Using the Apex Model to Support Refinement and Regionalization of Southern P Assessment Tools.

A multistate research program has the objective to coordinate and advance phosphorus (P) management in the region by ensuring that most southern P assessment tools have been tested, based on guidance in the 2011 NRCS 590 standard, and compared to water quality data. Research objectives include to compare predictions of P-Index assessment tools against fate and transport water quality models (APEX, APLE and TBET) for both calibrated and uncalibrated model conditions and the use of water quality data (monitored or predicted by model) to guide refinement of P Indices. Results are expected to show the assessment for P loss vulnerability estimated by different southern P Indices and the performance of the evaluated models before and after calibration and validation procedures for the proposed scenarios.

Field testing of the Agricultural Policy/Environmental eXtender (APEX) model has been performed using data for runoff, sediment and P losses from small watersheds (6.8-26 ha) and plots (0.02-0.8 ha) in 8 generalized physiographic regions in 5 southern states. Initial uncalibrated predictions of APEX evidenced that model’s default settings underestimated observed values of runoff, sediment and P losses. Preliminary calibrated parameters have been observed to fall into ranges proposed in other APEX simulation studies. Estimated model efficiency coefficients ((R²>0.7) and Nash-Sutcliffe (NSE> 0.42)) values are representative for very accurate model predictions. However, P and sediment loads predicted during irrigation events could importantly affect model efficiency (R²>0.47 and NSE>0.35). In addition to accurate water quantity and quality prediction, APEX has been able to predict the effects of the different agricultural practices on physical and chemical soil properties (e.g. pH, soil P).