Review of the A nnual P hosphorus L oss E stimator tool – a new model for estimating phosphorus losses at the field scale

Models that estimate P ‐loss from agricultural land to surface waters are important tools used by soil scientists, catchment scientists and land managers, to help identify high‐risk areas and determine measures that can reduce such losses. A widely used method for predicting P‐loss from agricultural land is the ‘Phosphorus Index’ ( PI ) tool, developed by Lemunyon and Gilbert [ J ournal of P roduction A griculture (1993), 6 , 483–486], which requires relatively basic input data and provides a qualitative estimate of the risk of P ‐loss from agricultural fields. The PI delivers a single numerical score, typically expressed as a risk factor ranging from ‘low’ to ‘very high’. Across the USA , state‐wide adaptations to the original PI resulted in widely differing indices. In an attempt to reduce these interstate discrepancies, the US Department for Agriculture developed the Annual Phosphorus Loss Estimator ( APLE ; Vadas et al ., Journal of Environmental Quality (2009), 38 , 1645–1653) tool to provide a more transparent and quantitative (rather than qualitative) output for both particulate and dissolved P fractions. This short review compares the latest incarnation of the PI in the APLE tool with other P management indices and explores the various features and opportunities included in the model's inbuilt equations.