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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

View larger version (73K): [in a new window]   Fig. 1. River basins of Minnesota within the study area.

View larger version (40K): [in a new window]   Fig. 2. Total P monitoring data collected in Minnesota summarized by watershed. (a) Percentage of samples exceeding 0.25 mg/L total P collected from streams. (b) Median total P concentrations (µg/L) collected from lakes.

View larger version (34K): [in a new window]   Fig. 3. Modified P Index transport site characteristics included in the modified P Index for watersheds of Minnesota. (a) Potential soil erosion losses (Mg/ha). (b) Observed runoff values interpolated from 327 United States Geological Survey (USGS) long-term gauging stations (cm/yr). (c) Cropland and pastureland area within 91.4 m of drainage ditches and perennial streams normalized by the watershed area.

View larger version (34K): [in a new window]   Fig. 4. Modified P index source factors for watersheds of Minnesota. (a) Bray-1 extractable P levels (mg P/kg). (b) Fertilizer P application rate normalized by reported total fertilized land (kg P/ha). (c) Organic P application rate normalized by reported total cropland (kg P/ha).

View larger version (75K): [in a new window]   Fig. 5. Modified P Index ratings for watersheds of Minnesota.

View larger version (18K): [in a new window]   Fig. 6. Relationship between water quality data and modified P Index rating for selected watersheds of Minnesota. (a) Relationship between modified P Index rating and percentage of samples exceeding 0.25 mg/L total P for watersheds within the Upper Mississippi River, Lower Mississippi River, Minnesota River, St. Croix River, and Cedar River Basins. (b) Relationship between modified P Index rating and median total P concentrations of lakes. (c) Relationship between modified P Index rating and percentage of samples exceeding 0.25 mg/L total P for watersheds within the Red River Basin.