Model Simulation of Runoff and Nutrient Lossses from Three Adjacent Row-crop Watersheds in the Claypan Region

Watershed models have become useful and necessary to evaluate impacts of conservation management practices on non-point source pollution (NPSP), as in situ studies at the watershed scale have inherent problems such as high costs due to their large scale and complex nature, private ownership of land, and results not timely enough to avoid negative consequences of current practices (Korfmacher, 2001). The Agricultural Policy Environmental eXtender (APEX) model is a distributed, continuous, daily time-step, hydrologic/water quality model (Williams et al., 2008) and has been used to examine effectiveness of vegetative filter strips in controlling sediments and pollutants at a national scale (Arnold et al., 1998). The goal of this study was to evaluate the long-term effects of grass waterways in reducing NPSP from three adjacent small watersheds in Missouri with claypan soils under a corn/soybean rotation. Objectives were to determine the APEX sensitive parameters for crop yields, runoff, sediment, and nutrients; to calibrate and validate the APEX model for these outputs; and to use the calibrated model to quantify the reduction in NPSP from grass waterways.