Wisconsin’s cropland phosphorus loss risk assessment for nutrient management planning (P Index) has been revised to account for the probability and potential volume of runoff events following manure applications. This P Index uses a simplified modeling approach to estimate annual unit area phosphorus runoff losses for its runoff loss risk ranking.  Analysis of simulated rainfall experiments has shown that both dissolved and total phosphorus concentrations as well as phosphorus loads in the first runoff event after an unincorporated manure application were significantly related to the ratio of runoff to rainfall across trials with varying soils, field conditions, manure rates, and manure types. Because of this finding, the P Index modifications include use of a runoff to rainfall ratio for the season of a planned manure application. The ratio is calculated using long-term precipitation data and a RUSLE2-generated runoff curve number for the field following the manure application. The resulting risk assessment value ranks phosphorus loss risks appropriately when compared to runoff monitoring data from cropped fields in Wisconsin.