Using the Annual Phosphorus Loss Estimator (APLE) Model to Evaluate the University of Maryland Phosphorus Management Tool.

Maryland’s phosphorus site index (PSI) has been used to guide management decisions to minimize the potential for P loss from agricultural fields in Maryland since the adoption of the Water Quality Improvement Act of 1998.  The PSI was recently revised and renamed the University of Maryland Phosphorus Management Tool (UM-PMT).  The most significant change was the transition from a multiplicative model to an additive component model. The original PSI summed all P source factors and all P transport factors separately, and then multiplied the summed source factors by the summed transport factors. In the UM-PMT independent components comprised of the P source and the specific P transport pathways are calculated for surface-dissolved, subsurface-dissolved, and sediment-bound P, and then all three components are summed to provide the total score. The modifications to the PSI were based on a dataset of 391 fields visited statewide where soil samples, field characteristics, and nutrient management plan data have been collected. The Annual Phosphorus Loss Estimator (APLE) model is a field-scale P loss quantification tool and has been used to assign weighting coefficients to P indices in other states, and was used to evaluate the UM-PMT and assign weighting coefficients to the sediment-bound P and surface-dissolved P components. A simulated dataset of 10,000 points across Maryland was created to perform the calibration. The dataset contained all variables required to run the PSI, UM-PMT, and APLE. Values were assigned to points using a uniform distribution and then the points were joined with county-specific soils data. Scaling coefficients were derived using this simulated dataset by regressing APLE output against UM-PMT scores for sediment-bound and surface-dissolved P loss risk. Maryland PSI, UM-PMT, and a revised UM-PMT that includes scaling coefficients were then compared using a real dataset collected from 391 field sites spread across Maryland.