@article{matthieu_bowman_thapa_cassel_rufty_2011, title={Turfgrass Root Response to Subsurface Soil Compaction}, volume={42}, ISSN={["1532-2416"]}, DOI={10.1080/00103624.2011.622826}, abstractNote={Soil compaction prevents turfgrass roots from growing deep into the soil and may limit access to water and nutrients. The objective of this study was to characterize the ability of turfgrass roots to penetrate a compacted subsurface layer. Seven turfgrasses were grown in soil columns. Each column was divided into three sections with the top and bottom packed to a bulk density of 1.6 g cm−3, and the middle (treatment) layer packed to 1.6, 1.7, 1.8, 1.9, or 2.0 g cm−3. Subsurface compaction reduced root mass for two of the species, and inhibited deep root growth in all seven species, with the greatest reduction occurring between 1.7 and 1.8 g cm−3. There appears to be little difference between species in ability to penetrate compacted soils, suggesting that soil preparation and routine management practices, rather than grass selection, is the more viable way to handle soil compaction problems in turf.}, number={22}, journal={COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS}, author={Matthieu, Donald E. and Bowman, Daniel C. and Thapa, Bir B. and Cassel, D. Keith and Rufty, Thomas W.}, year={2011}, pages={2813–2823} }
 @article{arya_heitman_thapa_bowman_2010, title={Predicting Saturated Hydraulic Conductivity of Golf Course Sands from Particle-Size Distribution}, volume={74}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2009.0022}, abstractNote={This research developed a model of saturated hydraulic conductivity for golf course and athletic field media. The model was developed from saturated flow data in packed sand cores, for which a pore‐size distribution was derived from particle‐size distribution, bulk density, and measured soil water characteristic data. The pores were first assumed to form an idealized structure, consisting of non‐tortuous capillary tubes of uniform shape and size, and the Hagen–Poiseuille flow equation was applied to compute idealized saturated flow. The idealized saturated flows were compared with saturated flows derived from the measured saturated hydraulic conductivity data. Subsequently, an empirical relationship was established between the two in the form: Qt(m) = c + dQt(h–p), where Qt(m) is the saturated flow through the natural‐structure sand cores and Qt(–p) is the saturated flow through the idealized pore structure for the same core. In our study, parameters c and d had values of −1.675 and 0.308, respectively, and the r2 of the regression had a value of 0.871. The model was applied to 14 golf course sands and produced excellent results with minor anomalies.}, number={1}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Arya, Lalit M. and Heitman, J. L. and Thapa, B. B. and Bowman, D. C.}, year={2010}, pages={33–37} }
 @article{arya_bowman_thapa_cassel_2008, title={Scaling soil water characteristics of golf course and athletic field sands from particle-size distribution}, volume={72}, ISSN={["0361-5995"]}, DOI={10.2136/sssaj2006.0232}, abstractNote={The soil water characteristic (SWC) of sands is an important hydraulic parameter in designing golf courses and athletic fields. A modified version of the Arya–Paris model of the soil water characteristic was adapted to 14 golf course media that contained no to minor amounts of clay and silt. In this model, the particle‐size distribution curve is divided into a number of fractions and the natural pore length, Li(n), is scaled using the diameter of spherical particles as the length unit. The scaled pore length is given by 2Ri, where ni is the number of spherical particles in the ith fraction, 2Ri is the particle diameter, and αi is the scaling parameter, which is calculated using the relationship log= a + blogni Although the model adapted well, there were concerns about the sensitivity of predicted SWCs to uncertainties in parameters a and b Consequently, we developed and evaluated a procedure to predict Li(n) directly from straight pore lengths, Li(c) in counterpart cubic close‐packed assemblages of spherical particles, using the relationship logLi(n) = c + dlogLi(c) Predicted pressure heads using both procedures were similar with best‐fit parameters. When uncertainties were imposed on Parameters a, b and c, d, however, SWCs using the latter procedure showed far less sensitivity, as measured by the root mean square residuals (RMSRs). In addition, for sand materials grouped together on the basis of similarity in particle‐size distribution and bulk density, replacing individual best‐fit parameters by the group mean parameters did not have significant effects on predicted pressure heads.}, number={1}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Arya, Lalit A. and Bowman, Daniel C. and Thapa, Bir B. and Cassel, D. Keith}, year={2008}, pages={25–32} }
 @article{thapa_cassel_garrity_2001, title={Animal powered tillage translocated soil affects nutrient dynamics and soil properties at Claveria, Philippines}, volume={56}, number={1}, journal={Journal of Soil & Water Conservation}, author={Thapa, B. B. and Cassel, D. K. and Garrity, D. P.}, year={2001}, pages={14–21} }
 @article{thapa_garrity_cassel_mercado_2000, title={Contour grass strips and tillage affect corn production on Philippine steepland oxisols}, volume={92}, ISSN={["1435-0645"]}, DOI={10.1007/s100870050012}, number={1}, journal={AGRONOMY JOURNAL}, author={Thapa, BB and Garrity, DP and Cassel, DK and Mercado, AR}, year={2000}, pages={98–105} }
 @article{thapa_cassel_garrity_1999, title={Assessment of tillage erosion rates on steepland Oxisols in the humid tropics using granite rocks}, volume={51}, ISSN={["1879-3444"]}, DOI={10.1016/s0167-1987(99)00040-9}, abstractNote={Soil translocation by tillage may be an important factor in land degradation in the humid tropics. The objective of this study was to evaluate tillage-induced soil translocation on an Oxisol with 25% and 36% slopes in Claveria, Philippines for three tillage systems: contour moldboard plowing (CMP), moldboard plowing up and downslope (UMP), and contour ridge tillage (CRT). Small rocks 3–4 cm in “diameter” were used as soil movement detection units (SMDU). The SMDUs were placed at 10 cm intervals in a narrow 5-cm-deep trench near the upper boundary of each plot, the position of each rock recorded, and the trench backfilled. Five tillage operations used to produce one corn crop were performed during a one month period: two moldboard plowing operations for land preparation (except for CRT), one moldboard plowing for corn planting, and two inter-culture (inter-row cultivation) operations. After these operations, over 95% of the SMDU were recovered manually and their exact locations recorded. Mean annual soil flux for the 25% slope was 365 and 306 kg m−1 y−1 for UMP and CMP, respectively. For the 36% slope, comparable values were 481 and 478 kg m−1 y−1. Estimated tillage erosion rates for the 25% slope were 456 and 382 Mg ha−1 y−1 for UMP and CMP, respectively, and increased to 601 and 598 Mg ha−1 y−1, respectively, for the 36% slope. The mean displacement distance, mean annual soil flux, and mean annual tillage-induced soil loss for both slopes were reduced by approximately 70% using CRT compared to CMP and UMP.}, number={3-4}, journal={SOIL & TILLAGE RESEARCH}, author={Thapa, BB and Cassel, DK and Garrity, DP}, year={1999}, month={Aug}, pages={233–243} }
 @article{thapa_cassel_garrity_1999, title={Ridge tillage and contour natural grass barrier strips reduce tillage erosion}, volume={51}, DOI={10.1016/s0167-1987(99)00047-1}, abstractNote={Large amounts of soil are eroded annually from tilled, hilly upland soils in the humid tropics. Awareness has been increasing that much of this erosion may be due to tillage operations rather than water-induced soil movement. This field study estimated soil translocation and tillage erosion for four tillage systems on Oxisols with slope gradients of 16–22% at Claveria, Misamis Oriental, Philippines. Soil movement was estimated using `soil movement tracers' (SMT) which consisted of painted 12-mm hexagonal steel nuts. The SMT were buried in three replicate plots of the following tillage treatments: (1) contour moldboard plowing in the open field (MP-open); (2) contour ridge tillage in the open field (RT-open); (3) contour moldboard plowing plus contour natural grass barrier strips (MP-strip); and (4) contour natural grass barrier strips plus ridge tillage (RT-strip). Two hundred SMT were placed at the 5-cm depth at 5-cm spacings on 10 rows and 20 columns in two microplots within each plot. The microplots were oriented with the boundaries running downslope and along the contour of each 8-m-wide × 38-m-long (downslope) tillage plot. After tilling the land for four successive corn (Zea mays L.) crops (20 tillage operations), the SMT were manually excavated and their positions recorded. Recovery of SMT ranged from 82% to 85%. Displacement of SMT was directly related to slope length, percent slope, and tillage method. Mean displacement distance of SMT during the four corn growing seasons was 3.3 m for MP-open, 1.8 m for RT-open, 1.5 m for the RT-strip, and 2.2 m for MP-strip. Based on tillage operations associated with two corn crops per year, mean annual soil flux was estimated to be 241, 131, 158 and 112 kg m−1 for MP-open, RT-open MP-strip, and RT-strip, respectively. Compared to the mean annual soil loss for MP-open of 63 Mg ha−1, soil loss was reduced by 30%, 45%, and 53% for the MP-strip, RT-open, and RT-strip systems, respectively. Both ridge tillage and natural grass barrier strips reduced soil displacement, soil translocation flux, and tillage erosion rates.}, number={3-4}, journal={Soil & Tillage Research}, author={Thapa, B. B. and Cassel, D. K. and Garrity, D. P.}, year={1999}, pages={341–356} }