two additional years of data. Performance statistics indicated that the model with calibrated input data accurately predicted daily water table depths with Nash-Sutcliffe modeling efficiency (EF) values of 0.68 and 0.72, daily drainage rates (EF = 0.73 and 0.49), and monthly drainage volumes (EF = 0.87 and 0.77) for the two-year validation period.}, number={4}, journal={Transactions of the ASABE}, author={Skaggs, R. W. and Youssef, M. A. and Chescheir, G. M.}, year={2012}, pages={1509–1522} }
 @article{tian_youssef_skaggs_amatya_chescheir_2012, title={Modeling water, carbon, and nitrogen dynamics for two drained pine plantations under intensive management practices}, volume={264}, ISSN={["1872-7042"]}, DOI={10.1016/j.foreco.2011.09.041}, abstractNote={This paper reports results of a study to test the reliability of the DRAINMOD-FOREST model for predicting water, soil carbon (C) and nitrogen (N) dynamics in intensively managed forests. The study site, two adjacent loblolly pine (Pinus taeda L.) plantations (referred as D2 and D3), are located in the coastal plain of North Carolina, USA. Controlled drainage (with weir and orifice) and various silvicultural practices, including nitrogen (N) fertilizer application, thinning, harvesting, bedding, and replanting, were conducted on the study site. Continuous collection of hydrological and water quality data (1988–2008) were used for model evaluation. Comparison between predicted and measured hydrologic variables showed that the model accurately predicted long-term subsurface drainage dynamics and water table fluctuations in both loblolly pine plantations. Predicted mean and standard deviation of annual drainage matched measured values very well: 431 ± 217 vs. 436 ± 231 mm for D2 site and 384 ± 152 vs. 386 ± 160 mm for D3 site. Nash–Sutcliffe coefficients (NSE) were above 0.9 for drainage predictions on annual and monthly basis and above 0.86 for predictions of daily water table fluctuations. Compared to measurements in other similar studies, the model also reasonably estimated long-term dynamics of organic matter pools on forest floor and in forest soil. Predicted mean and standard deviation of annual nitrate exports were comparable to measured values: 1.6 ± 1.3 vs. 1.5 ± 1.5 kg ha−1 for D2 site, and 1.4 ± 1.3 vs. 1.3 ± 1.1 kg ha−1 for D3 site, respectively. Predicted nitrate export dynamics were also in excellent agreement with field measurements as indicated by NSE above 0.90 and 0.84 on annual and monthly bases, respectively. The model, thus successfully tested, was applied to predicted hydrological and biogeochemical responses to drainage water management and silvicultural practices. Specifically, the model predicted reduced rainfall interception and ET after clear cutting, both of which led to increased water yield and elevated water table, as expected. The model also captured temporary changes in nitrogen transformations following forest harvesting, including increased mineralization, nitrification, denitrification, and decreased plant uptake. Overall, this study demonstrated that DRAINMOD-FOREST can predict water, C and N dynamics in drained pine forests under intensive management practices.}, journal={FOREST ECOLOGY AND MANAGEMENT}, author={Tian, Shiying and Youssef, Mohamed A. and Skaggs, R. Wayne and Amatya, Devendra M. and Chescheir, George M.}, year={2012}, month={Jan}, pages={20–36} }
 @article{tian_youssef_skaggs_amatya_chescheir_2012, title={Temporal Variations and Controlling Factors of Nitrogen Export from an Artificially Drained Coastal Forest}, volume={46}, ISSN={["1520-5851"]}, DOI={10.1021/es3011783}, abstractNote={Nitrogen losses in drainage water from coastal forest plantations can constrain the long term sustainability of the system and could negatively affect adjacent nutrient sensitive coastal waters. Based on long-term (21 years) field measurements of hydrology and water quality, we investigated the temporal variations and controlling factors of nitrate and dissolved organic nitrogen (DON) export from an artificially drained coastal forest over various time scales (interannual, seasonal, and storm events). According to results of stepwise multiple linear regression analyses, the observed large interannual variations of nitrate flux and concentration from the drained forest were significantly (p < 0.004) controlled by annual mean water table depth, and annual drainage or precipitation. Annual precipitation and drainage were found to be dominant factors controlling variations of annual DON fluxes. Temporal trends of annual mean DON concentration could not be explained explicitly by climate or hydrologic factors. No significant difference was observed between nitrogen (both nitrate and DON) export during growing and nongrowing seasons. Nitrate exhibited distinguished export patterns during six selected storm events. Peak nitrate concentrations during storm events were significantly (p < 0.003) related to 30-day antecedent precipitation index and the minimum water table depth during individual events. The temporal variations of DON export within storm events did not follow a clear trend and its peak concentration during the storm events was found to be significantly (p < 0.006) controlled by the short-term drying and rewetting cycles.}, number={18}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Tian, Shiying and Youssef, Mohamed A. and Skaggs, R. Wayne and Amatya, Devendra M. and Chescheir, George M.}, year={2012}, month={Sep}, pages={9956–9963} }
 @article{amatya_douglas-mankin_williams_skaggs_nettles_2011, title={Advances in forest hydrology: Challenges and opportunities}, volume={54}, DOI={10.13031/2013.40672}, abstractNote={Forests are an integral component of the landscape, and maintaining their functional integrity is fundamental for the sustainability of ecosystems and societies alike. Tools, innovations, and practices, analogous to those developed to improve agricultural production and quantify environmental impacts, are needed to ensure the sustainability of these forested landscapes as well as the ecosystem goods and services they produce. This article introduces ten technical articles on critical ecohydrologic processes, protection and restoration, and the effects of management practices on the hydrology and water quality of forests and forested wetlands, using both monitoring and modeling approaches. Prepared by experts in forest science, forest and agricultural hydrology, and water management, the studies reported in this special collection are concentrated in the Atlantic Coastal plain and focus on forests with shallow water tables. Experimental studies describe the effects of riparian vegetation harvest, human disturbance, and future climatic change on groundwater, the significance of emergent vegetation after harvest, and long-term hydrologic water balance of a managed pine forest. Modeling studies use the SWAT model to predict streamflow dynamics of a less disturbed, coastal forested watershed, and DRAINMOD to determine the impacts of minor silvicultural drainage on wetland hydrology and to improve wetland restoration. Finally, a study describes potential uncertainties associated with infrequent water sampling of nutrient loads from drained forested watersheds. This introductory article summarizes these studies of shallow water table forests and relates them to the broader field of forest hydrology, including its challenges and opportunities, while identifying pressing issues of land use and climate change. The results from these studies should help guide management and restoration of forest wetland ecosystems and direct future forest hydrologic research, including research in large prior converted agricultural landscapes.}, number={6}, journal={Transactions of the ASABE}, author={Amatya, D. M. and Douglas-Mankin, K. R. and Williams, T. M. and Skaggs, R. W. and Nettles, J. E.}, year={2011}, pages={2049–2056} }
 @article{skaggs_phillips_chescheir_trettin_2011, title={Effect of minor drainage on hydrology of forested wetlands}, volume={54}, DOI={10.13031/2013.40665}, abstractNote={Results of a simulation study to determine the impacts of minor drainage for silviculture on wetland hydrology are presented in this article. Long-term DRAINMOD simulations were conducted to determine the threshold drainage intensity (ditch depth and spacing) that removes wetland hydrology from forested wetlands. Analyses were conducted for 13 soil series and profile combinations at ten locations from Norfolk, Virginia, to Baton Rouge, Louisiana, in the Atlantic and Gulf coastal states. Threshold ditch spacings (LT) were obtained for five ditch depths for all combinations of soil profiles and locations. Analysis of the results showed that LT can be approximated as LT = , where T is the horizontal hydraulic transmissivity of the soil profile, and C is a coefficient dependent on ditch depth and geographic location. The C values for all combinations of ditch depth and location are given in this article. The threshold spacings can be used as benchmarks to directly evaluate the impact of drainage alternatives on wetland hydrology. They were also used herein to determine T25 inputs for previously developed methods to predict the lateral impact of a single ditch on wetland hydrology. Lateral impacts were determined and presented for a 0.9 m (3 ft) deep drainage ditch for all soils and locations considered. The T25 values presented can be used to determine lateral impacts for other ditch depths and soils. The analyses in this study were conducted for a surface depressional storage of 5 cm. More work is needed to define T25 values for smaller surface storages, including those smaller values needed for application to agricultural cropland.}, number={6}, journal={Transactions of the ASABE}, author={Skaggs, R. W. and Phillips, B. D. and Chescheir, G. M. and Trettin, C. C.}, year={2011}, pages={2139–2149} }
 @article{skaggs_chescheir_fernandez_amatya_diggs_2011, title={Effects of land use on soil properties and hydrology of drained coastal plain watersheds}, volume={54}, DOI={10.13031/2013.39037}, abstractNote={Some of the world's most productive cropland requires artificial or improved drainage for efficient agricultural production. Soil hydraulic properties, such as hydraulic conductivity and drainable porosity, are conventionally used in design of drainage systems. While it is recognized that these soil properties vary over a relatively wide range within a given soil series, it is generally assumed they can be approximated based on soil type, independent of crop or land use. Effects of land use on hydrology of drained soils in the North Carolina lower coastal plain were investigated by comparing hydrologic measurements on drained agricultural cropland, drained forest land (Loblolly pine), and an undrained forested wetland. Higher ET on the drained pine forest site resulted in reduced drainage outflow and deeper water tables compared to the agricultural site. Measurements for the wetland site showed water tables near the surface but annual outflows similar to the drained forest site. Field effective hydraulic conductivity in the top 70 cm of the drained forest site was more than two orders of magnitude greater than that of corresponding layers of the soil on the agricultural site. Drainable porosity, based on measured soil water characteristics, was also much higher for the forested sites. Long term (50-year) DRAINMOD simulations predicted average annual drainage outflow of 51.4 cm for the agricultural field as compared to 37.6 cm for the forested site. The difference resulted primarily from greater ET predicted for the forested site. Because of the high hydraulic conductivity of the surface layers and large surface depressional storage, predicted surface runoff from the forested site was nil, compared to an average annual runoff of 13 cm for the drained cropland site. Results of long-term simulations were used to analyze these effects for the widely variable seasonal and annual weather conditions of eastern North Carolina.}, number={4}, journal={Transactions of the ASABE}, author={Skaggs, R. W. and Chescheir, G. M. and Fernandez, G. P. and Amatya, D. M. and Diggs, J.}, year={2011}, pages={1357–1365} }
 @article{birgand_appelboom_chescheir_skaggs_2011, title={Estimating Nitrogen, Phosphorus, and Carbon Fluxes in Forested and Mixed-Use Watersheds of the Lower Coastal Plain of North Carolina: Uncertainties Associated with Infrequent Sampling}, volume={54}, ISSN={2151-0040}, url={http://dx.doi.org/10.13031/2013.40668}, DOI={10.13031/2013.40668}, abstractNote={Assessing the impact of a land use change or the water quality improvement provided by a treatment system almost always involves computation of the difference in nutrient loads before and after implementation, or upstream and downstream of the system studied. Reporting meaningful values on mass balance or differences in nutrient loads implies that the uncertainty in the computed loads is several times smaller than the difference itself. This may imply very small uncertainties for the nutrient load measurements. The level of uncertainty induced by infrequent sampling on annual loads was investigated for a suite of nutrients in runoff from a forested watershed and a mixed land use watershed in the lower coastal plain of North Carolina. Reference data were used to simulate discrete sampling and to calculate new annual load estimators, which were then compared to the reference loads to calculate the level of uncertainty. Uncertainties depended on the watershed and the nutrients and other constituents, but their level was generally found to be high, around ±20% and ±40% or more for weekly and monthly sampling for most nutrients. This was generally attributed to the short periods of active flow in these watersheds and the flashiness of flow associated with subsurface drainage. The results suggest that to obtain uncertainties of ±2% or ±5% for nitrogen forms, 100 or more than 200 samples over six months of the year might be necessary in the forested and mixed-use watersheds of the lower coastal plain.}, number={6}, journal={Transactions of the ASABE}, publisher={American Society of Agricultural and Biological Engineers (ASABE)}, author={Birgand, F. and Appelboom, T. W. and Chescheir, G. M. and Skaggs, R. W.}, year={2011}, pages={2099–2110} }
 @article{sampson_amatya_lawson_skaggs_2011, title={Leaf area index (lai) of loblolly pine and emergent vegetation following a harvest}, volume={54}, DOI={10.13031/2013.40664}, abstractNote={Forests provide goods and services to society and, often, refugia for plants and animals; forest managers utilize silviculture to provide ecosystem services and to create habitat. On the Coastal Plain of North Carolina, forest management objectives typically include wood fiber production but may also include the maintenance of environmental quality and, sometimes, species diversity. Silvicultural prescriptions alter stand structure and development trajectories by influencing the competitive interactions among plant species for site resources. Early site intervention may include nutrient additions and/or vegetation control; in coastal loblolly pine (Pinus taeda L.) stands, herbaceous and arborescent species can dominate the site leaf area index (LAI) for many years after a harvest (followed by planting). LAI is an important structural and functional component of a forest stand. Many eco-hydrologic and water quality models do not accurately account for LAI as the process driver to evapotranspiration (ET), and thus they ignore the ecophysiological effects of LAI on site water balance and nutrient loading. We measured LAI of emergent vegetation following a harvest, mechanical site preparation, and then pine planting for a drained loblolly pine plantation in coastal North Carolina. For six years monthly, growing season estimates of LAI were obtained using a LI-COR LAI 2000 Plant Canopy Analyzer (PCA) for control (D1), thinned (D3), and harvested (D2) watersheds. In this article, we present results from the D2 treatment. In D2, we harvested all emergent vegetation in 18 randomly placed 1 m2 clip plots for three growing seasons where we estimated LAI using species-pooled estimates of specific leaf area and total leaf dry mass (i.e., LAICLIP); PCA measurements were recorded prior to clipping (LAIPCA). We also simulated loblolly pine seedling growth and development using the biogeochemical process model SECRETS-3PG to examine site differentiation in LAI. Four years post-harvest maximum LAICLIP exceeded 8 m2 m-2 (projected area basis). LAIPCA underestimated LAICLIP; LAICLIP = 1.436 LAIPCA (r2 = 0.53; p < 0.0001; n = 195). Corrected LAIPCA estimates exceeded simulated pine LAI (LAISIM) for ~4.5 years post-planting. Emergent vegetation dominated the site for nearly five years and likely exerted a strong influence over site water balance and nutrient use during early stand development.}, number={6}, journal={Transactions of the ASABE}, author={Sampson, D. A. and Amatya, D. M. and Lawson, C. D. B. and Skaggs, R. W.}, year={2011}, pages={2057–2066} }
 @article{caldwell_vepraskas_gregory_skaggs_huffman_2011, title={Linking plant ecology and long-term hydrology to improve wetland restoration success}, volume={54}, DOI={10.13031/2013.40662}, abstractNote={Although millions of dollars are spent restoring wetlands, failures are common, in part because the planted vegetation cannot survive in the restored hydrology. Wetland restoration would be more successful if the hydrologic requirements of wetland plant communities were known so that the most appropriate plants could be selected for the range of projected hydrology at the site. Here we describe how hydrologic models can be used to characterize the long-term hydrology of wetland plant communities, and we show how these results can be used to define wetland design criteria. In our study, we quantified differences in long-term (40-year) hydrologic characteristics of the pond pine woodland (PPW), nonriverine swamp forest (NRSF), high pocosin (HP), and bay forest (BF) plant communities native to the North Carolina Coastal Plain. We found that the median water level was 8 cm below the land surface in PPW and 9, 2, and 8 cm above the land surface for NRSF, HP, and BF, respectively. When the land surface was inundated, the median duration of inundation was 91 d year-1 for PPW and 317, 243, and 307 d year-1 for NRSF, HP, and BF, respectively. Our models suggested that the PPW received an average of 15% of its water input from groundwater inflow, whereas the other communities we modeled did not appear to receive groundwater inflow. Using these results and soil organic layer thickness, we developed and propose design criteria linking soil, vegetation, and hydrology parameters that should contribute to improved restoration success.}, number={6}, journal={Transactions of the ASABE}, author={Caldwell, P. V. and Vepraskas, Michael and Gregory, J. D. and Skaggs, R. W. and Huffman, R. L.}, year={2011}, pages={2129–2137} }
 @article{amatya_skaggs_2011, title={Long-term hydrology and water quality of a drained pine plantation in North Carolina}, volume={54}, DOI={10.13031/2013.40667}, abstractNote={Long-term data provide a basis for understanding natural variability, reducing uncertainty in model inputs and parameter estimation, and developing new hypotheses. This article evaluates 21 years (1988-2008) of hydrologic data and 17 years (1988-2005) of water quality data from a drained pine plantation in eastern North Carolina. The plantation age was 14 years at the beginning of the investigation (1988) and 34 years at the end (2008). The 21-year average rainfall of 1517 mm was 9% higher than the 50-year (1951-2000) long-term average of 1391 mm observed at the nearest U.S. Weather Bureau station in Morehead City, North Carolina. Annual rainfall varied from 852 mm in the driest year (2001) to 2331 mm in the wettest year (2003) during the study period and was affected by several hurricanes and tropical storms. The runoff coefficient (ROC; drainage outflow expressed as a fraction of rainfall) varied from 0.05 in the driest year to as high as 0.56 in the wettest year (2003), with an average ROC of 0.32. Annual outflow (runoff) on this watershed was primarily subsurface flow to drainage ditches and was strongly correlated with rainfall (R2 = 0.81). Outflows were greater, more continuous, and longer in winter than in other seasons. Outflow in winter was 59% of rainfall on average. March was the only month that never produced zero outflow. The lowest mean outflow occurred in the spring and was significantly different from the other three seasons. Consistent with theory for subsurface drainage, outflow from this poorly drained land is dependent on water table elevation and occurs when the water table is within about 1.1 m of the surface. The water table tended to be close to the surface during the winter and early spring with low ET demands, and during summer with hurricanes and tropical storms producing large outflows, but was drawn down to depths much deeper than the drains during long dry periods in summer and fall. As a result, annual outflow and annual average water table depth were only weakly correlated (R2 = 0.52). There was no relationship (R2 = 0.01) between the annual average water table depth and the annual average evapotranspiration (ET), calculated as the difference between annual rainfall and outflow. The estimated average annual ET of 1005 mm was close to the Penman-Monteith based average annual potential ET (PET) of 1010 mm for a grass reference. Although nitrogen (N) levels in the drainage water were elevated after fertilization of the stand in late 1988, these elevated levels declined substantially by 1995. Average annual concentrations of total N ranged from 0.51 to 2.23 mg L-1 with a long-term average of 1.10 mg L-1. Annual average values for total P ranged from 0.01 to 0.12 mg L-1 with an average of 0.04 mg L-1. The highest average annual concentrations for N and P occurred in 1989 (N) and 1990 (P) following fertilization in spring of 1989. The average annual total N and P loadings were 6.5 5.3 kg ha-1 and 0.17 0.11 kg ha-1, respectively. Both concentrations and annual loadings were similar to other forested sites in the region. These long-term data should be useful for assessing the effects of land use change and management treatments on the hydrology and water quality of similar lands in the coastal region.}, number={6}, journal={Transactions of the ASABE}, author={Amatya, D. M. and Skaggs, R. W.}, year={2011}, pages={2087–2098} }
 @article{phillips_skaggs_chescheir_2010, title={A method to determine lateral effect of a drainage ditch on wetland hydrology: Field testing}, volume={53}, DOI={10.13031/2013.32599}, abstractNote={An approximate method was previously developed to predict the lateral effect of a drainage ditch on wetland hydrology. The method predicts the lateral distance of influence of a single ditch constructed through, or adjacent to, a wetland in terms of T25 values, which are dependent on climatological conditions. The lateral effect, or distance of influence, is defined as the width of a strip adjacent to the ditch that is drained such that it no longer satisfies wetland hydrologic criteria. T25 represents the time required for the water table to be drawn down by drainage from the surface to a depth of 25 cm at the location on the landscape that will just barely satisfy the wetland hydrologic criterion. Data to test the method were collected at two wetland mitigation sites in eastern North Carolina: Mildred Woods in Edgecombe County and ABC near Pinetown in Beaufort County. The approximate method predicted lateral effects of 42.6, 7.2, and 14.1 m for Mildred Woods, ABC shallow ditch, and the ABC deep ditch, respectively. Compared to direct interpolation of 3-year average field results for Mildred Woods (41 m) and the deep ditch (12 m), the method performed well. The lateral effect predicted by the method for the shallow ditch at the ABC site was at least two times that measured in the field (<3.75 m). In this case, the ditch was located in a tight clay layer, which substantially reduced the effective transmissivity of the profile and the lateral effect of the ditch on the hydrology of adjacent wetlands.}, number={4}, journal={Transactions of the ASABE}, author={Phillips, B. D. and Skaggs, R. W. and Chescheir, G. M.}, year={2010}, pages={1087–1096} }
 @article{skaggs_youssef_gilliam_evans_2010, title={Effect of controlled drainage on water and nitrogen balances in drained lands}, volume={53}, DOI={10.13031/2013.35810}, abstractNote={Field studies have shown that subsurface drainage systems can be managed to conserve water and reduce losses of nitrogen (N) to surface waters. The practice, called controlled drainage (CD) or drainage water management (DWM), is a viable alternative for reducing N loads from drained cropland, including millions of acres in the Midwest. This article reviews past studies on the effect of CD on drainage volumes and N losses for a wide range of soils and climatological conditions and uses simulations to examine mechanisms affecting the practice. Results published in the literature show that CD has reduced drainage volumes and N losses in drainage waters by 17% to over 80%, depending on soil properties, crops, drainage intensities, control strategies, and location. This study resulted in the following conclusions. CD reduces subsurface drainage and raises water tables, while increasing ET, seepage, and surface runoff. Seepage, which depends on soil properties and site conditions, is an important factor that often governs the effectiveness of CD. Experiments to determine the effect of CD on drainage volumes and N losses should be conducted on the field or watershed scale so that impacts of seepage are properly represented. Increases in ET in response to CD are important but are rarely greater than 10%. The effect of this increase in water use on drainage water loss is also less than 10% for most locations. CD reduces N losses in drainage water by about the same percentage as its effect on subsurface drainage volume in most cases. The effect of CD on N loss to surface waters depends on denitrification, both in the profile and in reduced zones along seepage paths. For soils that do not develop reduced zones, the effect of CD on N loss may be substantially less than its effect on drainage volume.}, number={6}, journal={Transactions of the ASABE}, author={Skaggs, R. W. and Youssef, M. A. and Gilliam, J. W. and Evans, R. O.}, year={2010}, pages={1843–1850} }
 @article{sun_noormets_gavazzi_mcnulty_chen_domec_king_amatya_skaggs_2010, title={Energy and water balance of two contrasting loblolly pine plantations on the lower coastal plain of North Carolina, USA}, volume={259}, ISSN={["1872-7042"]}, DOI={10.1016/j.foreco.2009.09.016}, abstractNote={During 2005–2007, we used the eddy covariance and associated hydrometric methods to construct energy and water budgets along a chronosequence of loblolly pine (Pinus taeda) plantations that included a mid-rotation stand (LP) (i.e., 13–15 years old) and a recently established stand on a clearcut site (CC) (i.e., 4–6 years old) in Eastern North Carolina. Our central objective was to quantify the differences in both energy and water balances between the two contrasting stands and understand the underlining mechanisms of environmental controls. We found that the LP site received about 20% more net radiation (Rn) due to its lower averaged albedo (α) of 0.25, compared with that at the CC (α = 0.34). The mean monthly averaged Bowen ratios (β) at the LP site were 0.89 ± 0.7, significantly (p = 0.02) lower than at the CC site (1.45 ± 1.2). Higher net radiation resulted in a 28% higher (p = 0.02) latent heat flux (LE) for ecosystem evapotranspiration at the LP site, but there was no difference in sensible heat flux (H) between the two contrasting sites. The annual total evapotranspiration (ET) at the LP site and CC site was estimated as 1011–1226 and 755–855 mm year−1, respectively. The differences in ET rates between the two contrasting sites occurred mostly during the non-growing seasons and/or dry periods, and they were small during peak growing seasons or wet periods. Higher net radiation and biomass in LP were believed to be responsible to the higher ET. The monthly ET/Grass Reference ET ratios differed significantly across site and season. The annual ET/P ratio for the LP and CC were estimated as 0.70–1.13 and 0.60–0.88, respectively, indicating higher runoff production from the CC site than the LP site. This study implied that reforestation practices reduced surface albedos and thus increased available energy, but they did not necessarily increase energy for warming the atmosphere in the coastal plain region where soil water was generally not limited. This study showed the highly variable response of energy and water balances to forest management due to climatic variability.}, number={7}, journal={FOREST ECOLOGY AND MANAGEMENT}, author={Sun, G. and Noormets, A. and Gavazzi, M. J. and McNulty, S. G. and Chen, J. and Domec, J. -C. and King, J. S. and Amatya, D. M. and Skaggs, R. W.}, year={2010}, month={Mar}, pages={1299–1310} }
 @article{noormets_sun_mcnulty_gavazzi_chen_domec_king_amatya_skaggs_2010, title={Energy and water balance of two contrasting loblolly pine plantations on the lower coastal plain of North Carolina, USA (vol 259, pg 1299, 2010)}, volume={260}, number={1}, journal={Forest Ecology and Management}, author={Noormets, A. and Sun, G. and McNulty, S. G. and Gavazzi, M. J. and Chen, J. and Domec, J. C. and King, J. S. and Amatya, D. M. and Skaggs, R. W.}, year={2010}, pages={169–169} }
 @article{beltran_amatya_youssef_jones_callahan_skaggs_nettles_2010, title={Impacts of Fertilization on Water Quality of a Drained Pine Plantation: A Worst Case Scenario}, volume={39}, ISSN={["1537-2537"]}, DOI={10.2134/jeq2008.0506}, abstractNote={Intensive plantation forestry will be increasingly important in the next 50 yr to meet the high demand for domestic wood in the United States. However, forest management practices can substantially influence downstream water quality and ecology. This study analyses, the effect of fertilization on effluent water quality of a low gradient drained coastal pine plantation in Carteret County, North Carolina using a paired watershed approach. The plantation consists of three watersheds, two mature (31‐yr) and one young (8‐yr) (age at treatment). One of the mature watersheds was commercially thinned in 2002. The mature unthinned watershed was designated as the control. The young and mature‐thinned watersheds were fertilized at different rates with Arborite (Encee Chemical Sales, Inc., Bridgeton, NC), and boron. The outflow rates and nutrient concentrations in water drained from each of the watersheds were measured. Nutrient concentrations and loadings were analyzed using general linear models (GLM). Three large storm events occurred within 47 d of fertilization, which provided a worst case scenario for nutrient export from these watersheds to the receiving surface waters. Results showed that average nutrient concentrations soon after fertilization were significantly (α = 0.05) higher on both treatment watersheds than during any other period during the study. This increase in nutrient export was short lived and nutrient concentrations and loadings were back to prefertilization levels as soon as 3 mo after fertilization. Additionally, the mature‐thinned watershed presented higher average nutrient concentrations and loadings when compared to the young watershed, which received a reduced fertilizer rate than the mature‐thinned watershed.}, number={1}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Beltran, Bray J. and Amatya, Devendra M. and Youssef, Mohamed and Jones, Martin and Callahan, Timothy J. and Skaggs, R. Wayne and Nettles, Jami E.}, year={2010}, pages={293–303} }
 @article{salazar_joel_wesstrom_linner_skaggs_2010, title={Modelling discharge from a coastal watershed in southeast Sweden using an integrated framework}, volume={24}, ISSN={["0885-6087"]}, DOI={10.1002/hyp.7815}, abstractNote={The field hydrology model DRAINMOD integrated with Arc Hydro in geographical information system (GIS) framework (Arc Hydro–DRAINMOD) was used to simulate the hydrological response of a coastal watershed in southeast Sweden. Arc Hydro–DRAINMOD uses a distributed approach to route water from each field edge to the watershed outlet. In the framework the Arc Hydro data model was used to describe the stream network in the watershed and to connect the individual simulated DRAINMOD-field outflow time series from each plot using Arc Hydro schema-links features, which were summed at Arc Hydro schema-nodes features along the stream network to generate the stream network flow. Hydrology data collected during six periods between 2003 and 2008 were used to test Arc Hydro–DRAINMOD and its performance was evaluated by considering uncertainties in model inputs using generalized likelihood uncertainty estimation (GLUE). The GLUE estimates obtained (uncertainty bands 5% and 95%) agreed satisfactorily with measured monthly discharges. The percentage of time in which the observed discharges were bracketed by the uncertainty bands was 88% in calibration periods and 75% in validation periods. Although monthly time step simulations showed good agreement with observed discharges during the two main discharge events in spring, the contradictory daily time step results indicate that the watershed response simulations on a daily basis need to be improved. The uncertainty analysis showed that in periods of higher discharge, such as spring periods, the uncertainty in prediction was higher. It is important to note that these uncertainty estimations using the GLUE procedure include the uncertainties in measured discharge values, model inputs, boundary conditions and model structures. It was estimated that stream baseflow represented 42% of the total watershed discharge, but further research is needed to confirm this. These results show that the new Arc Hydro–DRAINMOD framework is applicable for predicting discharge from artificially drained watersheds in southeast Sweden. Copyright © 2010 John Wiley & Sons, Ltd.}, number={26}, journal={HYDROLOGICAL PROCESSES}, author={Salazar, Osvaldo and Joel, Abraham and Wesstrom, Ingrid and Linner, Harry and Skaggs, R. Wayne}, year={2010}, month={Dec}, pages={3837–3851} }
 @article{appelboom_chescheir_birgand_skaggs_gilliam_amatya_2010, title={Temperature Coefficient for Modeling Denitrification in Surface Water Sediments Using the Mass Transfer Coefficient}, volume={53}, ISSN={2151-0040}, url={http://dx.doi.org/10.13031/2013.29578}, DOI={10.13031/2013.29578}, abstractNote={Watershed modeling has become an important tool for researchers. Modeling nitrate transport within drainage networks requires quantifying the denitrification within the sediments in canals and streams. In a previous study, several of the authors developed an equation using a term called a mass transfer coefficient to mathematically describe sediment denitrification. This equation takes into account the effect that water column nitrate concentration and flow depth have on denitrification in the sediments. Water column temperature also has a marked effect on the rate of denitrification in the sediments. In the present study, a relationship between denitrification rate and temperature was developed. This relationship was inserted into the original mathematical relationship to improve its ability to predict nitrate removal due to denitrification within drainage networks. The modified equation was tested by comparing predicted and measured nitrate concentrations over time in denitrification tanks at various temperatures. Results show that the modified equation increased the accuracy of predicting nitrate removal by denitrification in drainage canals. Overall Nash-Sutcliffe model efficiency values ranged from 0.72 to 0.76 for the original equation and from 0.90 to 0.97 for the equation developed in this study. The effective temperature range for the equation is 0C to 40C. The equation has also only been tested under stagnant/low-flow conditions.}, number={2}, journal={Transactions of the ASABE}, publisher={American Society of Agricultural and Biological Engineers (ASABE)}, author={Appelboom, T. W. and Chescheir, G. M. and Birgand, F. and Skaggs, R. W. and Gilliam, J. W. and Amatya, D.}, year={2010}, pages={465–474} }
 @article{salazar_wesstrom_youssef_skaggs_joel_2009, title={Evaluation of the DRAINMOD-N II model for predicting nitrogen losses in a loamy sand under cultivation in south-east Sweden}, volume={96}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2008.08.008}, abstractNote={The DRAINMOD–N II model (version 6.0) was evaluated for a cold region in south-east Sweden. The model was field-tested using four periods between 2002 and 2004 of climate, soil, hydrology and water quality data from three experimental plots, planted to a winter wheat–sugarbeet–barley–barley crop rotation and managed using conventional and controlled drainage. DRAINMOD–N II was calibrated using data from a conventional drainage plot, while data sets from two controlled drainage plots were used for model validation. The model was statistically evaluated by comparing simulated and measured drain flows and nitrate–nitrogen (NO3–N) losses in subsurface drains. Soil mineral nitrogen (N) content was used to evaluate simulated N dynamics. Observed and predicted NO3–N losses in subsurface drains were in satisfactory agreement. The mean absolute error (MAE) in predicting NO3–N drainage losses was 0.16 kg N ha−1 for the calibration plot and 0.21 and 0.30 kg N ha−1 for the two validation plots. For the simulation period, the modelling efficiency (E) was 0.89 for the calibration plot and 0.49 and 0.55 for the validation plots. The overall index of agreement (d) was 0.98 for the calibration plot and 0.79 and 0.80 for the validation plots. These results show that DRAINMOD–N II is applicable for predicting NO3–N losses from drained soil under cold conditions in south-east Sweden.}, number={2}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Salazar, Osvaldo and Wesstrom, Ingrid and Youssef, Mohamed A. and Skaggs, R. Wayne and Joel, Abraham}, year={2009}, month={Feb}, pages={267–281} }
 @inproceedings{appelboom_chescheir_skaggs_gilliam_amatya_2008, title={Nitrogen balance for a plantation forest drainage canal on the North Carolina coastal plain}, volume={51}, DOI={10.13031/2013.25239}, abstractNote={Human alteration of the nitrogen cycle has led to increased riverine nitrogen loads, contributing to the eutrophication of lakes, streams, estuaries, and near-coastal oceans. These riverine nitrogen loads are usually less than the total nitrogen inputs to the system, indicating nitrogen removal during transport through the drainage network. A two-year monitoring study quantified the ammonium, nitrate, and organic-N inputs, outputs, and inferred in-stream processes responsible for nitrogen transformations and removal in a 1900 m reach of a drainage canal located in a managed pine plantation. Total nitrogen inputs to the canal section were 527.8 kg in 2001 and 1422.7 kg in 2002. Total nitrogen discharge at the outlet was 502 kg in 2001 and 1458 kg in 2002. The mass balance of nitrogen inputs and outputs indicated a loss of 25.8 kg (5.1%) of total nitrogen from the system in 2001, and a gain of 35.3 kg (2.4%) of total nitrogen to the system in 2002. Variability in the input and output estimates was high, especially for groundwater exchange. Different hydrologic and nitrogen inputs and outputs were identified and quantified, but measurement variability obscured any potential nitrogen removal from the system.}, number={4}, booktitle={Transactions of the ASABE}, author={Appelboom, T. W. and Chescheir, G. M. and Skaggs, R. W. and Gilliam, J. W. and Amatya, D. M.}, year={2008}, pages={1215–1233} }
 @article{fernandez_chescheir_skaggs_amatya_2007, title={Application of DRAINMOD-GIS to a lower coastal plain watershed}, volume={50}, DOI={10.13031/2013.22635}, abstractNote={This article reports a case study for applying DRAINMOD-GIS, a DRAINMOD-based lumped parameter watershed model, to Chicod Creek watershed, a 11100 ha coastal plain watershed in North Carolina that is not intensively instrumented or documented. The study utilized the current database of land use, topography, stream network, soil, and weather data available to state and federal agencies. Methods for collecting, evaluating, and formatting watershed data for model input are described. The study demonstrated that the lumped parameter model may be used to characterize the hydrology and water quality of Chicod Creek. Hydrology predictions were within 5% of the measured data. Predicted mean monthly nitrate-nitrogen (NO3-N) loads compared well with the measured data. Mean annual delivery ratios of each field ranged from 81% to 99% with a watershed mean of 90%. Application of the model to evaluate the effects of changing land use is presented.}, number={2}, journal={Transactions of the ASABE}, author={Fernandez, G. and Chescheir, G. M. and Skaggs, R. W. and Amatya, D. M.}, year={2007}, pages={439–447} }
 @article{skaggs_2007, title={Criteria for calculating drain spacing and depth}, volume={50}, DOI={10.13031/2013.23971}, abstractNote={Agricultural and biological engineers have had a leading role in developing drainage design methods that span the range from simple drainage equations to complex computer simulation models. While current efforts are focused on development of complex models to quantify crop and drainage water quality response to design and management alternatives, there is still a place for simple drainage equations. Application of drainage design equations is limited by lack of design criteria for most locations in the U.S. This article reports results of a simulation study to determine design criteria for the steady-state Hooghoudt equation and the transient van Schilfgaarde equation. Simulations were conducted to determine drain spacings that maximize profit for three drain depths on four soils at five locations in eastern U.S. Drainage design rates (DDR) for the Hooghoudt equation and the time required for 30 cm water table drawdown (TDD30) were determined for each of the optimum drain spacings. Results were analyzed to develop equations for predicting design criteria (DDR and TDD30) in terms of growing season rainfall, drain depth, profile transmissivity, and drainable porosity for the humid region of the U.S. These criteria may be used, with their respective equations, to estimate drain depth and spacing for any location in eastern U.S.}, number={5}, journal={Transactions of the ASABE}, author={Skaggs, R. W.}, year={2007}, pages={1657–1662} }
 @article{grace_skaggs_cassel_2007, title={Influence of thinning loblolly pine (Pinus taeda L.) on hydraulic properties of an organic soil}, volume={50}, DOI={10.13031/2013.22640}, abstractNote={The impact of forest operations on soil properties has been a concern in forest management over the past 30 years. The objective of this study was to evaluate the impact of forest thinning operations on soil hydraulic properties of a shallow organic (Belhaven series) soil in the Tidewater region of North Carolina. Soil physical properties were evaluated in a nested design by collecting soil cores from an unthinned control and following a 40 ha fifth-row thinning with selection performed on a 14-year-old loblolly pine plantation in April 2001. Thinning decreased saturated hydraulic conductivity and drained volumes for a given water table depth; however, changes in bulk density were not detected. Saturated hydraulic conductivity determined by the constant head method before thinning was 100 cm h-1. Thinning resulted in a 3-fold decrease (from 100 to 32 cm h-1) in saturated hydraulic conductivity. The thinned watershed had less drainage at low pressures and greater retained water contents under increased soil water tensions in comparison with the control. Drained volume on the thinned watershed for a water table depth of 200 cm under drained to equilibrium conditions was reduced by 60% in comparison to drained volume for the control watershed. The reductions in ksat, drained volumes, and drainable porosity will likely result in shallower water tables and increased runoff for the thinned watershed.}, number={2}, journal={Transactions of the ASABE}, author={Grace, J. M. and Skaggs, R. W. and Cassel, D. K.}, year={2007}, pages={517–522} }
 @article{birgand_skaggs_chescheir_gilliam_2007, title={Nitrogen Removal in Streams of Agricultural Catchments—A Literature Review}, volume={37}, ISSN={1064-3389 1547-6537}, url={http://dx.doi.org/10.1080/10643380600966426}, DOI={10.1080/10643380600966426}, abstractNote={Excess nutrient loads have been recognized to be the major cause of serious water quality problems recently encountered in many estuaries and coastal waters of the world. Agriculture has been recognized in many regions of the world to be the largest single source of nitrogen emissions to the aquatic environments, and best management practices have been proposed to reduce nutrient losses at the field edge. As a result, there is growing awareness that nutrient management must be handled at the watershed scale. However, the key to nutrient management at the watershed scale is the understanding and quantification of the fate of nutrients both at the field scale and after they enter the aquatic environment. There has been widespread evidence since the late 1970s that nitrogen can be removed from water during its downstream transport in watersheds or basins. Although this information is becoming crucial, no overview has been proposed, so far, to qualitatively as well as quantitatively summarize available information in the literature. For this reason, we propose a review on the biogeochemical processes involved in nitrogen removal in streams, the rates of removal reported, and the factors influencing those rates. Nitrogen removal rates in agricultural streams should be expected to vary between 350 and 1250 mg N m−2 day−1. Mass transfer coefficients (coefficient evaluating intrinsic ability of a stream to remove nitrogen) values in agricultural streams may vary between 0.07 and 0.25 m day−1, although these values correspond to values obtained from reach scale studies. Reviewing values obtained from different measurement scales has revealed that results from incubations or experiments performed in the laboratory clearly underestimate mass transfer coefficients compared to those reported at the reach scale, from severalfold to more than one order of magnitude. Nitrogen removal rates and efficiency in streams are the highest in the summer, and this is critical for receiving ecosystems, which are most sensitive to external inputs at this period of the year. Removal efficiency is the lowest in winter in temperate climates due to high flow and loading combined with lowest removal rates. In-stream processes, on an annual basis, may remove at the watershed scale as much as 10 to 70% of the total N load to the drainage network.}, number={5}, journal={Critical Reviews in Environmental Science and Technology}, publisher={Informa UK Limited}, author={Birgand, Françoisx and Skaggs, R. Wayne and Chescheir, George M. and Gilliam, J. Wendell}, year={2007}, month={Jun}, pages={381–487} }
 @article{bechtold_koehne_youssef_lennartz_skaggs_2007, title={Simulating nitrogen leaching and turnover in a subsurface-drained grassland receiving animal manure in Northern Germany using DRAINMOD-N II}, volume={93}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2007.06.001}, abstractNote={The primary objective of this study was to evaluate the nitrogen fate and transport simulation model DRAINMOD-N II for drained permanent grassland soils. Since the plant component of DRAINMOD-N II was developed for annual row crop systems, it was modified using an empirical approach to consider perennial grasses. The model was field-tested using a 12-year (1989–2000) data set of field hydrology, non-reactive tracer transport, and carbon (C) and nitrogen (N) dynamics from a tile drained grassland research site (Infeld, North-West Germany). Model calibration was performed using the first half of the data set, followed by validation using the second half. Model simulations were visually and statistically compared to field measurements. Modified DRAINMOD-N II successfully predicted drain flow during the validation period. The model could also simulate the observed dynamics of weakly preferential tracer transport by using a high longitudinal dispersivity. Furthermore, the model well described organic carbon (OC) dynamics during calibration. Since there were no OC measurements during the second half of the study, OC model predictions were not validated. Simulated and measured N losses via drain flow were in agreement during model calibration. The model, however, substantially under-predicted N loads during the validation period. Two reasons were hypothesized for the relatively poor performance of DRAINMOD-N II during the validation period. First, the simplistic approach used in the modified DRAINMOD-N II to quantify grass biomass yield did not take into account the effect of soil water dry or wet stresses on plant growth and biomass yield. This hypothesis could not be tested since there were no measurements of grass biomass yield. Secondly, the effect of the calibration errors in N process rates increased with the difference in precipitation patterns between the calibration and validation periods. This is despite the fact that calibrated transformation rates were within published ranges. While inconclusive, these results indicates that a more robust approach for quantifying grass biomass and N uptake may be needed for the current version of DRAINMOD-N II in order to successfully simulate C and N dynamics in drained grassland.}, number={1-2}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Bechtold, Iris and Koehne, Sigrid and Youssef, Mohamed A. and Lennartz, Bernd and Skaggs, R. Wayne}, year={2007}, month={Oct}, pages={30–44} }
 @article{caldwell_vepraskas_skaggs_gregory_2007, title={Simulating the water budgets of natural Carolina bay wetlands}, volume={27}, ISSN={["0277-5212"]}, DOI={10.1672/0277-5212(2007)27[1112:stwbon]2.0.co;2}, number={4}, journal={WETLANDS}, author={Caldwell, Peter V. and Vepraskas, Michael J. and Skaggs, R. Wayne and Gregory, James D.}, year={2007}, month={Dec}, pages={1112–1123} }
 @article{stackelberg_chescheir_skaggs_amatya_2007, title={Simulation of the hydrologic effects of afforestation in the Tacuarembo River basin, Uruguay}, volume={50}, DOI={10.13031/2013.22636}, abstractNote={The Soil and Water Assessment Tool (SWAT) was used to simulate the hydrology of two small paired catchments in northern Uruguay. The control and treatment catchments (69 and 108 ha, respectively) were monitored for a three-year pretreatment period during which the land use was grassland with livestock grazing. Subsequently, the treatment catchment was planted (57% afforested) with loblolly pine (Pinus taeda). The objectives of the modeling study were to simulate the hydrologic response of the two catchments during the pretreatment period and predict the hydrologic effects of converting the native pasture to pine plantation. SWAT models of the two catchments were calibrated and validated using data measured during the pretreatment period. The model predicted outflows from the catchments reasonably well as compared to observed outflows during the years with above average rainfall (5% to -13% error). Model efficiency (E) for daily outflow volumes was greater than 0.71, indicating a good fit between simulated and observed results. A 33-year continuous simulation was performed on three land uses: grassland with livestock grazing, grassland without grazing, and pine treatment. The conversion of the catchments from the baseline pasture condition with grazing resulted in a predicted reduction in average annual water yield from the catchments of 15% for native grassland without grazing, and 23% for pine trees. A maximum predicted hydrologic effect was estimated by maximizing the model parameter that increases the ability of pine trees to withdraw water from the ground. For this condition, the model predicted a 30% reduction in mean annual water yield from the afforested catchment.}, number={2}, journal={Transactions of the ASABE}, author={Stackelberg, N. O. and Chescheir, G. M. and Skaggs, R. W. and Amatya, D. M.}, year={2007}, pages={455–468} }
 @article{burchell_skaggs_lee_broome_chescheir_osborne_2007, title={Substrate organic matter to improve nitrate removal in surface-flow constructed wetlands}, volume={36}, ISSN={["0047-2425"]}, DOI={10.2134/jeq2006.0022}, abstractNote={A wetland mesocosm experiment was conducted in eastern North Carolina to determine if organic matter (OM) addition to soils used for in-stream constructed wetlands would increase NO3−–N treatment. Not all soils are suitable for wetland substrate, so OM addition can provide a carbon and nutrient source to the wetland early in its development to enhance denitrification and biomass growth. Four batch studies, with initial NO3−–N concentrations ranging from 30 to 120 mg L−1, were conducted in 2002 in 21 surface-flow wetland mesocosms. The results indicated that increasing the OM content of a Cape Fear loam soil from 50 g kg−1 (5% dry wt.) to 110 g kg−1 (11% dry wt.) enhanced NO3−–N wetland treatment efficiency in spring and summer batch studies, but increases to 160 g kg−1 (16% dry wt.) OM did not. Wetlands constructed with dredged material from the USACE Eagle Island Confined Disposal Facility in Wilmington, NC, with initial OM of 120 g kg−1 (12% dry wt.), showed no improvement in NO3−–N treatment efficiency when increased to 180 g kg−1 (18% dry wt.), but did show increased NO3−–N treatment efficiency in all batch studies when increased to 220 g kg−1 (22% dry wt.). Increased OM addition and biosolids to the Cape Fear loam and dredged material blends significantly increased biomass growth in the second growing season when compared to no OM addition. Results of this research indicate that increased OM in the substrate will reduce the area required for in-stream constructed wetlands to treat drainage water in humid regions. It also serves as a demonstration of how dredged material can be used successfully in constructed wetlands, as an alternative to costly storage by the USACE.}, number={1}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Burchell, Michael R., II and Skaggs, R. Wayne and Lee, Charles R. and Broome, Steven and Chescheir, George M. and Osborne, Jason}, year={2007}, pages={194–207} }
 @article{fernandez_chescheir_skaggs_amatya_2006, title={DRAINMOD-GIS: A lumped parameter watershed scale drainage and water quality model}, volume={81}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2005.03.004}, abstractNote={A watershed scale lumped parameter hydrology and water quality model that includes an uncertainty analysis component was developed and tested on a lower coastal plain watershed in North Carolina. Uncertainty analysis was used to determine the impacts of uncertainty in field and network parameters of the model on the predicted outflows and nitrate–nitrogen loads at the outlet of the watershed. The model, which links DRAINMOD field hydrology and a spatially distributed routing model using a kernel function, accurately predicted the outlet flows and nitrate–nitrogen loads from a lower coastal plain watershed. Model predictions were within 1% of both measured outflows and nitrate–nitrogen loads. Uncertainty analysis indicated that uncertainty in stream velocities, decay coefficient and field exports significantly contributed to the uncertainty in the predicted outlet flows, loads and mean watershed delivery ratio.}, number={1-2}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Fernandez, GP and Chescheir, GM and Skaggs, RW and Amatya, DM}, year={2006}, month={Mar}, pages={77–97} }
 @article{skaggs_youssef_chescheir_2006, title={Drainage design coefficients for eastern United States}, volume={86}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2006.06.007}, abstractNote={The development of drainage simulation models has made it possible to quantitatively describe the performance of drainage systems, including the effects of design parameters on yields. While this was a primary goal of drainage researchers 40 years ago, it is no longer sufficient. Currently, the effect of drainage on nutrient loads and surface water quality is of equal or greater importance to production goals. Limited field data and modeling results indicate that nitrogen (N) loss in drainage water is proportional to subsurface drainage intensity (DI). This implies that the drainage system should be tailored to soil and site conditions, such that the DI is as small as possible. While simulation models can be used to determine drain depth and spacing required to maximize yields or profits for a specific site, the modeling expertise and/or the extensive data required are often not available. Simple approaches are still needed for estimating drain spacing and depth. The Hooghoudt equation has been used for many years for this purpose. Its application requires knowledge of the design drainage rate, which has not been defined for many locations in the US. A simulation study was conducted to determine the drain spacing corresponding to predicted maximum economic return for corn production on four soils at 10 locations in eastern United States. The drainage intensity (cm/day) corresponding to the optimum drain spacing was defined as the design drainage rate (DDR). DDR varied with growing season rainfall and ranged from an average (across the four soils) of 0.58 cm/day at Toledo, OH to 1.61 cm/day at Baton Rouge, LA. Variation of DDR among the four soils was least for the low rainfall locations and highest where growing season rainfall was high. The regression equation, DDR = 0.004P − 1.1, may be used to estimate DDR (cm/day) in terms of growing season rainfall, P (mm). These results were obtained for a drain depth of 1 m and surface depressional storage of 2 cm. Additional research is needed to test the relationships, and/or develop new equations, for additional drain depths and surface storages.}, number={1-2}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Skaggs, R. Wayne and Youssef, M. A. and Chescheir, G. M.}, year={2006}, month={Nov}, pages={40–49} }
 @article{youssef_skaggs_chescheir_gilliam_2006, title={Field evaluation of a model for predicting nitrogen losses from drained lands}, volume={35}, ISSN={["1537-2537"]}, DOI={10.2134/jeq2005.0249}, abstractNote={The N simulation model, DRAINMOD-N II, was field-tested using a 6-yr data set from an artificially drained agricultural site located in eastern North Carolina. The test site is on a nearly flat sandy loam soil which is very poorly drained under natural conditions. Four experimental plots, planted to a corn (Zea mays)-wheat (Triticum aestivum L.)-soybean (Glycine max.) rotation and managed using conventional and controlled drainage, were used in model testing. Water table depth, subsurface drainage, and N concentration in drain flow were measured and meteorological data were recorded continuously. DRAINMOD-N II was calibrated using the data from one plot; data sets from the other three plots were used for model validation. Simulation results showed an excellent agreement between observed and predicted nitrate-nitrogen (NO(3)-N) losses in drainage water over the 6-yr period and a reasonable agreement on an annual basis. The agreement on a monthly basis was not as good. The Nash-Sutcliffe modeling efficiency (EF) for monthly predictions was 0.48 for the calibration plot and 0.19, 0.01, and -0.02 for the validation plots. The value of the EF for yearly predictions was 0.92 for the calibration plot and 0.73, 0.62, and -0.10 for the validation plots. Errors in predicting cumulative NO(3)-N losses over the 6-yr period were remarkably small; -1.3% for the calibration plot, -8.1%, -2.8%, and 4.0% for the validation plots. Results of this study showed the potential of DRAINMOD-N II for predicting N losses from drained agricultural lands. Further research is needed to test the model for different management practices and soil and climatological conditions.}, number={6}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Youssef, Mohamed A. and Skaggs, R. Wayne and Chescheir, George M. and Gilliam, J. Wendell}, year={2006}, pages={2026–2042} }
 @article{grace_skaggs_chescheir_2006, title={Hydrologic and water quality effects of thinning loblolly pine}, volume={49}, DOI={10.13031/2013.20484}, abstractNote={Forest operations such as harvesting, thinning, and site preparation can affect the hydrologic behavior ofwatersheds on poorly drained soils. The influence of these operations conducted on organic soil sites can be more pronouncedthan on mineral soil sites due to the differences in bulk density and soil moisture relationships that exist between mineral andorganic soils. This article reports the results of a study to evaluate the effect of thinning on the hydrology and water qualityof an artificially drained pine plantation watershed on organic soils in eastern North Carolina. Outflow, water table depth,and water quality were monitored over a 3-year study period from paired 40 ha and 16 ha 15-year-old loblolly pine (Pinustaeda L.) plantations located in Washington County near Plymouth, North Carolina. Thinning increased daily outflow andpeak flow rates based on a paired-watershed study design. Mean daily outflow doubled and peak flow rates increased 40%on the thinned watershed in relation to the control. Treatment effects were also observed on nutrient loads following thethinning operation. Phosphorous, TKN, and TSS loads increased following thinning, while nitrate-nitrogen loads decreasedfollowing thinning. These differences in hydrologic behavior are primarily attributed to the reduction in evapotranspiration that resulted from thinning.}, number={3}, journal={Transactions of the ASABE}, author={Grace, J. M. and Skaggs, R. W. and Chescheir, G. M.}, year={2006}, pages={645–654} }
 @misc{boyer_alexander_parton_li_butterbach-bahl_donner_skaggs_del gross_2006, title={Modeling denitrification in terrestrial and aquatic ecosystems at regional scales}, volume={16}, ISSN={["1939-5582"]}, DOI={10.1890/1051-0761(2006)016[2123:mditaa]2.0.co;2}, abstractNote={Quantifying where, when, and how much denitrification occurs on the basis of measurements alone remains particularly vexing at virtually all spatial scales. As a result, models have become essential tools for integrating current understanding of the processes that control denitrification with measurements of rate-controlling properties so that the permanent losses of N within landscapes can be quantified at watershed and regional scales. In this paper, we describe commonly used approaches for modeling denitrification and N cycling processes in terrestrial and aquatic ecosystems based on selected examples from the literature. We highlight future needs for developing complementary measurements and models of denitrification. Most of the approaches described here do not explicitly simulate microbial dynamics, but make predictions by representing the environmental conditions where denitrification is expected to occur, based on conceptualizations of the N cycle and empirical data from field and laboratory investigations of the dominant process controls. Models of denitrification in terrestrial ecosystems include generally similar rate-controlling variables, but vary in their complexity of the descriptions of natural and human-related properties of the landscape, reflecting a range of scientific and management perspectives. Models of denitrification in aquatic ecosystems range in complexity from highly detailed mechanistic simulations of the N cycle to simpler source-transport models of aggregate N removal processes estimated with empirical functions, though all estimate aquatic N removal using first-order reaction rate or mass-transfer rate expressions. Both the terrestrial and aquatic modeling approaches considered here generally indicate that denitrification is an important and highly substantial component of the N cycle over large spatial scales. However, the uncertainties of model predictions are large. Future progress will be linked to advances in field measurements, spatial databases, and model structures.}, number={6}, journal={ECOLOGICAL APPLICATIONS}, author={Boyer, Elizabeth W. and Alexander, Richard B. and Parton, William J. and Li, Changsheng and Butterbach-Bahl, Klaus and Donner, Simon D. and Skaggs, R. Wayne and Del Gross, Stephen J.}, year={2006}, month={Dec}, pages={2123–2142} }
 @article{grace_skaggs_cassel_2006, title={Soil physical changes associated with forest harvesting operations on an organic soil}, volume={70}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2005.0154}, abstractNote={The influence of forest operations on forest soil and water continues to be an issue of concern in forest management. Research has focused on evaluating forest operation effects on numerous soil and water quality indicators. However, poorly drained forested watersheds with organic soil surface horizons have not been extensively investigated. A study was initiated in the Tidewater region of North Carolina to gain a better understanding of the impact of harvesting operations on poorly drained organic soils. Soils on the study site, having >80% organic matter (OM) content to a depth of 60 cm below the soil surface, were classified as shallow organic soils. Soil physical properties were examined by collecting soil cores from control and treatment watersheds in a nested design. Compaction caused by the harvest operation increased bulk density (Db) from 0.22 to 0.27 g cm−3, decreased saturated hydraulic conductivity (ksat) from 397 to 82 cm h−1, and decreased the drained volume for a given water table depth. However, Db following the harvest remained low at 0.27 g cm−3 The drained volume at equilibrium following the lowering of the water table from the soil surface to a depth of 200 cm was reduced by 10% from that of control watershed as a result of harvesting.}, number={2}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Grace, JM and Skaggs, RW and Cassel, DK}, year={2006}, pages={503–509} }
 @article{fernandez_chescheir_skaggs_amatya_2005, title={Development and testing of watershed-scale models for poorly drained soils}, volume={48}, DOI={10.13031/2013.18323}, abstractNote={Two watershed-scale hydrology and water quality models were used to evaluate the cumulative impacts of landuse and management practices on downstream hydrology and nitrogen loading of poorly drained watersheds. Field-scalehydrology and nutrient dynamics are predicted by DRAINMOD in both models. In the first model (DRAINMOD-DUFLOW),field-scale predictions are coupled to the canal/stream routing and in-stream water quality model DUFLOW, which handlesflow routing and nutrient transport and transformation in the drainage canal/stream network. In the second model(DRAINMOD-W), DRAINMOD was integrated with a new one-dimensional canal and water quality model. The hydrologyand hydraulic routing components of the models were tested using data from a 2950 ha drained managed forest watershedin the coastal plain of eastern North Carolina. Both models simulated the hydrology and nitrate-nitrogen (NO3-N) loadingof the watershed acceptably. Simulated outflows and NO3-N loads at the outlet of the watershed were in good agreement withthe temporal trend for five years of observed data. Over a five-year period, total outflow was within 1% of the measured value.Similarly, NO3-N load predictions were within 1% of the measured load. Predictions of the two models were not statisticallydifferent at the 5% level of significance.}, number={2}, journal={Transactions of the ASAE}, author={Fernandez, G. P. and Chescheir, G. M. and Skaggs, R. W. and Amatya, D. M.}, year={2005}, pages={639–652} }
 @article{skaggs_youssef_chescheir_gilliam_2005, title={Effect of drainage intensity on nitrogen losses from drained lands}, volume={48}, DOI={10.13031/2013.20103}, abstractNote={Agricultural drainage is a primary source of excessive nitrogen (N) in surface waters, leading to significant waterquality problems in streams and estuaries in many locations around the world. Although there have been only a few fieldstudies of the effect of drain depth and spacing on N loss to surface waters, the data that exist indicate that N loss increasesas subsurface drains are placed closer together. Some simulation model studies agree with these trends, others do not.Published field data from Indiana and North Carolina were plotted as a function of drainage intensity (DI), which was definedas the steady-state drainage rate when the water table at a point midway between the drains is coincident with the surface.Trends for NO3-N loss as a function of DI were similar for soils in the two states in spite of large differences in weather andsoil conditions. These data indicated that the magnitude of NO3-N loss in drainage waters is strongly dependent on DI.Simulations were conducted to examine effects of drain depth, spacing, and soil properties on processes that affect NO3-Nloss from drained soils. The use of DI explained or normalized the effect of these variables on some of the processes but notothers. Results showed that, in addition to its effect on DI, drain depth appears to have a significant impact on NO3-N losses.}, number={6}, journal={Transactions of the ASAE}, author={Skaggs, R. W. and Youssef, M. A. and Chescheir, G. M. and Gilliam, J. W.}, year={2005}, pages={2169–2177} }
 @article{shelby_chescheir_skaggs_amatya_2005, title={Hydrologic and water-quality response of forested and agricultural lands during the 1999 extreme weather conditions in eastern North Carolina}, volume={48}, DOI={10.13031/2013.20104}, abstractNote={This study evaluated hydrologic and water-quality data collected on a coastal-plain research watershed duringa series of hurricanes and tropical storms that hit coastal North Carolina in 1999, including hurricanes Dennis, Floyd, andIrene. During September and October 1999, the research watershed received approximately 555 mm of rainfall associatedwith hurricanes. This was the wettest such period in a 49-year historical weather record (1951-1999). Prior to the hurricanes,the watershed experienced a dry late winter, spring, and summer (565 cm for Feb.-Aug.). This was the third driest such periodin the 49-year record. Maximum daily flow rates measured across the research watershed were greater during hurricane Floydthan for any other time in a four-year (1996-1999) study of the watershed. Daily flows observed for an agriculturalsubwatershed were generally greater than for a forested subwatershed throughout the study, and during the hurricanes of1999. Daily nutrient loads measured across the research watershed were greater during hurricane Floyd than for any othertime in the study. In general, the two-month period of hurricanes produced total nitrogen and total phosphorus loads nearlyequal to average loads for an entire year. Total annual nitrogen export from an agricultural subwatershed was 18 kg/ha in1999, with 11 kg/ha (61%) lost during September and October. Total annual nitrogen export from a forested subwatershedwas 15 kg/ha in 1999, with 10 kg/ha (67%) lost during September and October. The nitrogen export observed in the forestedsubwatershed was high compared to other forested areas, likely due to the highly permeable organic soils in the watershed.Total annual phosphorus export from an agricultural subwatershed was 0.9 kg/ha in 1999, with 0.7 kg/ha (78%) lost duringthe hurricanes/tropical storms. Total annual phosphorus load from a forested subwatershed was 0.1 kg/ha in 1999, with 74%of the load exported during the months of September and October. Hurricanes and floods occur with some regularity in NorthCarolina, but the effects are infrequently documented. This study provides information that will contribute to greaterunderstanding of how watersheds respond to these events.}, number={6}, journal={Transactions of the ASAE}, author={Shelby, J. D. and Chescheir, G. M. and Skaggs, R. W. and Amatya, D. M.}, year={2005}, pages={2179–2188} }
 @article{wang_youssef_skaggs_atwood_frankenberger_2005, title={Sensitivity analyses of the nitrogen simulation model, DRAINMOD-N II}, volume={48}, DOI={10.13031/2013.20106}, abstractNote={A two-step global sensitivity analysis was conducted for the nitrogen simulation model DRAINMOD-N II to assessthe sensitivity of model predictions of NO3-N losses with drainage water to various model inputs. Factors screening usingthe LH-OAT (Latin hypercube sampling - one at a time) sensitivity analysis method was performed as a first step considering48 model parameters; then a variance-based sensitivity analysis was conducted for 20 model parameters, which were theparameters ranked 1 to 14 by the LH-OAT method, five organic carbon (OC) decomposition parameters, and the empiricalshape factor of the temperature response function for the nitrification process. DRAINMOD-N II simulated a continuous cornproduction on a subsurface drained sandy loam soil using a 40-year (1951-1990) eastern North Carolina climatologicalrecord. Results from the first 20-year period of the simulations were used to initialize the soil organic matter pools, and resultsfrom the last 20-year period of the simulations were considered for the sensitivity analyses. Both yearly and 20-year averagemodel predictions of NO3-N losses through drainage flow were used in the analyses. Both sensitivity analysis methodsindicated that DRAINMOD-N II is most sensitive to denitrification parameters, especially those controlling temperatureeffect on process rate. Results also indicated that the model is mildly sensitive to the parameters controlling OC decompositionand associated N mineralization/immobilization. The use of different sensitivity analysis methods with dissimilar theoreticalfoundations increases the confidence in key parameters identification. More efforts should be focused on quantifying keyparameters for more accurate model predictions.}, number={6}, journal={Transactions of the ASAE}, author={Wang, X. and Youssef, M. A. and Skaggs, R. W. and Atwood, J. D. and Frankenberger, J. R.}, year={2005}, pages={2205–2212} }
 @article{burchell_skaggs_chescheir_gilliam_arnold_2005, title={Shallow subsurface drains to reduce nitrate losses from drained agricultural lands}, volume={48}, DOI={10.13031/2013.18518}, abstractNote={Nitrate losses from subsurface drainage systems remain an important environmental concern. Data were collectedfrom two drainage systems near Plymouth, North Carolina, to evaluate the effect of subsurface drain depth on nitrate-nitrogen(NO3--N) losses. Drains in plot 1 were 1.5 m deep and 25 m apart, and drains in plot 2 were 0.75 m deep and 12.5 m apart.Both plots received swine wastewater applications. Overall, the shallow drainage system had 42% less outflow than thedeeper drainage system. Lower NO3--N concentrations were observed in the shallow groundwater beneath the shallowdrainage plots as a result of higher water tables and likely increased denitrification. However, NO3--N concentrations in thedrainage water from the shallow drains were not reduced. On average, NO3--N export from the shallow subsurface drainswas 8 kg ha-1 in 2001 and 27 kg ha-1 in 2002. Nitrate export from the deeper drains was 6 kg ha-1 in 2001 and 37 kg ha-1in 2002. Decreased export observed in 2002 from the shallow subsurface drainage system was significant at the 10% level,but not for the entire 21-month period. Longer-term field studies, which incorporate variable climatological events, areneeded to conclude whether shallower drain depth will reduce NO3--N export from subsurface drainage systems.}, number={3}, journal={Transactions of the ASAE}, author={Burchell, Michael and Skaggs, R. W. and Chescheir, G. M. and Gilliam, J. W. and Arnold, L. A.}, year={2005}, pages={1079–1089} }
 @article{youssef_skaggs_chescheir_gilliam_2005, title={The  nitrogen simulation model, DRAINMOD-N II}, volume={48}, DOI={10.13031/2013.18335}, abstractNote={DRAINMOD-N II is a field-scale, process-based model that was developed to simulate nitrogen dynamics and
turnover in the soil-water-plant system under different management practices and soil and environmental conditions. It is
an enhanced version of the nitrogen (N) simulation model, DRAINMOD-N, that simulates a more complete N cycle, adds a
carbon (C) cycle, and operates at different levels of complexity. Processes considered in the model include atmospheric
deposition, application of mineral N fertilizers including urea and anhydrous ammonia (NH3), soil amendment with organic
N (ON) sources including plant residues and animal waste, plant uptake, organic C (OC) decomposition and associated N
mineralization/immobilization, nitrification, denitrification, NH3 volatilization, and N losses via subsurface drainage and
surface runoff. Nitrogen pools considered in the model are nitrate-nitrogen (NO3-N), ammoniacal nitrogen (NHx-N) and ON.
DRAINMOD-N II includes a submodel that simulates C dynamics in the soil-plant system using a C cycle similar to that of
the CENTURY model. A simplified approach is used to simulate temporal changes in soil pH; consequently, the model
determines the composition of the NHx-N pool and, if necessary, changes its operation mode. DRAINMOD-N II simulates
N reactive transport using a finite difference solution to a multiphase form of the one-dimensional advection-
dispersion-reaction equation. Model output includes daily concentrations of NO3-N and NHx-N in soil solution and drain
flow, daily OC content of the top 20 cm soil layer, and cumulative rates of simulated N processes.}, number={2}, journal={Transactions of the ASAE}, author={Youssef, M. A. and Skaggs, R. W. and Chescheir, G. M. and Gilliam, J. W.}, year={2005}, pages={611–626} }
 @article{vepraskas_he_lindbo_skaggs_2004, title={Calibrating hydric soil field indicators to long-term wetland hydrology}, volume={68}, ISSN={["0361-5995"]}, DOI={10.2136/sssaj2004.1461}, abstractNote={Jurisdictional wetlands are required to be saturated to the surface for 5% or more of the growing season in 5 out of 10 yr, but practical field methods for confirming this are lacking. This study determined whether hydric soil field indicators were related to wetland hydrology requirements. Water table levels were monitored daily for 2.5 yr in a toposequence of nine soil plots that included well to poorly drained members (Oxyaquic Paleudults and Typic Albaqualfs). Monitoring data were used to calibrate a hydrologic model that simulated water table levels from inputs of hourly rainfall data. Forty years of rainfall data were then used with the model to compute long‐term daily water‐table levels in each plot. These data were summarized as “saturation events”, which are the frequency that water tables were at or above preselected depths for at least 21 d. Twenty‐one days was the average period needed for Fe reduction to begin in these saturated soils. This condition must occur for hydric soil field indicators to form. Regression equations were developed to relate saturation events to percentages of redoximorphic features. The r 2 values for relationships between percentages of redoximorphic features and saturation events were >0.80 for depths of 15 cm, and >0.90 for depths between 30 and 90 cm. Results showed that the depleted matrix field indicator, in which redox depletions occupy >60% of the horizon, occurred in soils that were saturated for 21 d or longer at least 9 yr out of 10. This indicated the depleted matrix indicator occurred in soils that were saturated nearly twice as long, and more frequently, than the minimum requirements needed to meet wetland hydrology requirements.}, number={4}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Vepraskas, MJ and He, X and Lindbo, DL and Skaggs, RW}, year={2004}, pages={1461–1469} }
 @article{amatya_chescheir_fernandez_skaggs_gilliam_2004, title={DRAINWAT-based methods for estimating nitrogen transport in poorly drained watersheds}, volume={47}, DOI={10.13031/2013.16100}, abstractNote={Methods are needed to quantify effects of land use and management practices on nutrient and sediment loads atthe watershed scale. Two methods were used to apply a DRAINMOD-based watershed-scale model (DRAINWAT) to estimatetotal nitrogen (N) transport from a poorly drained, forested watershed. In both methods, in-stream retention or losses of Nwere calculated with a lumped-parameter model, which assumes that N concentration decreases exponentially with residence(or travel) time in the canals. In the first method, daily field outflows predicted by DRAINWAT were multiplied by averageN concentrations to calculate daily loads at the field edge. Travel time from the field edge to the watershed outlet was computedfor each field for each day based on daily velocities predicted by DRAINWAT for each section of the canal-stream network.The second lumped-parameter method was similar but used predicted annual outflow to obtain annual load at the fieldedge. The load was transported to the watershed outlet, and the in-stream N loss was determined by using a constant averagevelocity (obtained by long-term DRAINWAT simulations), independent of season, for the entire canal-stream network. Themethods were applied on a 2,950 ha coastal forested watershed near Plymouth, North Carolina, to evaluate daily, monthly,and annual export of nitrogen for a five-year (1996-2000) period. Except for some late spring and hurricane events, predicteddaily flows were in good agreement with measured results for all five years (Nash-Sutcliffe coefficient, E = 0.71 to 0.85).Estimates of monthly total N load were in much better agreement (E = 0.76) with measured data than were the daily estimates(E = 0.19). Annual nitrogen load was predicted within 17% of the measured value, on average, and there was no difference(. = 0.05) between measured and estimated monthly and annual loads. The estimates of annual N loads using travel time witha daily velocity yielded better results than with the constant average velocity. The estimated delivery ratio (load at theoutlet/load at the field edge) for total N was shown to vary widely among individual fields depending on their location in thewatershed and distance from the outlet. Both of the methods investigated can potentially be used with GIS in predictingimpacts of land management practices on total N loads from poorly drained watersheds.}, number={3}, journal={Transactions of the ASAE}, author={Amatya, D. M. and Chescheir, G. M. and Fernandez, G. P. and Skaggs, R. W. and Gilliam, J. W.}, year={2004}, pages={677–687} }
 @article{reyes_skaggs_bengtson_2004, title={GLEAMS-SWT with nutrients}, volume={47}, DOI={10.13031/2013.15871}, abstractNote={This article introduces the Groundwater Loading Effects of Agricultural Management Systems with Subsurfacedrainage and Water Table (GLEAMS-SWT) with nutrients model. This version contains GLEAMS nutrient component andadditional routines to predict nitrogen loss from subsurface drainage. Comparisons of GLEAMS-SWT and GLEAMS nitrogenand phosphorus loss predictions with six years of measured nitrogen and phosphorus losses from nonsubsurface andsubsurface drained plots located at Ben Hur Research Farm in Baton Rouge, Louisiana, showed that: (1) GLEAMS-SWToverpredicted total (ammonia and nitrate both in solution and solids) surface nitrogen (TSN) loss by 10%, and GLEAMSunderpredicted TSN loss by 38% for the nonsubsurface drained plot; (2) GLEAMS-SWT overpredicted TSN loss by 31%, andGLEAMS overpredicted it by 7% for the subsurface drained plot; (3) both models severely underpredicted total (solution andsolids) surface phosphorus losses for the nonsubsurface and subsurface drained plots; and (4) GLEAMS-SWT overpredictedtotal nitrogen loss in subsurface drainage nearly 7-fold.}, number={1}, journal={Transactions of the ASAE}, author={Reyes, M. R. and Skaggs, R. W. and Bengtson, R. L.}, year={2004}, pages={129–132} }
 @article{ha_evans_luo_skaggs_2004, title={Modification and use of DRAINMOD to evaluate a lagoon effluent land application system}, volume={47}, DOI={10.13031/2013.15869}, abstractNote={Traditionally, lagoon design has considered waste inflow, sludge accumulation, individual event rainfall
associated with the 25-year, 24-hour storm, and sufficient temporary storage to handle excess rainfall during non-irrigation
periods. Excess rainfall was defined as the “average” or “normal” rainfall in excess of evaporation during the non-irrigation
(drawdown) period. North Carolina experienced a series of tropical storms and hurricanes in 1995 that resulted in several
lagoon overtoppings; however, none of the storms individually satisfied the 25-year, 24-hour criterion. These storms raised
questions as to whether the 25-year, 24-hour criterion presented the appropriate design constraint to prevent lagoon
overtopping or whether the cumulative impact of prolonged rainy periods (referred to herein as “chronic” rainfall) was a
greater threat. To evaluate the validity of existing lagoon design criteria and emergency action measures proposed by the
North Carolina Soil and Water Conservation Commission, the irrigation component of the field hydrology model DRAINMOD
was modified to consider animal waste lagoon constraints of chronic rainfall, crop nitrogen utilization, and emergency lagoon
operational measures. The modified DRAINMOD was used to evaluate lagoon design and operational guidelines in effect
in eastern North Carolina at the time of the 1995 lagoon breaches and the proposed 1999 emergency measures. Model
simulation results showed that prolonged wet periods in the winter that result in high moisture surplus are the most likely cause
of excessively high lagoon stage or overflow. To minimize the occurrence of elevated lagoon stage and eliminate the risk of
overflow, model results also showed that the design temporary storage criterion should be increased to account for chronic
rainfall excess between drawdown periods. Intense storms with short durations, such as the catastrophic design (25-year,
24-hour) storm, mainly occurred in the summer and usually posed no risks to lagoon overflow because these events typically
occurred at a time when lagoons were traditionally drawn down to their lowest allowable stage. Using a constant average
nitrogen concentration for lagoon wastewater resulted in fewer irrigation applications, which in turn resulted in more
frequent high lagoon stage and more overflows. Lagoon spills resulting from extreme weather conditions could be avoided
by applying wastewater more frequently and temporarily suspending the crop nitrogen limit in wet years without exceeding
soil hydraulic limits.}, number={1}, journal={Transactions of the ASAE}, author={Ha, Z. and Evans, R. O. and Luo, W. and Skaggs, R. W.}, year={2004}, pages={47–58} }
 @article{he_vepraskas_lindbo_skaggs_2003, title={A method to predict soil saturation frequency and duration from soil color}, volume={67}, ISSN={["0361-5995"]}, DOI={10.2136/sssaj2003.0961}, number={3}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={He, X and Vepraskas, MJ and Lindbo, DL and Skaggs, RW}, year={2003}, pages={961–969} }
 @article{haan_skaggs_2003, title={Effect of parameter uncertainty on DRAINMOD predictions: I. Hydrology and yield}, volume={46}, DOI={10.13031/2013.13968}, abstractNote={The computerbased hydrologic model DRAINMOD can be used to predict the effect of drainage design on therate of subsurface drainage and on crop yield. An uncertainty analysis was conducted to quantitatively assess the variabilityin model outputs caused by parameter uncertainty. The analysis was based on an experimental field at the Tidewater ResearchStation in Plymouth, North Carolina. As a first step in the uncertainty analysis, a sensitivity test was conducted to determinewhich parameters in the model have the most influence on the model objective functions. Firstorder approximation andMonte Carlo simulation were used to determine the effect of the uncertainty in the most sensitive parameters on theuncertainty in the model objective functions. Objective functions evaluated were: average annual subsurface drainagevolume; SEW30 (a measure of stress caused by excessive soil water in the top 30 cm) during the growing season; and relativeyield for both conventional and controlled drainage. Nine parameters found to significantly affect model output were usedin the uncertainty analysis. The firstorder approximation showed that in the case of conventional drainage, lateral saturatedhydraulic conductivity accounted for 81% of the uncertainty in terms of variance for predicted annual subsurface drainagevolume, 81% for growing season SEW30, and 71% for relative yield. For controlled drainage, lateral saturated hydraulicconductivity contributed 62% of the uncertainty in terms of the variance in predicted annual subsurface drainage volume,69% in growing season SEW30, and 62% in relative yield. The Monte Carlo simulation showed similar results. Improving theknowledge of these most influential parameters will help to reduce the uncertainty in DRAINMOD predictions for theseobjective functions.}, number={4}, journal={Transactions of the ASAE}, author={Haan, P. K. and Skaggs, R. W.}, year={2003}, pages={1061–1067} }
 @article{haan_skaggs_2003, title={Effect of parameter uncertainty on DRAINMOD predictions: II. Nitrogen loss}, volume={46}, DOI={10.13031/2013.13969}, abstractNote={Reducing nitrate levels in sensitive coastal waters has become a national priority. Agriculture has been targeted
as a significant contributor to this problem. Controlled drainage is recognized as one way to reduce nitrate losses from
agricultural fields requiring subsurface drainage. The computer–based hydrology model DRAINMOD can be used to predict
the effect of drainage design on the rate of subsurface drainage and the water quality related to nitrates in these waters. An
uncertainty analysis of the model was conducted to quantitatively assess the variability in the model predictions for average
annual nitrate loss through subsurface drainage, surface runoff, and denitrification caused by parameter uncertainty.
First–order approximation and Monte Carlo simulation were used to estimate the effect of the uncertainty in the most sensitive
parameters (dispersivity, bulk density, % nitrogen uptake by corn, mineralization rate constant, and denitrification rate
constant) on the uncertainty in the model objective functions. The denitrification rate constant accounted for greater than
70% of the uncertainty in terms of the variance for all objective functions under both conventional and controlled drainage.
Reducing the uncertainty in influential parameters can reduce the uncertainty in DRAINMOD predictions for nitrate loss.}, number={4}, journal={Transactions of the ASAE}, author={Haan, P. K. and Skaggs, R. W.}, year={2003}, pages={1069–1075} }
 @article{amatya_skaggs_gilliam_hughes_2003, title={Effects of orifice-weir outlet on hydrology and water quality of a drained forested watershed}, volume={27}, number={2}, journal={Southern Journal of Applied Forestry}, author={Amatya, D. M. and Skaggs, R. W. and Gilliam, J. W. and Hughes, J. H.}, year={2003}, pages={130–142} }
 @article{skaggs_chescheir_2003, title={Effects of subsurface drain depth on nitrogen losses from drained lands}, volume={46}, DOI={10.13031/2013.12974}, abstractNote={A simulation study was conducted to determine effects of drain depth on nitrogen (N) loss in drainage water.Simulations were conducted for drain depths of 0.75, 1.0, 1.25, and 1.5 m for a Portsmouth sandy loam at Plymouth, NorthCarolina. A wide range of drain spacing was considered for each depth. Corn yields were predicted and an economic analysiswas conducted to determine the drain spacing giving maximum predicted profit for each depth. Results showed that nitrogenlosses from subsurface drains can be reduced by placing the drains at shallow depths. In order to satisfy agriculturalproduction requirements, shallow drains must be placed closer together than deeper drains. While predicted agriculturalprofits for the shallow drains are reduced somewhat compared to the deeper drains, overall profits are substantially increasedwhen the cost of removing N from drainage water is considered.}, number={2}, journal={Transactions of the ASAE}, author={Skaggs, R. W. and Chescheir, G. M.}, year={2003}, pages={237–244} }
 @book{chescheir_lebo_amatya_hughes_gilliam_skaggs_herrmann_2003, title={Hydrology and water quality of forested lands in eastern North Carolina}, publisher={Raleigh, N.C. : N.C. Agricultural Research Service, N.C. State University}, author={Chescheir, G. M. and Lebo, M. E. and Amatya, D. M. and Hughes, J. and Gilliam, J. W. and Skaggs, R. W. and Herrmann, R. B.}, year={2003} }
 @article{sun_mcnulty_amatya_skaggs_swift_shepard_riekerk_2002, title={A comparison of the watershed hydrology of coastal forested wetlands and the mountainous uplands in the Southern US}, volume={263}, ISSN={["0022-1694"]}, DOI={10.1016/S0022-1694(02)00064-1}, abstractNote={Hydrology plays a critical role in wetland development and ecosystem structure and functions. Hydrologic responses to forest management and climate change are diverse in the Southern United States due to topographic and climatic differences. This paper presents a comparison study on long-term hydrologic characteristics (long-term seasonal runoff patterns, water balances, storm flow patterns) of three watersheds in the southern US. These three watersheds represent three types of forest ecosystems commonly found in the lower Atlantic coastal plain and the Appalachian upland mountains. Compared to the warm, flat, and shallow groundwater dominated pine flatwoods on the coast, the inland upland watershed was found to have significantly higher water yield, Precipitation/Hamon's potential evapotranspiration ratio (1.9 for upland vs 1.4 and 0.9 for wetlands), and runoff/precipitation ratio (0.53±0.092 for upland vs 0.30±0.079 and 0.13±0.094 for wetlands). Streamflow from flatwoods watersheds generally are discontinuous most of the years while the upland watershed showed continuous flows in most years. Stormflow peaks in a cypress–pine flatwoods system were smaller than that in the upland watershed for most cases, but exceptions occurred under extreme wet conditions. Our study concludes that climate is the most important factor in determining the watershed water balances in the southern US. Topography effects streamflow patterns and stormflow peaks and volume, and is the key to wetland development in the southern US.}, number={1-4}, journal={JOURNAL OF HYDROLOGY}, author={Sun, G and McNulty, SG and Amatya, DM and Skaggs, RW and Swift, LW and Shepard, JP and Riekerk, H}, year={2002}, month={Jun}, pages={92–104} }
 @article{he_vepraskas_skaggs_lindbo_2002, title={Adapting a drainage model to simulate water table levels in coastal plain soils}, volume={66}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2002.1722}, abstractNote={Seasonal saturation in soils is expensive and time consuming to document, but the information is needed for land use assessments. Hydrologic models can be used to assess saturation occurrence quickly if the models are calibrated for individual sites. This study determined whether a drainage model (DRAINMOD) could predict water table levels in soils with and without a perimeter ditch. Water table levels were monitored for up to 3 yr at two toposequences that contained a total of 21 soil plots (3 m by 3 m). Soils included Typic Paleudults, Aquic Paleudults, and Umbric Paleaquults. Each plot was instrumented with a recording well to monitor daily water table levels. DRAINMOD was calibrated for each soil plot using measurements of in situ saturated hydraulic conductivity, soil water characteristic, depth to impermeable layer, depth of rooting, and rainfall. A plot's water table fluctuation was simulated by a system of virtual drains whose distance and depth were adjusted to produce simulated water table fluctuations in line with those actually measured. Further calibration adjusted drainable porosity in the upper 20 cm of the soil, depressional storage, evapotranspiration rate, and depth to impermeable layer. Adjustments were made by iteration to minimize the absolute average deviation between simulated and measured water table levels. Calibration had to be done by plot. Average absolute deviations were generally <20 cm for periods ranging from 1 to 3 yr. The results showed that DRAINMOD could be adapted to simulate water table levels in landscapes that do not contain a network of parallel drains.}, number={5}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={He, X and Vepraskas, MJ and Skaggs, RW and Lindbo, DL}, year={2002}, pages={1722–1731} }
 @article{el-sadek_feyen_skaggs_berlamont_2002, title={Economics of nitrate losses from drained agricultural land}, volume={128}, DOI={10.1061/(asce)0733-9372(2002)128:4(376)}, abstractNote={Some of the highest losses of nitrate to surface waters come from drained agricultural land. This research studied, for Belgian farming conditions, (i) the effect of subsurface drainage density on nitrate losses and (ii) the economics of nitrate losses, using the nitrogen version of the program DRAINMOD-N. DRAINMOD was used to simulate the performance of the drainage system of the Hooibeekhoeve experiment, situated in the sandy region of the Kempen (Belgium) for a 14-year (1985–1998) period. A continuous cropping with maize was assumed. Daily NO3-N losses were predicted for a range of drain spacings and depths, two drainage strategies (conventional and controlled), and three fertilizer application rates (225, 275, and 325 kg N ha-1). Losses of N in subsurface drainage were assumed to occur almost entirely in the NO3-N form. Losses of organic and inorganic N in the form of NO3-N in surface runoff are small and were neglected. Hydrologic results indicated that increasing drain spacing or decreasing drain de...}, number={4}, journal={Journal of Environmental Engineering (New York, N.Y.)}, author={El-Sadek, A. and Feyen, J. and Skaggs, W. and Berlamont, J.}, year={2002}, pages={376–383} }
 @article{appelboom_chescheir_skaggs_hesterberg_2002, title={Management practices for sediment reduction from forest roads in the coastal plains}, volume={45}, DOI={10.13031/2013.8529}, abstractNote={Sediment has been identified as one of the most important nonpoint source pollutants of surface waters. Inforested areas, the predominant source of sediment is from the construction and maintenance of access roads, which contributeas much as 90% of the total eroded sediments. Seven different road management practices were studied to determine theireffectiveness in reducing sediment production from forest roads on nearly flat lands in the lower coastal plains of NorthCarolina. One practice utilized a continuous berm along the roadside, while the other six practices had a noncontinuousberm with different combinations of road surface gravel and roadside vegetative strips. Runoff samples collected duringeleven different rainfall events of varying intensity and duration were analyzed for sediment content. The rainfall amount,intensity, infiltration, and antecedent rainfall conditions were combined into a single energy rating to assist in the overallanalysis. The results of the study showed that a continuous berm maintained along the edge of a forest road can reduce totalsediment loss by an average of 99% compared to the same type road without the presence of a continuous berm. When acontinuous berm is not present, graveling the road surface can reduce the total loss of sediment from roads by an averageof 61% compared to a nongraveled road surface. A 90 cm wide grass strip on the edge of the driving surface can reduce totalsediment loss by an average of 56% compared to a road without a grass strip.}, number={2}, journal={Transactions of the ASAE}, author={Appelboom, T. W. and Chescheir, G. M. and Skaggs, R. W. and Hesterberg, Dean}, year={2002}, pages={337–344} }
 @article{fernandez_chescheir_skaggs_amatya_2002, title={Watgis: A GIS-based lumped parameter water quality model}, volume={45}, DOI={10.13031/2013.8822}, abstractNote={A Geographic Information System (GIS)based, lumped parameter water quality model was developed to estimatethe spatial and temporal nitrogenloading patterns for lower coastal plain watersheds in eastern North Carolina. The modeluses a spatially distributed delivery ratio (DR) parameter to account for nitrogen retention or loss along a drainage network.Delivery ratios are calculated from time of travel and an exponential decay model for instream dynamics. Travel times fromany point in the drainage network to the watershed outlet are obtained from simulations using a combined physically basedfield hydrology and drainage canal routing model (DRAINMODDUFLOW). Nitrogen load from contributing areas in thewatershed delivered to the main watershed outlet is obtained as the product of field export with the corresponding deliveryratio. The total watershed load at the outlet is the combined loading of the individual fields. Nitrogen exports from sourceareas are measured. The lumped water quality model is integrated within a GIS framework with menu interface, displayoptions, and statistical procedures. Within this framework, the model can be used as a screening tool to analyze the effectsof different land and water management practices on downstream water quality. A description of the model is presented alongwith the results from the evaluation of the model to characterize the seasonal and annual export of nitrogen from a drainedforested watershed near Plymouth, North Carolina. Results of the study showed that the lumped parameter model canreasonably predict the loads at the outlet of the watershed. Predicted loads for 1997 were highly correlated with the observedloads (correlation coefficients of 0.99, 0.90, and 0.96 for nitratenitrogen, TKN, and total nitrogen respectively). Sensitivityand uncertainty analyses indicated that predicted outlet loads were sensitive to field flow predictions and exportconcentrations. Overall, the results indicate that the lumped parameter model can be an effective tool for describing themonthly nitrogen loads from a poorly drained coastal plain watershed.}, number={3}, journal={Transactions of the ASAE}, author={Fernandez, G. P. and Chescheir, G. M. and Skaggs, R. W. and Amatya, D. M.}, year={2002}, pages={593–600} }
 @article{luo_skaggs_chescheir_2001, title={DRAINMOD modifications for cold conditions}, volume={43}, ISBN={0001-2351}, DOI={10.13031/2013.3057}, abstractNote={The field hydrology model DRAINMOD was modified to include freezing and thawing, and snowmeltcomponents. Based on daily hydrologic predictions of the original model, the modified DRAINMOD numerically solvesthe heat flow equation to predict soil temperature. When freezing conditions are indicated by below zero temperatures, themodel calculates ice content in the soil profile and modifies soil hydraulic conductivity and infiltration rate accordingly.Recorded precipitation is separated as rain and snow when daily average air temperature is above or below a rain/snowdividing base temperature. Snow is predicted to accumulate on the ground until air temperature rises above a snowmeltbase temperature. Soil surface temperature is recalculated when snow cover exists. Daily snowmelt water is added torainfall, which may infiltrate or run off depending on soil freezing condition. The modified DRAINMOD predictions ofsoil temperature agreed well with field observations at Plymouth, North Carolina, Truro, Nova Scotia, and Lamberton,Minnesota. Assuming air temperature as the soil surface boundary condition increased the variability of soil temperaturepredictions at shallow depths, agreement with field measurements was still good. The method of using average airtemperature as an indicator to separate snow and rain worked very well for Carsamba, Turkey. At Truro, Nova Scotia,however, the method was not as successful, and several snow events were predicted as rainfall and vice versa. Comparedwith the original version of DRAINMOD, the modified version predicts fewer drainage flow events in winter monthsbecause of snow accumulation on the surface. Subsurface drainage and/or surface runoff resulting from snowmelt arepredicted when air temperature rises, the snow melts, and the soil begins to thaw.}, number={6}, journal={Transactions of the ASAE}, author={Luo, W. and Skaggs, R. W. and Chescheir, G. M.}, year={2001}, pages={1569–1582} }
 @article{sun_mcnulty_shepard_amatya_riekerk_comerford_skaggs_swift_2001, title={Effects of timber management on the hydrology of wetland forests in the southern United States}, volume={143}, ISSN={["0378-1127"]}, DOI={10.1016/s0378-1127(00)00520-x}, abstractNote={Abstract The objectives of this paper are to review the hydrologic impacts of various common forest management practices that include harvesting, site preparation, and drainage. Field hydrological data collected during the past 5–10 years from ten forested wetland sites across the southern US are synthesized using various methods including hydrologic simulation models and Geographic Information Systems. Wetland systems evaluated include red river bottoms, black river bottoms, pocosins, wet mineral flats, cypress domes, and pine flatwoods. Hydrologic variables used in this assessment include water table level, drainage, and storm flow on different spatial and temporal scales. Wetland ecosystems have higher water storage capacity and higher evapotranspiration than uplands. Hydrologic impacts of forest management are variable, but generally minor, especially when forest best management practices are adopted. A conceptually generalized model is developed to illustrate the relative magnitude of hydrologic effects of forest management on different types of wetlands in the southern US. This model suggests that in addition to soils, wetland types, and management practice options, climate is an important factor in controlling wetland hydrology and the magnitude of disturbance impacts. Bottomland wetlands, partial harvesting, and warm climate usually offer conditions that result in low hydrologic impact.}, number={1-3}, journal={FOREST ECOLOGY AND MANAGEMENT}, author={Sun, G and McNulty, SG and Shepard, JP and Amatya, DM and Riekerk, H and Comerford, NB and Skaggs, W and Swift, L}, year={2001}, month={Apr}, pages={227–236} }
 @article{amatya_skaggs_2001, title={Hydrologic modeling of a drained pine plantation on poorly drained soils}, volume={47}, number={1}, journal={Forest Science}, author={Amatya, D. M. and Skaggs, R. W.}, year={2001}, pages={103–114} }
 @article{luo_skaggs_madani_cizikci_mavi_2001, title={Predicting field hydrology in cold conditions with DRAINMOD}, volume={44}, DOI={10.13031/2013.6247}, abstractNote={The field hydrology model DRAINMOD was modified for cold conditions, where soil freezing, thawing, and snowaccumulation affect the hydrologic cycle. Field observations of water table depth and subsurface drainage from Carsamba,Turkey; Truro, Nova Scotia; and Lamberton, Minnesota, were used to test the modified DRAINMOD. At Carsamba, Turkey,the modified DRAINMOD correctly predicted the timing and magnitude of drainage events resulting from snow accumulationand subsequent snow melt. Although the variable weather pattern and nature of rain/snow mixture at the Nova Scotia sitemade it difficult to describe hydrology during winter months, the modified DRAINMOD predicted the timing and magnitudeof most drainage flow events more accurately. Continuous longterm model simulations at Lamberton, Minnesota, weregenerally in good agreement with drainage measurements. The temperaturebased approach used in the model caused someunusual rainfall events to be missed during winter. Errors from snow event predictions contributed to errors in hydrologicpredictions. Overall, the modified DRAINMOD gave reasonable predictions of field hydrology. The simplified snow algorithmprovided a timely prediction of snow accumulation and melting.}, number={4}, journal={Transactions of the ASAE}, author={Luo, W. and Skaggs, R. W. and Madani, A. and Cizikci, S. and Mavi, A.}, year={2001}, pages={825–834} }
 @article{borin_morari_bonaiti_paasch_skaggs_2000, title={Analysis of DRAINMOD performances with different detail of soil input data in the Veneto region of Italy}, volume={42}, ISSN={["0378-3774"]}, DOI={10.1016/S0378-3774(99)00044-X}, abstractNote={The deterministic field scale model DRAINMOD was tested in an experiment on subsurface pipe drainage in the Veneto region, in an environment of NE Italy characterised by the presence of a shallow water table in a flat area. The objective was to determine whether a minimal set of field data would suffice to use DRAINMOD for predictive purposes. The measured water table depth and drain outflow from a 5-year field experiment were compared to those determined by DRAINMOD using three levels of detail in input data. The results indicated that even very limited input data (texture and porosity of the top 30 cm of soil) gave good predictions. More elaborate data improved the estimates.}, number={3}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Borin, M and Morari, F and Bonaiti, G and Paasch, M and Skaggs, RW}, year={2000}, month={Jan}, pages={259–272} }
 @article{sun_amatya_mcnulty_skaggs_hughes_2000, title={Climate change impacts on the hydrology and productivity of a pine plantation}, volume={36}, ISSN={["1752-1688"]}, DOI={10.1111/j.1752-1688.2000.tb04274.x}, abstractNote={ABSTRACT: There are increasing concerns in the forestry community about global climate change and variability associated with elevated atmospheric CO2. Changes in precipitation and increases in air temperature could impose additional stress on forests during the next century. For a study site in Carteret County, North Carolina, the General Circulation Model, HADCM2, predicts that by the year 2099, maximum air temperature will increase 1.6 to 1.9°C, minimum temperature will increase 2.5 to 2.8°C, and precipitation will increase 0 to 10 percent compared to the mid-1990s. These changes vary from season to season. We utilized a forest ecosystem process model, PnET-II, for studying the potential effects of climate change on drainage outflow, evapotranspiration, leaf area index (LAI) and forest Net Primary Productivity (NPP). This model was first validated with long term drainage and LAI data collected at a 25-ha mature loblolly pine (Pinus taeda L.) experimental watershed located in the North Carolina lower coastal plain. The site is flat with poorly drained soils and high groundwater table. Therefore, a high field capacity of 20 cm was used in the simulation to account for the topographic effects. This modeling study suggested that future climate change would cause a significant increase of drainage (6 percent) and forest productivity (2.5 percent). Future studies should consider the biological feedback (i.e., stomata conductance and water use efficiency) to air temperature change.}, number={2}, journal={JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION}, author={Sun, G and Amatya, DM and McNulty, SG and Skaggs, RW and Hughes, JH}, year={2000}, month={Apr}, pages={367–374} }
 @article{amatya_gregory_skaggs_2000, title={Effects of controlled drainage on storm event hydrology in a loblolly pine plantation}, volume={36}, ISSN={["1093-474X"]}, DOI={10.1111/j.1752-1688.2000.tb04258.x}, abstractNote={ABSTRACT: A paired watershed approach was utilized to study the effects of three water management regimes on storm event hydrology in three experimental watersheds in a drained loblolly pine (Pinus taeda L.) plantation in eastern North Carolina. The regimes were: (1) conventional drainage, (2) controlled drainage (CD) to reduce outflows during spring fish recruitment, and (3) controlled drainage to reduce outflows and conserve water during the growing season. Data from two pit-treatment years and three years of CD treatment with raised weirs at the watershed outlet are presented. CD treatment resulted in rises in water table elevations during the summer. But the rises were small and short-lived due to increased evapotranspiration (ET) rates as compared to the spring treatment with lower ET demands. CD treatment had no effect on water tables deeper than 1.3 m. CD treatments, however, significantly (α= 0.05) reduced the stoning outflows for all events, and peak outflow rates for most of the events depending upon the outlet weir level. In some events, flows did not occur at all in watersheds with CD. When event outflows occurred, duration of the event was sharply reduced because of reduced effective ditch depth. Water table depth at the start of an event influenced the effect of CD treatment on storm event hydrology.}, number={1}, journal={JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION}, author={Amatya, DM and Gregory, JD and Skaggs, RW}, year={2000}, month={Feb}, pages={175–190} }
 @book{skaggs_schilfgaarde_1999, title={Agricultural drainage}, ISBN={0891181415}, publisher={Madison, Wis.: American Society of Agronomy, Inc.; Crop Science Society of America, Inc.; Soil Science Society of America, Inc.}, author={Skaggs, R. W. and Schilfgaarde, J. van}, year={1999} }
 @inbook{skaggs_chescheir_1999, title={Application of simulation models}, DOI={10.2134/agronmonogr38.c16}, booktitle={Agricultural drainage: Chapter 15  (Agronomy ; no. 38).}, publisher={Madison, Wis.: American Society of Agronomy, Inc.: Crop Science Society of America, Inc.: Soil Science Society of America, Inc.}, author={Skaggs, R. W. and Chescheir, G. M.}, editor={R.W. Skaggs and Schilfgaarde, J.Editors}, year={1999}, pages={537–564} }
 @inbook{skaggs_1999, title={Drainage simulation models}, DOI={10.2134/agronmonogr38.c14}, booktitle={Agricultural drainage: Chapter 13  (Agronomy ; no. 38).}, publisher={Madison, Wis.: American Society of Agronomy, Inc.: Crop Science Society of America, Inc.: Soil Science Society of America, Inc.}, author={Skaggs, R. W.}, editor={R.W. Skaggs and Schilfgaarde, J.Editors}, year={1999}, pages={469–500} }
 @article{skaggs_chescheir_1999, title={Drains in depth. Studies focus on nitrogen losses from agricultural lands}, volume={6}, number={9}, journal={Resource, Engineering & Technology for a Sustainable World}, author={Skaggs, R. W. and Chescheir, G. M.}, year={1999}, pages={13} }
 @inproceedings{fernandez_chescheir_amatya_skaggs_1999, title={GIS-based water quality lumped parameter model}, number={1999}, booktitle={Proceedings of the Mini-Conference, Advances in Water Quality Modeling}, publisher={St. Joseph, MI: ASAE}, author={Fernandez, G.P. and Chescheir, G.M. and Amatya, D.M. and Skaggs, R.W.}, year={1999}, pages={65–70} }
 @inbook{skaggs_schilfgaarde_1999, title={Introduction}, DOI={10.2134/agronmonogr38.c1}, booktitle={Agricultural drainage: Chapter 1  (Agronomy ; no. 38).}, publisher={Madison, Wis.: American Society of Agronomy, Inc.: Crop Science Society of America, Inc.: Soil Science Society of America, Inc.}, author={Skaggs, R. W. and Schilfgaarde, J. van}, editor={R.W. Skaggs and Schilfgaarde, J.Editors}, year={1999}, pages={3–10} }
 @inproceedings{sun_amatya_mcnulty_skaggs_hughes_1999, title={Potential impact of climate change on the hydrology and productivity of a drained loblolly pine plantation in North Carolina}, booktitle={Proceedings: Specialty Conference on Potential Consequences of Climate Variability and Change to Water Resources of the United States: May 10-12, 1999, Atlanta, Georgia  (American Water Resources Association technical publication series ; TPS-99-1).}, publisher={Herndon, VA: American Water Resources Association}, author={Sun, G. and Amatya, D. M. and McNulty, S. G. and Skaggs, R. W. and Hughes, J. H.}, year={1999}, pages={403–408} }
 @inproceedings{birgand_chescheir_skaggs_gilliam_1999, title={Quantification and effects of in-stream processes in the ditches and canals of the Lower Coastal Plain of North Carolina}, number={1999}, booktitle={Proceedings of the Mini-Conference, Advances in Water Quality Modeling}, publisher={St. Joseph, MI: ASAE}, author={Birgand, F. and Chescheir, G. M. and Skaggs, R. W. and Gilliam, J. W.}, year={1999}, pages={45–50} }
 @inproceedings{amatya_chescheir_fernandez_skaggs_1999, title={Testing of a watershed scale hydrologic/water quality model for poorly drained soils}, number={1999}, booktitle={Proceedings of the Mini-Conference, Advances in Water Quality Modeling}, publisher={St. Joseph, MI: ASAE}, author={Amatya, D. M. and Chescheir, G. M. and Fernandez, G. P. and Skaggs, R. W.}, year={1999}, pages={33–39} }
 @inbook{skaggs_1999, title={Water table management: Subirrigation and controlled drainage}, DOI={10.2134/agronmonogr38.c21}, booktitle={Agricultural drainage (Agronomy ; no. 38.)}, publisher={Madison, Wis.: American Society of Agronomy, Inc.: Crop Science Society of America, Inc.: Soil Science Society of America, Inc}, author={Skaggs, R. W.}, editor={R.W. Skaggs and Schilfgaarde, J.Editors}, year={1999}, pages={695–718} }
 @inbook{fernandez_skaggs_chescheir_amatya_1999, title={Watershed scale GIS based lumped parameter water quality model}, booktitle={Proceedings of 2nd Inter-Regional Conference on Environment-Water, Emerging Technologies for Sustainable Land Use and Water Management}, publisher={Lausanne, Switzerland: Presses Polytechniques et Universitaries Romandes}, author={Fernandez, G. P. and Skaggs, R. W. and Chescheir, G. M. and Amatya, D. M.}, editor={L. Amusy, S. Periera and Fritsch, M.Editors}, year={1999} }
 @inproceedings{fernandez_chescheir_skaggs_1998, title={DRAINMOD 5.0: A Windows version that considers crop yield, nitrogen and salinity}, booktitle={Drainage in the 21st century: Food production and the environment: Proceedings of the seventh International Drainage Symposium}, publisher={St. Joseph, Michigan: American Society of Agricultural Engineers}, author={Fernandez, G. P. and Chescheir, G. M. and Skaggs, R. W.}, year={1998}, pages={220–226} }
 @article{amatya_gilliam_skaggs_lebo_campbell_1998, title={Effects of controlled drainage on forest water quality}, volume={27}, ISSN={["0047-2425"]}, DOI={10.2134/jeq1998.00472425002700040029x}, abstractNote={Abstract The effects of controlled drainage (CD) on hydrology and water quality are presented for three eastern North Carolina watersheds in loblolly pine ( Pinus taeda L.). Timing of CD treatments were: the spring fish recruitment period in the downstream estuary in one watershed, and the summer‐fall period to facilitate tree growth in another watershed. A third watershed was maintained under conventional drainage throughout the study. It was demonstrated that seasonal controlled drainage can reduce both the total drainage outflows and corresponding sediment and nutrient exports. For example, CD reduced drainage outflows by as much as 88% during the summer‐fall and 39% during the spring with annual average reduction of 20 to 25%. Annual average total phosphorus (TP) and ammonium‐nitrogen (NH 4 ‐N) exports from watersheds under treatment were reduced by 7 to 72% as compared to the watershed under conventional drainage. For other nutrients and total suspended solids (TSS), concentrations were significantly different (α = 0.05) among the three watersheds during the winter when they were all under conventional drainage. This indicated characteristic differences unrelated to applied treatments. Taking these differences into account, the reductions in annual average export of TSS (up to 47%) and nitrate plus nitrite‐nitrogen (NO 3 +NO 2 ‐N) (up to 16%), total Kjeldahl nitrogen (TKN) (up to 45%) and total organic carbon (TOC) (up to 33%) from watersheds under treatment were directly attributed to reduction in outflows. Even though CD appeared to have increased concentrations of some of the nutrients analyzed, except for NH 4 ‐N, the applied treatments lowered the export of TSS and most nutrients measured. It is concluded that CD can be used to reduce TSS and nutrient exports from pine plantations, primarily through reduced drainage outflows.}, number={4}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Amatya, DM and Gilliam, JW and Skaggs, RW and Lebo, ME and Campbell, RG}, year={1998}, pages={923–935} }
 @inproceedings{amatya_chescheir_skaggs_fernandez_birgand_1998, title={Evaluation of a DRAINMOD based watershed scale model}, booktitle={Drainage in the 21st century: Food production and the environment: Proceedings of the seventh International Drainage Symposium}, publisher={St. Joseph, Michigan: American Society of Agricultural Engineers}, author={Amatya, D. M. and Chescheir, G. M. and Skaggs, R. W. and Fernandez, G. and Birgand, F.}, year={1998}, pages={211–219} }
 @book{skaggs_1998, title={Lumped parameter water quality modeling using a probabilistic approach}, number={1998 Apr. 1}, journal={Report (Water Resources Research Institute of the University of North Carolina)}, institution={Raleigh, NC: University of North Carolina Water Resources Research Institute}, author={Skaggs, W.}, year={1998} }
 @inproceedings{salem_skaggs_1998, title={Predicting drainage rates under varying water table conditions}, booktitle={Drainage in the 21st century: Food production and the environment: Proceedings of the seventh International Drainage Symposium}, publisher={St. Joseph, Michigan: American Society of Agricultural Engineers}, author={Salem, H. E. and Skaggs, R. W.}, year={1998}, pages={168–175} }
 @article{breve_skaggs_parsons_gilliam_1998, title={Using the DRAINMOD-N model to study effects of drainage system design and management on crop productivity, profitability and NO3-N losses in drainage water}, volume={35}, ISSN={["1873-2283"]}, DOI={10.1016/S0378-3774(97)00035-8}, abstractNote={The environmental impacts of agricultural drainage have become a critical issue. There is a need to design and manage drainage and related water table control systems to satisfy both crop production and water quality objectives. The model DRAINMOD-N was used to study long-term effects of drainage system design and management on crop production, profitability, and nitrogen losses in two poorly drained soils typical of eastern North Carolina (NC), USA. Simulations were conducted for a 20-yr period (1971–1990) of continuous corn production at Plymouth, NC. The design scenarios evaluated consisted of three drain depths (0.75, 1.0, and 1.25 m), ten drain spacings (10, 15, 20, 25, 30, 40, 50, 60, 80, and 100 m), and two surface conditions (0.5 and 2.5 cm depressional storage). The management treatments included conventional drainage, controlled drainage during the summer season and controlled drainage during both the summer and winter seasons. Maximum profits for both soils were predicted for a 1.25 m drain depth and poor surface drainage (2.5 cm depressional storage). The optimum spacings were 40 and 20 m for the Portsmouth and Tomotley soils, respectively. These systems however would not be optimum from the water quality perspective. If the water quality objective is of equal importance to the productivity objective, the drainage systems need to be designed and managed to reduce NO3–N losses while still providing an acceptable profit from the crop. Simulated results showed NO3–N losses can be substantially reduced by decreasing drain depth, improving surface drainage, and using controlled drainage. Within this context, NO3–N losses can be reduced by providing only the minimum subsurface drainage intensity required for production, by designing drainage systems to fit soil properties, and by using controlled drainage during periods when maximum drainage is not needed for production. The simulation results have demonstrated the applicability of DRAINMOD-N for quantifying effects of drainage design and management combinations on profits from agricultural crops and on losses of NO3–N to the environment for specific crop, soil and climatic conditions. Thus, the model can be used to guide design and management decisions for satisfying both productivity and environmental objectives and assessing the costs and benefits of alternative choices to each set of objectives.}, number={3}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Breve, MA and Skaggs, RW and Parsons, JE and Gilliam, JW}, year={1998}, month={Jan}, pages={227–243} }
 @article{breve_skaggs_parsons_gilliam_1997, title={DRAINMOD-N, a nitrogen model for artificially drained soils}, volume={40}, DOI={10.13031/2013.21359}, abstractNote={DRAINMOD-N, a quasi two-dimensional model that simulates the movement and fate of nitrogen in shallow
water table soils with artificial drainage, is described. Results of sensitivity analyses are presented and model predictions
are compared with results from VS2DNT, a more complex, two-dimensional model. The nitrogen transport component is
based on an explicit solution to the advective-dispersive-reactive (ADR) equation. Nitrate-nitrogen is the main N pool
considered. Functional relationships are used to quantify rainfall deposition, fertilizer dissolution, net mineralization,
denitrification, plant uptake, and surface runoff and subsurface drainage losses. 
A sensitivity analysis showed DRAINMOD-N predictions are most sensitive to the standard rate coefficients for
denitrification and mineralization and nitrogen content in rainfall. Simulated daily water table depths were within
0.121 m, cumulative subsurface drainage rates were within 0.016 m, and cumulative surface runoff rates were within
0.003 m, of those predicted by VS2DNT for a 250-day period. DRAINMOD-N predictions for NO3-N losses in subsurface
drainage water only differed from VS2DNT predictions by less than 2.6 kg ha–1. DRAINMOD-N predictions for
denitrification were within 8%, for plant uptake were within 15%, and for net mineralization were within 26%, of those
simulated by VS2DNT.}, number={4}, journal={Transactions of the ASAE}, author={Breve, M. A. and Skaggs, R. W. and Parsons, J. E. and Gilliam, J. W.}, year={1997}, pages={1067–1075} }
 @article{amatya_skaggs_gregory_1997, title={Evaluation of a watershed scale forest hydrologic model}, volume={32}, ISSN={["0378-3774"]}, DOI={10.1016/S0378-3774(96)01274-7}, abstractNote={A watershed scale hydrologic model (DRAINWAT) for drained forested lands was developed by coupling DRAINLOB, a field scale forestry version of DRAINMOD and the ditch and channel routing model section of FLD and STRM. The simulation model was tested with 5 years (1988–1992) of data collected on a 340 ha watershed located near Beaufort in eastern North Carolina. Testing of the model included comparison of observed and simulated daily, monthly, and annual outflows and hourly event hydrographs by three different evapotranspiration (ET) methods. Two of which (Teskey form and GS HR form) are based on the Penman-Monteith method and the third one on the Thornthwaite method. The average absolute deviation in observed and predicted daily outflows for a 5 year period was 0.94 mm day−1, when the Penman-Monteith methods were used to predict ET. The average absolute deviation in cumulative outflow when ET was predicted by the Thornthwaite method was, respectively, 23% and 50% higher compared with the values obtained with both forms of the Penman-Monteith method. Based on coefficient of determination (R2), coefficient of efficiency (E), and root mean square error (RMSE), Teskey and GS HR forms of the Penman-Monteith method performed better than the Thornthwaite method in predicting both daily and monthly outflows. However, the average daily deviations by all three methods were not significantly different at 5% level. Prediction errors in simulating monthly outflows were reduced compared with daily outflows. The predicted mean annual outflow volumes when the GS_HR and Thornthwaite methods were used for ET were in closest agreement with observed data. Statistics showed that errors resulting from use of the Thornthwaite method, with correction factors, were usually within acceptable limits given the large input data required by the Penman-Monteith ET methods. Model prediction of event hydrographs was satisfactory based on different statistical and graphical comparisons. Deviations in predicted and observed results are attributed to errors in both. Errors in the measured outflows occurred for some larger events due to weir submergence. Errors in the simulations resulted from errors in rainfall inputs, and from uncertainties in drainable porosity, hydraulic conductivity and estimates of ET due to a number of factors including approximations of leaf area index (LAI) and stomatal conductance parameters. The model performance as a whole was satisfactory given the complexity of the model, limitations of input data for the watershed, measurement errors in outflow and rainfall, and the fact that the model was not calibrated.}, number={3}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Amatya, DM and Skaggs, RW and Gregory, JD}, year={1997}, month={Mar}, pages={239–258} }
 @article{breve_skaggs_gilliam_parsons_mohammad_chescheir_evans_1997, title={Field testing of DRAINMOD-N}, volume={40}, DOI={10.13031/2013.21360}, abstractNote={This study was conducted to evaluate the performance of DRAINMOD-N, a nitrogen fate and transport modelfor artificially drained soils, based on a comparison between predicted and observed hydrologic and nitrogen variablesfor an experimental site in eastern North Carolina. The site consisted of six plots drained by subsurface drain tubes1.25 m deep and 23 m apart. Each plot was instrumented to measure water table depth, subsurface drainage, surfacerunoff and subirrigation rates. There were two replications of three water management treatments: conventionaldrainage, controlled drainage and subirrigation. Crops were winter wheat followed by soybean. Results showed themodel did a good job in describing the hydrology of the site. On average the predicted daily water table depths werewithin 0.13 m of observed during the 14-month study period. Differences between predicted and observed cumulativesubsurface drainage and surface runoff volumes were less than 0.10 and 0.09 m, respectively, for all treatments.Predictions for the movement and fate of nitrogen were also in good agreement with measured results. Simulated nitratenitrogen(NO3-N) losses in subsurface drainage water were within 1.5 kg/ha of the observed values for the 14-monthperiod. Differences between simulated and observed total NO3-N losses (subsurface drainage plus surface runoff) werewithin 3.0 kg/ha.<br><br>Results of this study indicated DRAINMOD-N could be used to simulate nitrogen losses in poorly drained soils withartificial drainage. The model, however, needs to be tested for longer periods of time and under different climaticconditions and soil types, before it can be recommended for general use.}, number={4}, journal={Transactions of the ASAE}, author={Breve, M. A. and Skaggs, R. W. and Gilliam, J. W. and Parsons, J E. and Mohammad, A. T. and Chescheir, G. M. and Evans, R. O.}, year={1997}, pages={1077–1085} }
 @article{giraud_faure_zimmer_lefeuvre_skaggs_1997, title={Hydrologic modeling of a complex wetland}, volume={123}, DOI={10.1061/(ASCE)0733-9437(1997)123:5(344)}, abstractNote={A coupled model was used to simulate the hydrology of a 22 km2 agricultural marsh characterized by a very dense channel network composed of 1,800 storage basins closely connected with 500 km of ditches. The model couples the field-scale drainage model SIDRA with the hydraulic model MAGE. The SIDRA-MAGE model simulates the hydrology of independent fields, either subsurface drained or undrained, and routes outflows from the fields through numerous basins, ditches, and canals to the outlet of the marsh. Evapotranspiration from surface water bodies colonized by aquatic plants, seepage from ditches, and management of water gates in the channel network are considered. Calibration and validation of the SIDRA, MAGE, and coupled SIDRA-MAGE models are not presented in the paper. The study focuses instead on the simulation of the hydrological effects of eight subsurface drainage schemes for a 72-h event. As the percentage of marsh drained increases from 0 to 88%, peak flow, mean flow, and total outflow volume at the marsh outlet are increased by 7, 27, and 27%, respectively. However, there is no significant changes in outflows at the marsh outlet when the percentage of marsh drained increases from 44 to 62%, even though flow from fields was predicted to increase by 8%. This is attributed, first, to a small change of the channel network efficiency, which is less than 8%, and, second, to the distribution of drained areas within the marsh. Drainage effects on the hydrology of the marsh were also simulated for summer conditions. Reduction of the channel network length and the area of basins by 72 and 84%, respectively, would reduce evapotranspiration losses from surface water bodies by 30% and save up to 11,000 m3/day of freshwater.}, number={5}, journal={Journal of Irrigation and Drainage Engineering}, author={Giraud, F. and Faure, J. B. and Zimmer, D. and Lefeuvre, J. C. and Skaggs, R. W.}, year={1997}, pages={344–353} }
 @article{amatya_skaggs_gregory_herrmann_1997, title={Hydrology of a drained forested Pocosin watershed}, volume={33}, ISSN={["0043-1370"]}, DOI={10.1111/j.1752-1688.1997.tb03530.x}, abstractNote={: In order to assess the effects. of silvicultural and drainage practices on water quality it is necessary to understand their impacts on hydrology. The hydrology of a 340 ha artificially drained forested watershed in eastern North Carolina was studied for a five-year period (1988–92). Effects of soils, beds and changes in vegetation on water table depth, evapotranspiration (ET) and drainage outflows were analyzed. Total annual outflows from the watershed varied from 29 percent of the rainfall during the driest year (1990) when mostly mature trees were present to as much as 53 percent during a year of normal rainfall (1992) after about a third of the trees were harvested. Annual ET from the watershed, calculated as the difference between annual rainfall and outflow, varied from 76 percent of the calculated potential ET for a dry year to as much as 99 percent for a wet year. Average estimated ET was 58 percent of rainfall for the five-year period. Flow rates per unit area were consistently higher from a smaller harvested block (Block B - 82 ha) of the watershed than from the watershed as a whole. This is likely due to time lags, as drainage water flows through the ditch-canal network in the watershed, and to timber harvesting of the smaller gaged block.}, number={3}, journal={JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION}, author={Amatya, DM and Skaggs, RW and Gregory, JD and Herrmann, RB}, year={1997}, month={Jun}, pages={535–546} }
 @article{skaggs_breve_gilliam_1995, title={Predicting effects of water table management on loss of nitrogen from poorly drained soils}, volume={4}, ISSN={["1873-7331"]}, DOI={10.1016/S1161-0301(14)80096-8}, abstractNote={Agricultural drainage and related water management practices affect the quality and quantity of water leaving the field and entering ground and surface waters. The design and management of drainage systems should consider these impacts, as well as effects on agricultural production. In many instances, water quality and environmental requirements have a priority greater than that of maximizing agricultural productivity and profits. This paper describes the application of a simulation model, DRAINMOD-N, to predict the effects of design and management of subsurface drainage systems on nitrogen losses and crop yields. DRAINMOD-N uses hydrologic predictions by DRAINMOD, including daily soil water fluxes, in numerical solutions to the advective-dispersive-reactive (ADR) equation to describe movement and fate of NO3-N in shallow water table soils. Simulations were conducted for maize production on a Portsmouth sandy loam soil (Thermic, Typic Umbraquult) in the North Carolina Coastal Plain. Agricultural production objectives could be satisfied with 1 m deep parallel drains spaced 40 m apart or less. Predicted losses of NO3-N were significantly affected by drainage design and management. Increasing drain spacing from 20 to 40 m decreased NO3-N losses by 47 per cent. Nitrate losses can be further reduced by placing a weir in the drainage outlet so as to raise the water level in the outlet and reduce subsurface drainge rates. This practice, called controlled drainage, can be applied in both the growing season and the nongrowing season, and can be varied in intensity (with season) by placing the weir closer to or further below the soil surface. Controlled drainage during both the growing season and winter months reduced NO3-N losses from an annual average of 21.8 kg ha−1 to 10.5 kg ha−1 (52 per cent) for a 30 m drain spacing, without reducing crop yields. Analysis of simulated results on a year-by-year basis showed that large losses of NO3-N via drainage water occur in years following droughts when crops remove little of the fertilizer N because of reduced yields. Losses to the environment under these circumstances can be reduced by increasing the intensity of drainage control to minimize subsurface drainage during the following year. In one year, for example, raising the weir in the drainage outlet to a 25 cm depth directly after harvest and holding it there until time for seedbed preparation (one month prior to planting) in the spring reduced predicted NO3-N losses by 77 per cent compared to conventional drainage and by 60 per cent compared to currently recommended controlled drainage practices. The effectiveness of intensive drainage control was not as great in other years, but reduced NO3-N losses by at least 20 per cent in the years analysed. Results of this study indicate simulation modelling can be used to design and guide the management of drainage systems to address both agricultural productivity and environmental objectives.}, number={4}, journal={EUROPEAN JOURNAL OF AGRONOMY}, author={Skaggs, RW and Breve, MA and Gilliam, JW}, year={1995}, pages={441–451} }
 @article{skaggs_breve_mohammad_parsons_gilliam_1995, title={Simulation of drainage water quality of DRAINMOD}, volume={9}, DOI={10.1007/bf00880867}, number={3}, journal={Irrigation and Drainage Systems}, author={Skaggs, R. W. and Breve, M. A. and Mohammad, A. T. and Parsons, J. E. and Gilliam, J. W.}, year={1995}, pages={259} }
 @article{skaggs_amatya_evans_parsons_1994, title={Characterization and evaluation of proposed hydrologic criteria for wetlands}, volume={49}, number={5}, journal={Journal of Soil & Water Conservation}, author={Skaggs, R. W. and Amatya, D. and Evans, R. O. and Parsons, J. E.}, year={1994}, pages={501} }
 @misc{skaggs_breve_gilliam_1994, title={HYDROLOGIC AND WATER-QUALITY IMPACTS OF AGRICULTURAL DRAINAGE}, volume={24}, ISSN={["1547-6537"]}, DOI={10.1080/10643389409388459}, abstractNote={While some of the world's most productive agriculture is on artificially drained soils, drainage is increasingly perceived as a major contributor to detrimental off‐site environmental impacts. Howe...}, number={1}, journal={CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY}, author={SKAGGS, RW and BREVE, MA and GILLIAM, JW}, year={1994}, pages={1–32} }
 @article{skaggs_nassehzadeh-tabrizi_1983, title={Optimum drainage for corn production}, number={274}, journal={Technical Bulletin (North Carolina Agricultural Experiment Station)}, author={Skaggs, R. W. and Nassehzadeh-Tabrizi, A.}, year={1983}, pages={41} }
 @inbook{skaggs_khaleel_1982, title={Infiltration}, booktitle={Hydrologic modeling of small watersheds}, publisher={St. Joseph, Mich.: American Society of Agricultural Engineers}, author={Skaggs, R. W. and Khaleel, R.}, editor={C. T. Haan, H. P. Johnson and Brakensiek, D. L.Editors}, year={1982}, pages={119} }
 @article{skaggs_nassehzadeh-tabrizi_foster_1982, title={Subsurface drainage effects on erosion}, volume={37}, number={3}, journal={Journal of Soil & Water Conservation}, author={Skaggs, R. W. and Nassehzadeh-Tabrizi, A. and Foster, G. R.}, year={1982}, pages={167} }
 @article{skaggs_gilliam_1981, title={Effect of drainage system design and operation on nitrate transport}, volume={24}, DOI={10.13031/2013.34366}, abstractNote={ABSTRACT THE computer simulation model, DRAINMOD, was modified to predict nitrate movement from artificial-ly drained soils with high water tables. Nitrate concentra-tions in surface runoff, subsurface drainage and seepage waters were assumed to be constant and independent of the drainage system design for a corn-soybean rotation on these high water table soils. Total annual nitrate outflow was determined for alternative drainage system designs and operational procedures. The results showed that trafficability and crop protection requirements can be satisfied by several different drainage system designs. For the poorly drained soil considered in this study, there was a three-fold difference in NO3-N outflow among systems that satisfied drainage objectives. The amount of nitrate that leaves the field through drainage waters can be reduced by using controlled drainage during the winter months and during the growing season. However if the controlled drainage systems are not used properly an overall increase in nitrate outflow may result.}, number={4}, journal={Transactions of the ASAE}, author={Skaggs, R. W. and Gilliam, J. W.}, year={1981}, pages={929} }
 @article{skaggs_fausey_nolte_1981, title={Water management model evaluation for North Central Ohio}, volume={24}, DOI={10.13031/2013.34365}, abstractNote={ABSTRACT DATA from a field drainage experiment in North Central Ohio were used to evaluate the reliability of the water management simulation model, DRAINMOD. Predicted and measured outflow volumes were compared for field plots with surface drainage alone, subsurface drainage alone and for combination plots with both sur-face and subsurface drainage. Comparisons were made for four replications of each treatment over a total of eight years. Inputs to the model were climatological, crop, soil property and drainage system parameter data for each treatment. Predicted and measured surface runoff and drainage volumes were in good agreement for all treatments. Devi-ations in predicted and measured volumes for individual drainage events were usually small and, in most cases, about the same magnitude as measured differences be-tween replications.}, number={4}, journal={Transactions of the ASAE}, author={Skaggs, R. W. and Fausey, N. R. and Nolte, B. H.}, year={1981}, pages={922} }
 @article{skaggs_1980, title={A water management model for artificially drained soils}, number={267}, journal={Technical Bulletin (North Carolina Agricultural Experiment Station)}, author={Skaggs, R. W.}, year={1980}, pages={54} }
 @article{skaggs_tang_1979, title={Effect of drain diameter, openings and envelopes on water table drawdown}, volume={22}, DOI={10.13031/2013.35014}, abstractNote={ABSTRACT NUMERICAL solutions to the Richards equation for saturated and unsaturated flow to parallel drains were used to determine the effects of drain tube diameter, openings and envelopes on water table draw-down. Conventional drain tubes with a finite number of openings in the walls were represented as completely open tubes with an effective radius, re. The effect of drain tube size and the use of envelopes on water table drawdown was determined for different soils, drain spacingS and profile depths. It was concluded that envelopes will allow an increase in drain spacing without reducing the design drawdown rate. However the allow-able spacing increase is smaller than previously reported in the literature. The midpoint drawdown rate was found to be relatively insensitive to changes of one or two sizes in the drain tube diameter. Finally it was shown that the effect of drain tube diameter on drainage rates is dependent on drain spacing and profile depth as well as hydraulic properties of the soil.}, number={2}, journal={Transactions of the ASAE}, author={Skaggs, R. W. and Tang, Y. K.}, year={1979}, pages={326} }
 @article{skaggs_hardjoamidjojo_wiser_1979, title={Simulation of crop response to surface and subsurface drainage systems}, number={79-2555}, journal={Paper (American Society of Agricultural Engineers)}, author={Skaggs, R. W. and Hardjoamidjojo, S. and Wiser, E. H.}, year={1979}, pages={13} }
 @article{skaggs_fausey_nolte_1979, title={Water management model evaluation for North Central Ohio}, number={79-2070}, journal={Paper (American Society of Agricultural Engineers)}, author={Skaggs, R. W. and Fausey, N. R. and Nolte, B. H.}, year={1979}, pages={25} }
 @article{skaggs_1979, title={Water movement factors important to design and operation of subirrigation systems}, number={79-2543}, journal={Paper (American Society of Agricultural Engineers)}, author={Skaggs, R. W.}, year={1979}, pages={16} }
 @book{skaggs_1978, title={A water management model for shallow water table soils}, number={134}, journal={A water management model for shallow water table soils}, publisher={Raleigh:  Water Resources Research Institute of the University of North Carolina}, author={Skaggs, R. W.}, year={1978}, pages={178} }
 @article{skaggs_1978, title={Effect of drain tube openings on water-table drawdown}, volume={104}, journal={Journal of the Irrigation and Drainage Division}, author={Skaggs, R. W.}, year={1978}, pages={13} }
 @article{skaggs_1973, title={Water table movement during subirrigation}, volume={16}, DOI={10.13031/2013.37678}, number={5}, journal={Transactions of the ASAE}, author={Skaggs, R. W.}, year={1973}, pages={988} }