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Using a Continuous Model for Refinement of Nutrient Risk Assessment Tools

Ramirez-Avila, J. J., Osmond, D., Radcliffe, D., Bolster, C., Ortega-Achury, S. L., Oldham, L., & Sharpley, A. (2017). Using a Continuous Model for Refinement of Nutrient Risk Assessment Tools. Proceedings of the 1st World Conference on Soil and Water Conservation under Global Change-CONSOWA. Lleida, Spain. 298-302.


Implementation of nutrient management plans should protect the environment, maintain crop productivity, and increase profitability. Nutrient management planning is a complex process requiring planners knowing what resources are available and what needs are to be met. The Natural Resources Conservation Service (NRCS) 590 standard sets the national minimum standards for nutrient management in the U.S. 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. A study was performed to compare predictions from edge of field models, including APEX, against measured P loss data to determine if models could be used for refinement or replacement of P Indices in the southern U.S. Uncalibrated and calibrated APEX model predictions were compared against measured water quality data from row crop and pasture fields in four southern states. Model performance was evaluated using the Nash-Sutcliffe efficiency (NSE) and percent bias (PBIAS) with critical values of NSE &#8805; 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), respectively. Comparisons were made on an event basis. Overall, uncalibrated and calibrated APEX models predicted runoff that met the performance criteria for the event-based predictions at most sites. However, neither the uncalibrated nor the calibrated model could accurately predict sediment, DP, or TP losses. Based on the analysis of key details about the observational data and model characteristics, it was concluded that the capability of APEX to predict P losses is limited and consequently, cannot be used to refine or replace P indices in the southern U.S.

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