The Wisconsin P Index is calculated with SNAP-Plus nutrient management planning software.
SNAP-Plus includes RUSLE2 erosion calculation software.
The WI P Index can not be calculated by hand because it requires particular pieces of information from the RUSLE2 erosion calculations that are not accessible to regular RUSLE2 users. This document explains the P Index calculations . This Addendum to Wisconsin P Index Calculations explains 2015 updates and corrections, including updates to the precipitation databases used in the P Index calculation: WI P Index Precipitation 2015.
The equations for the P Index have been tested with monitoring date for Wisconsin: Testing the Wisconsin P Index at the Field Scale
Not from Wisconsin? Peter Vadas at ARS has developed a spreadsheet for estimating annual runoff phosphorus losses that uses equations similar to those in the WI P Index. It can be used anywhere.