@article{liu_tian_youssef_birgand_chescheir_2022, title={Patterns of long-term variations of nitrate concentration - Stream discharge relationships for a drained agricultural watershed in Mid-western USA}, volume={614}, ISSN={["1879-2707"]}, DOI={10.1016/j.jhydrol.2022.128479}, abstractNote={Nitrate Concentration–discharge (C-Q) relationships have been used to infer nitrate sources, storage, reactions, and transport in watersheds, and to reveal key processes that control runoff chemistry. Yet, studies on long-term nitrate C-Q relationships are limited due to scarce high frequency (e.g., daily) concentration data. In this paper, using a long-term high-frequency dataset (1976–2019) comprising stream flow and nitrate concentrations, we quantitatively analyzed the long-term variations of event-scale hysteresis patterns (quantified by hysteresis index, HI, and flushing index, FI) to infer the leaching mechanisms of nitrate in an artificially drained agricultural watershed in Mid-western U.S. Our results revealed that most events exhibited anti-clockwise behaviors (HI < 0), regardless of whether nitrate was flushed or diluted during events. This means that water with high levels of nitrate-N reaches the stream network slower than water with lower nitrate concentrations. Long-term mean FI was close to zero but had strong seasonal patterns with dilution patterns observed during Winter and Summer, and flushing patterns during late Spring and Fall. The consistently negative HI values regardless of the FI value gave a strong indication of the preponderant role of the near-drain zone that usually exhibits accelerated leaching and less accumulation of nitrate in the soil profile in these drained agricultural watersheds. Both HI and FI depicted strong but opposite seasonality because of weather patterns and agricultural activities, particularly N fertilization. Overall, our findings suggest a little evidence of the role of deep groundwater and instead a strong evidence of the role of subsurface drainage as the primary pathway for nitrate transport in drained agricultural watersheds. Therefore, artificial drainage could dampen N legacy caused by the historically intensive N fertilization in drained agricultural landscapes.}, journal={JOURNAL OF HYDROLOGY}, author={Liu, Wenlong and Tian, Shiying and Youssef, Mohamed A. and Birgand, Francois P. and Chescheir, George M.}, year={2022}, month={Nov} }
 @article{amatya_tian_marion_caldwell_laseter_youssef_grace_chescheir_panda_ouyang_et al._2021, title={Estimates of Precipitation IDF Curves and Design Discharges for Road-Crossing Drainage Structures: Case Study in Four Small Forested Watersheds in the Southeastern US}, volume={26}, ISSN={["1943-5584"]}, DOI={10.1061/(ASCE)HE.1943-5584.0002052}, abstractNote={AbstractWe compared precipitation intensity-duration-frequency (PIDF) curves developed for four small forested watersheds to spatially interpolated estimates from the National Oceanic and Atmospher...}, number={4}, journal={JOURNAL OF HYDROLOGIC ENGINEERING}, author={Amatya, D. M. and Tian, S. and Marion, D. A. and Caldwell, P. and Laseter, S. and Youssef, M. A. and Grace, J. M. and Chescheir, G. M. and Panda, S. and Ouyang, Y. and et al.}, year={2021}, month={Apr} }
 @misc{liu_birgand_tian_chen_2021, title={Event-scale hysteresis metrics to reveal processes and mechanisms controlling constituent export from watersheds: A review}, volume={200}, ISSN={["1879-2448"]}, DOI={10.1016/j.watres.2021.117254}, abstractNote={Due to the increased availability of high-frequency measurements of stream chemistry provided by in situ sensors, researchers have gained more access to relationships between stream discharge and constituent concentrations (C-Q relationships) at event-scales. Existing studies reveal that event-scale C-Q relationships are mostly non-linear and exhibit temporal lags between peaks (or troughs) of hydrographs and chemographs, resulting in apparent hysteresis effects. In this paper, we summarize and introduce tools and methods in hysteresis analysis, especially the history and progresses of metrics to quantify hysteresis patterns. In addition, this paper provides a typical workflow to conduct event-scale hysteresis analysis, such as how to obtain the access to high-frequency measurements, existing methods to delineate storm events, approaches to classify and quantify hysteresis patterns, possible features/properties controlling hysteresis patterns, statistical methods to identify features at play, and strategies to deliver the inferences from hysteresis analysis. Lastly, we discuss some potential limitations that arise in the workflow and possible future work to address the challenges, including the development of advanced quantitative hysteresis metrics, generalized and standardized tools to delineate events and the integration of hysteresis analysis with numerical modeling. This paper aims to provide a critical overview of technical approaches for hysteresis analysis for researchers and hopefully foster their interests to advance our understanding of complex mechanisms in event-scale hydro-biogeochemical processes.}, journal={WATER RESEARCH}, author={Liu, Wenlong and Birgand, Francois and Tian, Shiying and Chen, Cheng}, year={2021}, month={Jul} }
 @article{liu_youssef_birgand_chescheir_tian_maxwell_2020, title={Processes and mechanisms controlling nitrate dynamics in an artificially drained field: Insights from high-frequency water quality measurements}, volume={232}, ISSN={["1873-2283"]}, DOI={10.1016/j.agwat.2020.106032}, abstractNote={Intensive agricultural activities, especially in artificially drained agricultural landscapes, generate a considerable amount of nutrient export, which has been identified as a primary cause of water quality impairment. Several management practices have been developed and installed in agricultural watersheds to reduce nutrient export, e.g. nitrate-nitrogen (NO3-N). Although published research reported considerable water quality benefits of these practices, there exist many unanswered questions regarding the inherent processes and mechanisms that control nitrate fate and transport from drained agricultural landscape. To advance our understanding of processes and mechanisms, we deployed two high-frequency sampling systems in a drained agricultural field to investigate the relationship between agricultural drainage and nitrate concentrations (C-Q relationship). Results indicated that the high-frequency measuring system was able to capture the rapidly changing C-Q relationships at the experimental site, e.g. hysteresis patterns. The 22 identified storm events exhibited anti-clockwise behavior with high variability of flushing/dilution effects. In addition, high drainage flows contributed far more nitrate loading compared with lower flows. For instance, the top 10 % of drainage flow exported more than 50 % of the nitrate lost via subsurface drainage during the monitoring period. Additionally, we observed that animal waste application was the most influential practice to change the C-Q relationship by increasing the size of soil nitrogen pools. The insights obtained from the high-frequency water quality measurements could help provide practical suggestions regarding the design and management of conservation practices, such as controlled drainage, bioreactors, and saturated buffers, to improve their nitrogen removal efficiencies. This subsequently leads to better nutrient management in drained agricultural lands.}, journal={AGRICULTURAL WATER MANAGEMENT}, author={Liu, Wenlong and Youssef, Mohamed A. and Birgand, Francois P. and Chescheir, George M. and Tian, Shiying and Maxwell, Bryan M.}, year={2020}, month={Apr} }
 @article{maxwell_birgand_schipper_christianson_tian_helmers_williams_chescheir_youssef_2019, title={Drying–Rewetting Cycles Affect Nitrate Removal Rates in Woodchip Bioreactors}, volume={48}, ISSN={0047-2425}, url={http://dx.doi.org/10.2134/jeq2018.05.0199}, DOI={10.2134/jeq2018.05.0199}, abstractNote={Woodchip bioreactors are widely used to control nitrogen export from agriculture using denitrification. There is abundant evidence that drying-rewetting (DRW) cycles can promote enhanced metabolic rates in soils. A 287-d experiment investigated the effects of weekly DRW cycles on nitrate (NO) removal in woodchip columns in the laboratory receiving constant flow of nitrated water. Columns were exposed to continuous saturation (SAT) or to weekly, 8-h drying-rewetting (8 h of aerobiosis followed by saturation) cycles (DRW). Nitrate concentrations were measured at the column outlets every 2 h using novel multiplexed sampling methods coupled to spectrophotometric analysis. Drying-rewetting columns showed greater export of total and dissolved organic carbon and increased NO removal rates. Nitrate removal rates in DRW columns increased by up to 80%, relative to SAT columns, although DRW removal rates decreased quickly within 3 d after rewetting. Increased NO removal in DRW columns continued even after 39 DRW cycles, with ∼33% higher total NO mass removed over each weekly DRW cycle. Data collected in this experiment provide strong evidence that DRW cycles can dramatically improve NO removal in woodchip bioreactors, with carbon availability being a likely driver of improved efficiency. These results have implications for hydraulic management of woodchip bioreactors and other denitrification practices.}, number={1}, journal={Journal of Environment Quality}, publisher={American Society of Agronomy}, author={Maxwell, Bryan M. and Birgand, François and Schipper, Louis A. and Christianson, Laura E. and Tian, Shiying and Helmers, Matthew J. and Williams, David J. and Chescheir, George M. and Youssef, Mohamed A.}, year={2019}, pages={93} }
 @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{maxwell_birgand_schipper_christianson_tian_helmers_williams_chescheir_youssef_2019, title={Increased Duration of Drying–Rewetting Cycles Increases Nitrate Removal in Woodchip Bioreactors}, volume={4}, ISSN={2471-9625}, url={http://dx.doi.org/10.2134/ael2019.07.0028}, DOI={10.2134/ael2019.07.0028}, abstractNote={Nitrate removal in woodchips increased linearly with drying–rewetting duration. Nitrate removal increased up to 172% in the longest drying–rewetting duration. Nitrate removal rates increased proportionally with dissolved organic C leaching.}, number={1}, journal={Agricultural & Environmental Letters}, publisher={American Society of Agronomy}, author={Maxwell, Bryan M. and Birgand, François and Schipper, Louis A. and Christianson, Laura E. and Tian, Shiying and Helmers, Matthew J. and Williams, David J. and Chescheir, George M. and Youssef, Mohamed A.}, year={2019} }
 @article{tian_fischer_chescheir_youssef_cacho_king_2018, title={Microtopography-induced transient waterlogging affects switchgrass (Alamo) growth in the lower coastal plain of North Carolina, USA}, volume={10}, ISSN={["1757-1707"]}, DOI={10.1111/gcbb.12510}, abstractNote={Very limited information is currently available on growth responses of switchgrass (lowland cultivars) to transient waterlogging in lowland or poorly drained areas. This study investigated impacts of microtopography‐induced transient waterlogging on switchgrass (Alamo cultivar) growth, represented by leaf‐level gas exchange and biomass yield, in an established experimental field located in the Atlantic coastal plain of North Carolina, USA. Intensive leaf‐level gas exchange measurements were conducted on switchgrass at paired spots with distinct elevations in three sub‐blocks. Aboveground biomass was randomly collected across the study field to explore the potential impacts of the transient waterlogging on biomass yield. The sum of excess water (SEW) was calculated based on measured instantaneous water table depth to generalize the relationship between biomass yield and intensity of transient waterlogging. Results showed significant (P ≤ 0.0001) treatment effects on leaf‐level gas exchange, characterized by evident reduction in both CO2 assimilation rate and stomatal conductance when water table was at or near the soil surface at low positions. Negative impacts of transient waterlogging on leaf‐level gas exchange became more evident with the increasing of elevation differences between paired subplots. Stomatal closure was found to be the main mechanism responsible for the decline of net assimilation under transient waterlogging. Aboveground biomass yields of switchgrass showed relatively high spatial variability and were positively and linearly correlated with microtopography (represented by elevation in the analysis) (P < 0.03, R2 > 0.77). Further analysis showed that biomass yields were negatively correlated with SEW (P < 0.001, R2 > 0.6) with an exponential relationship. Results of this study strongly demonstrated transient waterlogging could negatively affect switchgrass growth by suppressing leaf‐level gas exchange rates and ultimately reducing biomass yield. Findings from this study have critical implications for evaluating the economic viability of growing switchgrass on marginal lands that are subject to transient waterlogging stresses.}, number={8}, journal={GLOBAL CHANGE BIOLOGY BIOENERGY}, author={Tian, Shiying and Fischer, Milan and Chescheir, George M. and Youssef, Mohamed A. and Cacho, Julian F. and King, John S.}, year={2018}, month={Aug}, pages={577–591} }
 @article{tian_cacho_youssef_chescheir_fischer_nettles_king_2017, title={Switchgrass growth and pine-switchgrass interactions in established intercropping systems}, volume={9}, ISSN={["1757-1707"]}, DOI={10.1111/gcbb.12381}, abstractNote={Intercropping switchgrass (Panicum virgatum L.) with loblolly pine (Pinus taeda L.) has been proposed for producing bioenergy feedstock in the southeastern United States. This study investigated switchgrass growth and pine–switchgrass interactions at two established experimental fields (7‐year‐old Lenoir site and 5‐year‐old Carteret site) located on the coastal plain of eastern United States. Position effects (edge and center of switchgrass alley in intercropping plots) and treatment effects (intercropping vs. grass‐only) on aboveground switchgrass growth were evaluated. Interspecific interactions with respect to capturing resources (light, soil water, and nitrogen) were investigated by measuring photosynthetically active radiation (PAR) above grass canopy, soil moisture, and soil mineral nitrogen contents. Switchgrass growth was significantly (P = 0.001) affected by treatments in Lenoir and by position (P < 0.0001) in both study sites. Relative to the center, PAR above grass canopy at edge in both sites was about 48% less during the growing season. Soil water content during the growing season at the edge of grass alley was significantly (P = 0.0001) lower by 23% than at the center in Lenoir, while no significant (P = 0.42) difference was observed in Carteret, in spite of more grass growth at center at both sites. Soil mineral nitrogen content at the center of intercropping plots in Lenoir (no fertilization during 2015) was significantly (P < 0.07) lower than at the edge during the peak of growing season (June, July, and August), but not during early and late parts of growing season (May, September, and November). Position effects on soil water and mineral nitrogen were less evident under conditions with higher external inputs (rainfall and fertilization) and lower plant uptake during nongrowing seasons. Results from this study contributed to a better understanding of above‐ and belowground pine–switchgrass interactions which is necessary to properly manage this new cultivation system for bioenergy production in the southeastern United States.}, number={5}, journal={GLOBAL CHANGE BIOLOGY BIOENERGY}, author={Tian, Shiying and Cacho, Julian F. and Youssef, Mohamed A. and Chescheir, George M. and Fischer, Milan and Nettles, Jami E. and King, John S.}, year={2017}, month={May}, pages={845–857} }
 @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{tian_youssef_richards_liu_baker_liu_2016, title={Different seasonality of nitrate export from an agricultural watershed and an urbanized watershed in Midwestern USA}, volume={541}, ISSN={["1879-2707"]}, DOI={10.1016/j.jhydrol.2016.08.042}, abstractNote={Land use/land cover is a critical factor affecting temporal dynamics of nitrate export from watersheds. Based on a long-term (>30 years) water quality monitoring program in the Western Lake Erie area, United States, this study compared seasonal variation of nitrate export from an agricultural watershed and an urbanized watershed. A seasonality index was adapted to quantitatively characterize seasonal variation of nitrate export from the two watersheds. Results showed that monthly nitrate concentrations from the two watersheds exhibited different seasonal variation. Seasonality index of monthly nitrate loading for the agricultural watershed is approximately 3 times of that from the urbanized watershed and the difference is statistically significant (p < 0.0001). Meanwhile, calculated historical seasonality indexes of monthly nitrate loading for both watersheds exhibited significant (p < 0.05) decreasing trends according to the non-seasonal Mann-Kendall test. The identified differences in seasonal nitrate export from the two watersheds were mainly attributed to their distinct nitrogen sources, physical and biogeochemical settings. The declining seasonality index of monthly nitrate loading from the agricultural watershed could be partially caused by historical climate change in the study region, especially increased temperature during winter. Urbanization could be one key factor contributing to the declining seasonality index of monthly nitrate loading from the urbanized watershed. Information derived from this study have practical implications for developing proper management practices to mitigate nitrate pollution in Midwestern United States.}, journal={JOURNAL OF HYDROLOGY}, author={Tian, S. and Youssef, M. A. and Richards, R. P. and Liu, J. and Baker, D. B. and Liu, Y.}, year={2016}, month={Oct}, pages={1375–1384} }
 @article{skaggs_tian_chescheir_amatya_youssef_2016, title={Forest drainage}, DOI={10.1079/9781780646602.0124}, abstractNote={This chapter reviews the impacts of drainage on forest production and the hydrology of forested lands. It is established that drainage is used to improve access and yields on a small percentage of the world's forested lands. However, it has had a big impact on the millions of hectares on which it is applied. Drainage has increased timber yields on poorly drained peatlands and mineral soils in northern Europe, Canada and the southern USA.}, 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{golden_evenson_tian_amatya_sun_2016, title={Hydrological modelling in forested systems}, DOI={10.1079/9781780646602.0141}, abstractNote={This chapter provides a brief overview of forest hydrology modelling approaches for answering important global research and management questions. Many hundreds of hydrological models have been applied globally across multiple decades to represent and predict forest hydrological processes. The focus of this chapter is on process-based models and approaches, specifically 'forest hydrology models'; that is, physically based simulation tools that quantify compartments of the forest hydrological cycle. Physically based models can be considered those that describe the conservation of mass, momentum and/or energy.}, journal={Forest Hydrology: Processes, Management and Assessment}, author={Golden, H. E. and Evenson, G. R. and Tian, S. and Amatya, D. M. and Sun, G.}, year={2016}, pages={141–161} }
 @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 ( L.) in a loblolly pine ( 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, NH-N, and NO-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 NO-N concentration effects decreased from 0.18 to -0.09 mg L, and loads effects decreased from 0.86 to 0.49 kg ha. During the same period in site D3, the average NO-N concentration effects increased from 0.03 to 0.09 mg L, and loads effects increased from -0.26 to 1.24 kg ha. 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{tian_cacho_youssef_chescheir_nettles_2015, title={Switchgrass growth and morphological changes under established pine-grass agroforestry systems in the lower coastal plain of North Carolina, United States}, volume={83}, ISSN={["1873-2909"]}, DOI={10.1016/j.biombioe.2015.10.002}, abstractNote={Switchgrass (Panicum virgatum L.) intercropped with Loblolly pine (Pinus taeda L.) has been proposed as a potential biomass feedstock for biofuel production in the southeastern United States. This study investigated effects of treatments (intercropping vs. grass only) on biomass increment processes and morphological properties of switchgrass at two experimental plots (Lenoir1) located in the coastal plain of North Carolina. We also evaluated effects of trimming lower tree branches of pine trees on switchgrass growth at another watershed-scale site (Carteret7) in the same region. Results showed that biomass yield of intercropped switchgrass was reduced by adjacent trees and negatively affected by relative position of grass to trees at the 6th year after planting at Lenoir1. Relative grass-to-tree position was also found to be a significant (p < 0.001) factor affecting grass growth at Carteret7 site with tree age of 5 years old, which is irrespective to the trimming practice. Trimming lower tree branches did not significantly (p = 0.57) improve biomass yield of switchgrass at Carteret7. We also observed intercropped switchgrass typically had higher specific leaf area and grew taller compared to grass-only plots. Stem-to-leaf ratios of switchgrass were significantly (p = 0.02) affected by trees at Lenoir1, but not by trimming lower branches in Carteret7 and relative position of grass to trees at both study sites. Findings from this study are important for evaluating the viability of producing biofuel feedstocks using this proposed intercropping system in the southeastern United States.}, journal={BIOMASS & BIOENERGY}, author={Tian, Shiying and Cacho, Julian F. and Youssef, Mohamed A. and Chescheir, George M. and Nettles, Jami E.}, year={2015}, month={Dec}, pages={233–244} }
 @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{tian_youssef_amatya_vance_2014, title={Global sensitivity analysis of DRAINMOD-FOREST, an integrated forest ecosystem model}, volume={28}, ISSN={["1099-1085"]}, DOI={10.1002/hyp.9948}, abstractNote={Global sensitivity analysis is a useful tool to understand process‐based ecosystem models by identifying key parameters and processes controlling model predictions. This study reported a comprehensive global sensitivity analysis for DRAINMOD‐FOREST, an integrated model for simulating water, carbon (C), and nitrogen (N) cycles and plant growth in lowland forests. The analysis was carried out for multiple long‐term model predictions of hydrology, biogeochemistry, and plant growth. Results showed that long‐term mean hydrological predictions were highly sensitive to several key plant physiological parameters. Long‐term mean annual soil organic C content and mineralization rate were mainly controlled by temperature‐related parameters for soil organic matter decomposition. Mean annual forest productivity and N uptake were found to be mainly dependent upon plant production‐related parameters, including canopy quantum use efficiency and carbon use efficiency. Mean annual nitrate loss was highly sensitive to parameters controlling both hydrology and plant production, while mean annual dissolved organic nitrogen loss was controlled by parameters associated with its production and physical sorption. Parameters controlling forest production, C allocation, and specific leaf area highly affected long‐term mean annual leaf area. Results of this study could help minimize the efforts needed for calibrating DRAINMOD‐FOREST. Meanwhile, this study demonstrates the critical role of plants in regulating water, C, and N cycles in forest ecosystems and highlights the necessity of incorporating a dynamic plant growth model for comprehensively simulating hydrological and biogeochemical processes. Copyright © 2013 John Wiley & Sons, Ltd.}, number={15}, journal={HYDROLOGICAL PROCESSES}, author={Tian, Shiying and Youssef, Mohamed A. and Amatya, Devendra M. and Vance, Eric D.}, year={2014}, month={Jul}, pages={4389–4410} }
 @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={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={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 appli- cation. We review the applicability of five commonly used eco-hydrologic models (APEX, MIKESHE-DNDC, DRAIN- MOD-FOREST, REMM, and SWAT) in assessing N fate and transport in southern forest landscapes ( 50 km 2 ), although N routing below the subbasin level does not yet exist. Similarly, the dis- tributed 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 hy- drology 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 us- ing 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{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{tian_luo_jia_butalia_2010, title={Temporal variation of over-bank flooding of Wei River and its impact on a riparian wetland in Xi'an, China}, volume={24}, ISSN={["1099-1085"]}, DOI={10.1002/hyp.7591}, abstractNote={Over‐bank flooding is one of the driving forces controlling ecological integrity of riparian wetlands. Indentifying natural over‐bank flooding regime and its temporal variations is crucial for developing conservation and restoration plans and making water resources management policies for these ecosystems. Along the midstream of the Wei River in Xi'an, China lies the Jingwei riparian wetland, which was well preserved until the 1970s. Based on historical record of hydrological and morphological data of the Wei River from 1951 to 2000, we analysed temporal variations of over‐bank flooding frequency, duration, and timing in this paper. The natural annual over‐bank flooding regime was identified as having an occurrence frequency of 2·2 times a year and average duration of 5·3 days; these flooding events typically occur between June and September with occasional occurrence in late spring and late autumn. Over‐bank flooding occurrence frequency and duration decreased significantly during the 1990s, seasonal events of over‐bank floods were changed through reduced flooding frequency during summer and disappearing flooding events in late spring and late autumn. Further investigations showed that reduced discharge in the Wei River was the principal cause for these changes in over‐bank flooding dynamics. Our analysis also showed that decreased discharge of the Wei River during the 1990s was attributed near equally to disturbances from human activities and decreased regional precipitation. Results from this study may help reestablish natural over‐bank flooding dynamics in order to ensure successful restoration of Jingwei riparian wetland. Copyright © 2010 John Wiley & Sons, Ltd.}, number={10}, journal={HYDROLOGICAL PROCESSES}, author={Tian, Shiying and Luo, Wan and Jia, Zhonghua and Butalia, Rajpreet Singh}, year={2010}, month={May}, pages={1296–1307} }