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TECHNICAL REPORT
Landscape and Watershed Processes

Evaluation of the Phosphorus Index in Watersheds at the Regional Scale

Department of Soil, Water, and Climate, 1991 Upper Buford Circle, Univ. of Minnesota, St. Paul, MN 55108

* Corresponding author (dmulla{at}soils.umn.edu)

Received for publication November 27, 2000. 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.

Abbreviations: USGS, United States Geological Survey

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