As phosphorus (P)-based nutrient management planning becomes necessary for some farms in Wisconsin, it will be critical to have reliable, research-based planning tools. The Wisconsin PIndex provides one method for preparing P-based nutrient management plans. The P-Index was developed largely from small plot-scale data showing the relationships between various site and management variables and runoff P losses. This study was conducted to compare runoff composition measurements at the subwatershed scale with those obtained from natural runoff at the small plot (1 m2 ) scale. We intend to use this information to refine the Wisconsin P Index estimates of P concentrations in runoff. Sediment, soluble P, and total P in natural runoff from small plots located in two subwatersheds instrumented to measure and sample runoff events over a 12-month period were compared with similar measurements from the subwatersheds. The subwatersheds, cropped with either corn or alfalfa, were located on a Tama silt loam in southwest Wisconsin. The total dissolved P relationships at the two scales of measurement were very good with the corn having an R2 value of 0.86 and the alfalfa having a R2 of 0.91. The sediment P enrichment ratios varied by crop and were similar in the small plot and subwatershed runoff. The agreement of small plot and subwatershed runoff dissolved P and sediment P concentrations supports use of small plot data in constructing the Wisconsin P index.