Evaluation of the APEX model to simulate runoff quality from agricultural fields in the southern region of the United States

Authors: RAMIREZ-AVILA, JOHN - Mississippi State University; OSMOND, DEANNA - North Carolina State University; RADCLIFFE, DAVID - University Of Georgia; Bolster, Carl; ORTEGA-ACHURY, SANDRA - Mississippi State University; FORSBERG, ADAM - University Of Georgia; SHARPLEY, ANDREW - University Of Arkansas; OLDHAM, LARRY - Mississippi State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2017
Publication Date: 11/16/2017
Citation: Ramirez-Avila, J.J., Osmond, D., Radcliffe, D., Bolster, C.H., Ortega-Achury, S.L., Forsberg, A., Sharpley, A.N., Oldham, L. 2017. Evaluation of the APEX model to simulate runoff quality from agricultural fields in the southern region of the United States. Journal of Environmental Quality. 46:1357-1364. https://doi.org/10.2134/jeq2017.07.0258.
DOI: https://doi.org/10.2134/jeq2017.07.0258

Interpretive Summary: The phosphorus (P) Index (PI) is the risk assessment tool approved in the NRCS 590 standard used to target critical source areas and practices to reduce P losses. A recent revision of the 590 standard suggested using the Agricultural Policy/Environmental eXtender (APEX) model in place of the phosphorus index. In this study we first evaluated the accuracy of the APEX model by comparing model predictions with measured field-scale phosphorus losses collected at research locations throughout the South. Comparisons were made on an event basis and using long-term 25-yr simulations. Overall, the APEX model predicted runoff that met the performance criteria for both the event-based and long-term predictions at most sites. However, model predictions of dissolved and total P loss were poor. These results highlight the limitation of the APEX model for use in nutrient management planning and underscore the need for additional data collection and model development.

Technical Abstract: The phosphorus (P) Index (PI) is the risk assessment tool approved in the NRCS 590 standard used to target critical source areas and practices to reduce P losses. A revision of the 590 standard, suggested using the Agricultural Policy/Environmental eXtender (APEX) model to assess the risk of nitrogen and P loss. We compared uncalibrated and calibrated APEX model predictions against measured water quality data from row crop fields in North Carolina and Mississippi, and pasture fields in Arkansas and Georgia. Model performance was evaluated using the Nash Sutcliffe efficiency (NSE) and percent bias (PBIAS) with critical values of NSE = 0.30 and absolute value of PBIAS < 0.35, 0.6, 0.7, and 0.7 for runoff, sediment, dissolved P (DP) and total P (TP). Comparisons were made on an event basis and using long-term 25-yr simulations. Overall, both the uncalibrated and calibrated APEX models predicted runoff that met the performance criteria for both the event-based and long-term predictions at most sites. However, neither the uncalibrated nor the calibrated model could simulate sediment, DP, or TP losses. APEX tended to underpredict P losses from fields where manure was surface applied and this may have been due to the lack of a surface manure pool for P that was separate from the soil surface layer pool. The APEX model’s capability to predict P losses is limited and consequently, so is the potential for using APEX to refine or replace P indices in the southern region.