Modeling the impacts of agricultural best management practices on runoff, sediment, and crop yield in an agriculture-pasture intensive watershed

Best management practices (BMPs) are commonly used to control sediment yields. In this study, we modeled the Fort Cobb Reservoir watershed located in southwestern Oklahoma, USA using the Soil and Water Assessment Tool (SWAT) and evaluated the impacts of agricultural five different BMP scenarios on surface runoff, sediment load, and crop yield. The hydrological model with 43 sub-basins and 15,217 hydrological response units was calibrated (1991 ̶ 2000) and validated (2001 ̶ 2010) against the monthly observations of streamflow, sediment grab samples, and crop-yields. The coefficient of determination (R²), Nash-Sutcliffe efficiency (NS) and percentage bias (PB) were used to determine model performance with satisfactory values of R² (0.64) and NS (0.61) in the calibration period and a good model performance (R² = 0.79; NS = 0.62) in the validation period for streamflow. We found that contouring practice reduced surface runoff by more than 18% in both conservation tillage and no-till practices for all crops. In addition, contour farming with either conservation tillage or no-till practice reduced sediment yield by almost half. Compared to the conservation tillage practice, no-till system decreased sediment yield by 25.3% and 9.0% for cotton and grain sorghum, respectively. Using wheat as cover crop for grain sorghum generated the lowest runoff followed by its rotation with canola and cotton regardless of contouring. Converting all the crops in the watershed into Bermuda grass resulted significant reduction in sediment yield (72.5-96.3%) and surface runoff (6.8-38.5%). The model was capable of providing precise information for stakeholders to prioritize ecologically sound feasible BMPs at fields that are capable of reducing overland soil erosion and sediment delivery to channels while increasing crop yield.