Evaluation of the phosphorus index in watersheds at the regional scale.

Agricultural losses of phosphorus (P) in runoff are a primary cause of eutrophication in many freshwater systems. A modified version of the P Index originally developed jointly by the USDA (Agricultural Research Service [ARS], Cooperative State Research, Education, and Extension Service [CSREES], and Natural Resources Conservation Service [NRCS]) was used to prioritize P loss vulnerability at the regional scale from 60 watersheds located within Minnesota using readily available data related to the transport and sources of P. This modified version of the P Index was created for a regionally based analysis of the index. Validation of the P Index rating was conducted using long-term water quality monitoring data consisting of total P concentrations collected from watersheds and lakes. The modified version of the P Index produced a strong correlation between P Index rating and total P stream monitoring data in watersheds (r2 = 0.70) excluding the Red River Basin. An equally strong relationship was observed between P Index rating and lake water quality (r2 = 0.68) using the modified P Index. The P Index ratings for the Red River Basin showed good correlation with observed total P stream monitoring data (r2 = 0.51); however, the P Index ratings were smaller than in other basins. The results of this study suggest that, with certain limitations, the P Index can be used at the regional scale to prioritize P loss vulnerability using state and national databases. Regional P Index ratings represent the average risk for agricultural land within the entire watershed.