Identifying Critical Portions of the Landscape for Water Quality Protection Using GIS Terrain Analysis.

Terrain attributes can be calculated using readily available digital elevation models (DEMs) and GIS software.  This study investigates the application of terrain attributes to identify different critical source areas of contaminants from the landscape.  A full suite of terrain attributes were calculated using 3 (LIDAR) or 30 meter resolution DEMs for two watersheds in south central Minnesota.  Varying thresholds were applied to different attributes, such as slope steepness (S), stream power index (SPI), compound topographic index (CTI), specific catchment area (SCA), and curvature.  Combinations of these data layers as well as combinations with ancillary GIS data, such as SSURGO map units, resulted in spatial data layers identifying critical source areas of the landscape.  Field visits aided in determining attribute thresholds and data layer combinations. Low resolution DEMs were useful for delineating critical portions of the landscape that were controlled by broad landscape patterns.  The combination of S and SPI was useful for identifying riparian areas that had a high potential for delivery of sediment and phosphorus to surface waters, while the combination of CTI and SSURGO hydrologic class was useful for identifying upland areas that had a high potential for wetland restoration. High resolution DEMs were useful for delineating critical areas that were affected by structural field management practices such as terraces, culverts, grassed waterways, side-inlets, and drop structures.  Terrain analysis using a combination of low and high resolution DEMs was a satisfactory approach to identifying critical landscape areas for water resource protection.