Evolution of a Phosphorus Loss Risk Assessment Tool.

The Phosphorus Site Index (PSI) was developed as a phosphorus (P) loss risk assessment tool for use by farm managers and conservation planners to rank agricultural fields based on relative risk for phosphorus movement from agricultural production fields to surface water. This P-loss risk assessment tool has been continuously modified since its inception in 2000 in an attempt to increase the relative precision and accuracy. In 2012, major modifications were made to the PSI calculation, including a shift from a multiplicative model that produced a single average risk assessment for a given production field to a component model structure that generated field specific P-loss risk potential for three primary P-loss pathways. The resulting updated P-loss risk assessment tool was renamed the University of Maryland Phosphorus Management Tool (UM-PMT). The Annual P Loss Estimator (APLE), a validated field-scale P loss quantification model that runs on an annual time step, was used to evaluate the UM-PMT for accuracy. Modifications were made based on the evaluation and the final assessment tool was named the Phosphorus Management Tool 2 (PMT-2). Final score for PMT-2 correlated better with total P loss estimated by APLE than final score for UM-PMT. In a dataset of 382 agricultural fields sampled in Maryland, the PSI categorized most fields as low risk for P loss and the UM-PMT categorized most fields as high risk for P loss. The PMT-2 was an intermediate between PSI and UM-PMT for categorization of risk of P loss. Particulate P loss comprised a greater percentage of PMT-2 final score than UM-PMT final score.