Identifying critical portions of the landscape for water quality protection using GIS terrain analysis.

Abstract

Terrain attributes can be calculated using readily available digital elevation models (DEMs) and Geographic Information System (GIS) software. This study investigates the effectiveness of using terrain attributes to identify different critical source areas of contaminants on the landscape. A full suite of terrain attributes were calculated using 3 and 30-meter resolution DEMs for areas within the Le Sueur River Watershed in south central Minnesota, a watershed known to contribute disproportionate amounts of sediments and nutrients to the Minnesota River. Terrain attributes employed throughout this study included slope (S), flow accumulation (FA), compound topographic index (CTI), and stream power index (SPI). Thresholds applied to these attributes combined with ancillary GIS data, such as SSURGO soil data, resulted in spatial data layers identifying critical areas on the landscape. These critical areas accumulate flow along with associated contaminants and are hydrologically connected to nearby surface waters. 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, such as artificially drained upland depressions. High resolution DEMs were useful for delineating critical areas at the sub-field scale, such as ephemeral gullies. Terrain analysis using a combination of low and high resolution DEMs can rapidly identify critical landscape areas at various spatial scales for water resource protection, and best management practices (BMPs) can be applied to these critical areas to mitigate their detrimental effects on surface water quality.