Apex Model Simulation for Row Crop Watersheds with Agroforestry and Grass Buffers.

 Conducting field experiments to assess buffer influences on water quality are constrained by the large-scale nature of watersheds, high experimental costs, private ownership of land, and need for many years of collected data. Thus, watershed model simulation is a useful alternative to quantify these environmental benefits. The objective of this study was to use the Agricultural Policy/Environmental eXtender (APEX) model to simulate runoff, sediment and nutrient losses from row crop watersheds with agroforestry and grass. A paired watershed study under corn (Zea mays L.)-soybean [Glycine Max (L.) Merr.] rotation conducted with agroforestry buffers and contour grass buffers, has shown that buffers significantly reduce runoff, sediment and nutrient losses. The APEX model was used to simulate water, sediment and nutrient transport for these watersheds. The model was chosen not only for its uniqueness to simulate multiple sub-areas, but also for its ability to simulate the impact of filter strips on sediment and nutrient losses. Model calibration was completed using data from 1997 to 2002 and validation with data from 2003 to 2009. The calibration results were tested using coefficients of determination, Nash-Sutcliffe efficiency coefficients, and the ratio of the root mean square error to the observations standard deviation. The APEX model was found to adequately simulate average runoff, sediment and nutrient losses relative to measured values. Results of this study can be used to develop buffer guidelines to maximize environmental benefits which protect water quality by minimizing runoff, sediment, and nutrient losses from watersheds under corn-soybean management.