As phosphorus (P)-based nutrient management planning becomes necessary for some farms in Wisconsin, it will be critical to have research-based planning tools. The Wisconsin P-Index 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 management variables and runoff P losses. Little information is available on how small plot runoff data compares with field or sub-watershed scale measurements. This study was conducted to compare runoff composition measurements at the subwatershed scale with those obtained from natural runoff and rainfall simulations at the small plot (1 m²) scale. We intend to use this information to validate the Wisconsin P Index. Sediment, soluble P, and total P concentrations and volumes in natural and simulated rain 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 [Zea mays] (7.2 ha) or alfalfa [Medicago sativa] (12 ha), were located on a Tama silt loam (fine-silty, mixed, superactive, mesic Typic Argiudoll) in southwest Wisconsin. In spring 2004 rain simulations, runoff volumes were much higher in corn compared with the alfalfa cropping system, but the reverse was true in fall 2004. Subwatershed and small plot natural runoff sediment, soluble P, and total P concentrations are within an order of magnitude or less, despite large volume differences.