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Using a Phosphorus Loss Model to Evaluate and Improve Phosphorus Indices

Bolster, Carl H., Vadas, Peter A., Sharpley, Andrew N., Lory, John A.
Journal of Environmental Quality 2012 v.41 pp. 1758
agricultural land, agricultural management models, losses from soil, nonpoint source pollution, phosphorus, risk
In most states, the phosphorus (P) index (PI) is the adopted strategy for assessing a fi eld’s vulnerability to P loss; however, many state PIs have not been rigorously evaluated against measured P loss data to determine how well the PI assigns P loss risk—a major reason being the lack of fi eld data available for such an analysis. Given the lack of P loss data available for PI evaluation, our goal was to demonstrate how a P loss model can be used to evaluate and revise a PI using the Pennsylvania (PA) PI as an example. Our fi rst objective was to compare two diff erent formulations—multiplicative and component—for calculating a PI. Our second objective was to evaluate whether output from a P loss model can be used to improve PI weighting by calculating weights for modifi ed versions of the PA PI from model-generated P loss data. Our results indicate that several potential limitations exist with the original multiplicative index formulation and that a component formulation is more consistent with how P loss is calculated with P loss models and generally provides more accurate estimates of P loss. Moreover, using the PI weights calculated from the model-generated data noticeably improved the correlation between PI values and a large and diverse measured P loss data set. Th e approach we use here can be used with any P loss model and PI and thus can serve as a guide to assist states in evaluating and modifying their PI.