Performance of the Soil Vulnerability Index with respect to slope, digital elevation model resolution, and hydrologic soil group

Abstract

Soil erosion and nutrient loss from surface runoff and subsurface leaching are critical problems for cultivated land. Conservation initiatives show a persistent need for field-scale cropland vulnerability assessments to inform farm management options and prioritize efforts at watershed or regional scales. The Soil Vulnerability Index (SVI) was developed by USDA's Natural Resources Conservation Service (NRCS) to assess inherent vulnerability of cropland to surface runoff and leaching using readily available soil and topographic inputs: hydrologic soil group, slope, erodibility K-factor, coarse fragments, and organic carbon (C). The SVI has been evaluated in a few watersheds but requires further evaluation across a wider range of physiographic and climatic conditions. The objective of this study was to evaluate the ability of the SVI to correctly identify vulnerability class based on slope, digital elevation model (DEM) resolution, hydrologic soil group, and soil erodibility across 13 of USDA's Conservation Effects Assessment Project (CEAP) watersheds. The SVI classification was consistent with model output classification with a similarity rate of more than 70% when the SVI component corresponded to the primary route of loss for nutrients or sediment. Results showed that SVIs were consistent with local scientific expertise about the site vulnerability to runoff and leaching, and were particularly useful in areas with mixed slopes and hydrologic soil groups. In watersheds with uniform C or D hydrologic soil groups, the SVI was primarily driven by slope. In these cases, it was important to use a digital elevation map with 10 m resolution or higher to more finely distinguish vulnerability. In areas with uniform slopes and hydrologic soil group, and in areas with uniformly steep slopes, the SVI was not able to identify fields with greater or lower vulnerability than others. In these cases, vulnerability assessments required additional factors: depth of restrictive layer, clay content, slope length, and landscape position. While the SVI was able to categorize vulnerability correctly in mixed soil and slope conditions, findings from this project highlight the need for incorporating DEM-sourced slope and other factors like depth of restrictive layer, clay content, slope length, and landscape position into the SVI to ensure that the SVI is applicable to the broad range of geomorphic conditions found in the United States.