Estimation of Phosphorus Loss from Agricultural Land in the Heartland Region of the U.S.a. Using the Apex Model.

Accurate phosphorus (P) loss estimation from agricultural land is important for development of best management practices and protection of water quality.  The Agricultural Policy/Environmental Extender (APEX) model is a powerful simulation model designed to simulate edge-of-field water, sediment, and nutrient losses.  However, additional research is needed to determine the level of model calibration required for it to accurately simulate P loss.  The objectives of this study were to determine the accuracy of runoff, sediment, and P loss estimations with APEX using minimal parameter adjustment and full model calibration. The analysis was conducted for 18 watersheds at five sites representing a range of hydrologic conditions and including grazing, tilled row-crop, and no-till row-crop management systems. Model performance was based on the Nash Sutcliffe efficiency (NSE), the coefficient of determination (r²), and the regression slope between simulated and measured loads. Although the model performance for flow was acceptable with minimal parameter adjustment (NSE = 0.5), full calibration improved flow estimation (NSE = 0.6). Acceptable estimation of sediment and total phosphorus losses (NSE = 0.7 and 0.6, respectively) was obtained only after full calibration at each site. Future work will determine if the fully calibrated parameter sets can be used to develop a regional APEX parameterization. A regional parameterization could assist conservation and resource management agencies evaluate management impacts on P loss without requiring intensive and time-consuming calibration procedures.