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    Geographic Information Systems Lab

     

    Water Quality Risk

    The risk score for Water Quality ranges from 0-100, with larger values indicating areas that are more likely to contribute overland runoff than smaller values.  This risk was defined by two data sources: Stream Power Index and Proximity to Water

     

    Stream Power Index

     

    Stream Power Index

     

    Stream Power Index (SPI) measures the erosive power of overland flow as a function of local slope and upstream drainage area.  SPI was calculated statewide, but summarized by Terrain Zones, which represent physiographic regions of Minnesota with similar physiographic characteristics. The use of Terrain Zones removes bias from landscapes with extremely high relief.  Large SPI values (i.e. those in the 85thpercentile or higher) from each of the five terrain zones were used to create a critical area layer where overland erosion is likely to occur.  These critical SPI areas were summarized by SSURGO soil polygons: the proportion of SPI critical areas within each SSURGO polygon was used to assign a percentile rank to these polygons, the larger the proportion of critical SPI data, the larger risk score for that polygon.  This percentile rank represents 50 of the total 100 points for this risk layer. 

     

     

     

     

    Proximity to Water

     

    The remainder of points was determined by calculating proximity from SSURGO polygons to the nearest DNR 24k surface water feature (Lake or Intermittent/perennial stream).  A percentile rank of these proximity values assigned to each SSURGO polygon represents the remaining 50 points, where the highest risk scores are given to the polygons closest to water features.

     

    Proximity to Water

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