Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/11/2013
Publication Date: 4/15/2013
Citation: Senaviratne, A., Udawatta, R.P., Baffaut, C., Anderson, S.H. 2013. Agricultural Policy Environmental eXtender simulation of three adjacent row-crop watersheds in the claypan region. Journal of Environmental Quality. 42:726-736. Interpretive Summary: Scientists and soil and water managers need computer simulation tools such as the Agricultural Policy Environmental Extender (APEX) model to assess sediment and nutrient transport from agricultural fields and the effects of best management practices. APEX can simulate the effects of management practices such as tillage, fertilizer applications, and grass waterways in various parts of a field-size watershed. In this study, the model was used to simulate the long-term benefits of grass waterways for three adjacent no-till corn/soybean field-size watersheds at the Greenley Memorial Research Center, Knox County, Missouri. This area is characterized by soils that have a restrictive layer, which increases runoff and runoff transported pollutants compared to better drained soils. The model was first used to simulate crop yields, runoff, sediment, and phosphorus transport in these fields with a grass waterway and compare results to 1993-1997 data. Simulated crop yields and runoff were well predicted for the three watersheds. Phosphorus losses were well predicted for events with more than 2 inches of rain. Sediment and nitrogen predictions were not as good. Model simulations with and without the grass waterways showed that they provided a 22-36 percent reduction of phosphorus loadings over 20 years in the 3 fields. The study provides guidance on how to apply the APEX model for row crop watersheds in areas with similar soils. The results contribute to the validation of APEX across the United States and will be useful to scientists and natural resources managers in need of applying and evaluating this tool.
Technical Abstract: The Agricultural Policy Environmental Extender (APEX) model can simulate crop yields, and pollutant loadings in whole farms or small watersheds with variety of management practices. The study objectives were to identify sensitive parameters and parameterize, calibrate and validate the APEX model for three (East, Center, and West) adjacent field-size watersheds with claypan soils and no-till corn (Zea mays L.)/soybean [Glycine max (L.) Merr.] rotation, then use the model to evaluate the long-term benefits of grass waterways. ArcAPEX was used to create input files. The APEX0604 model was calibrated for daily events using measured data from the Center watershed from 1993 to 1997and validated with data from the West and East watersheds. Simulated crop yields were within ±13 percent of the measured yields of all watersheds. Daily runoff was simulated with r2 and Nash-Sutcliffe Coefficient (NSC) values ranging from 0.86-0.88 and 0.71-0.87, respectively. Goodness of fit indicators for total phosphorus (TP) simulations for > 50-mm rainfall events ranged between 0.60-0.62 and 0.50-0.58, for r2 and NSC, respectively. There were difficulties in calibrating the model for sediment and total nitrogen losses. Model simulations for average annual values for 20 years showed 22-36 percent reduction of TP loadings by the grass waterways. This study provides parameterization of the APEX model for row crop watersheds with claypan soils, with ranks of most sensitive parameters for runoff, sediment, and phosphorous losses. APEX provides a useful tool to evaluate best management practices for long-term benefits on runoff and TP losses.