Testing the Wisconsin P Index At the Field and Watershed Scales.

The Wisconsin P Index (WPI) gives an estimate of total phosphorus (P) delivered annually from a field or pasture to nearest surface water assuming long term average weather conditions. As use of the WPI is required for nutrient management planning in Wisconsin, we have limited its inputs to data that is readily available to consultants and farmers. Comparing the annual measured P loads for 86 site years from farm fields throughout Wisconsin shows that, despite these data limitations, WPI equations based on soil test P and manure and fertilizer P application rates provide good estimates of P concentrations in runoff and eroded sediment. When measured runoff volume, sediment mass, and precipitation volumes are used in the WPI equations, the predicted P load = 0.81 measured P + 0.12 (r²=0.82, Nash-Sutcliffe = 0.82).  Sediment-bound P loads are predicted more accurately by these equations (r² = 0.86) than dissolved P loads (r² = 0.58). This difference may be in part due to the lack of measured field-specific snow water volumes that are needed for calculating the WPI dissolved P loads on those monitored fields with winter manure applications.  In contrast to the concentration equations, testing the WPI components used for estimating eroded sediment (RUSLE2) and runoff volumes with field monitoring data has proven difficult as these components use long term average weather. In addition, a paired watershed project is testing whether the targeted implementation of management practices designed to meet a WPI standard can achieve stream water quality goals in a 19-mi² agricultural watershed.