Monday, November 13, 2006 - 3:15 PM

Watershed Scale Disproportionalities in Phosphorus Loss Potentials.

David Mulla, Dept of Soil, Water & Climate, Univ of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108 and Adam Birr, Minnesota Dept of Agriculture, 2300 Silver Creek Rd, Rochester, MN 55906.

This study uses the phosphorus index to estimate the relative impact of small proportions of the landscape on phosphorus pollution of surface waters.  The Minnesota Phosphorus Index (PI) is made up of three independent pathways used to characterize P delivery to a surface water body. The three pathways include 1) sediment-bound P from rainfall runoff, 2) soluble P from rainfall runoff, and 3) soluble P from snowmelt runoff.  The Minnesota PI was evaluated on 220 fields located within two watersheds in the south-central portion of the state.  Fifty-one percent of the study area had a PI site risk rating of low or very low (1 kg/ha/yr).  These areas are not deemed to be a significant risk for phosphorus pollution of surface water.  The remaining 32, 12, and 6% of the study area had a PI site risk rating of medium (3 kg/ha/yr), high (5 kg/ha/yr), and very high (7 kg/ha/yr), respectively.  Based on these estimates, 20%, 39%, 24% or 17% of the phosphorus loads at the mouth of the watershed can be attributed to fields with a very low or low, medium, high or very high PI rating, respectively.  Of particular interest is that 18% of the watershed area generates a disproportionate 41% of the phosphorus pollution.  Based on this information, implementing best management practices on a small proportion of the landscape (18%) should lead to large reductions in phosphorus loss to the mouth of the watershed.