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.