Phosphorus management at the watershed scale: a modification of the phosphorus index

We considered hydrologic and chemical factors controlling P export from a 39.5-ha mixed land use watershed in east-central Pennsylvania, focusing our evaluation on watershed vulnerability to P loss. The spatial variations of P source factors, soil P, and P inputs from fertilizer and manure were evaluated. Distribution of Mehlich-3 soil P on a 30-m grid over the watershed showed that soil P varied with land use. Soils in wooded areas had low Mehlich-3 P (<30 mg kg(-1)); grazed pasture had Mehlich-3 P values between 100 and 200 mg kg(-1); and cropped fields receiving manure and fertilizer applications were mostly >200 mg kg(-1). Phosphorus sources and transport controls on P loss were evaluated by examining in-stream P concentrations during storm hydrographs. Phosphorus concentrations decreased 50% downstream from headwaters to watershed outlet, and were more closely related to near-stream (within 60 m) distribution of high-P soils than to that of the whole watershed. This suggests that near-stream surface runoff and soil P are controlling P export from the watershed. Based on these findings, we modified the Phosphorus Index (PI), a user-oriented tool developed by the NRCS-USDA to identify critical source areas controlling P export from agricultural watersheds. The modification separately evaluates P source and transport factors, and incorporates the hydrologic return period to describe contributing areas. The modified PI was applied to the watershed to illustrate interactions between P source and transport processes controlling P export, and approaches for managing P loss.