Wisconsin's Phosphorus (P) Index is a surface-water-delivery-model P loss risk index for agricultural fields. The P Index is primarily based on relationships between field characteristics, management practices and runoff P concentrations observed in small-plot research. It is currently being verified through on-going, year-round in-field sub-watershed monitoring projects throughout the state. The monitored fields vary by crop, soil type, slope, tillage, soil test P, and manure and fertilizer applications. With twenty-one site-years of runoff monitoring on eighteen fields, there is a strong relationship (r² = 0.72) between P Index edge-of-field P loss risk estimates and measured annual P loads. In contrast, there is no relationship between soil test P values and monitored P loads for this dataset.