Technical Abstract: The Wisconsin Phosphorus Index (WPI) is one of several P indexes in the US that uses equations to describe actual P loss processes and estimate field-scale dissolved P (DP) and particulate P (PP) loss risk in units that can be directly compared to measurements. The WPI calculations use soil P concentration; applied manure and fertilizer P; and estimates of average annual erosion, frozen soil runoff, and rainfall (non-winter) runoff based on long-term precipitation patterns, soil, topography, and planned management. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from average annual estimates, the WPI and measured loads were not well correlated. When measured runoff and erosion for each field year was used in the WPI field loss calculations, however, the WPI accurately ranked annual TP loads with a Nash-Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that its methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment for managing runoff P losses.