Evaluation of the Soil Vulnerability Index for artificially drained cropland across eight Conservation Effects Assessment Project watersheds

The USDA Natural Resources Conservation Service (NRCS) has proposed the Soil Vulnerability Index (SVI) as a standard tool to classify inherent soil vulnerability of cropland to loss of sediment and nutrients by runoff and leaching. The tool uses soil properties and topography, and does not consider crop management, except for the presence of artificial surface or subsurface drainage. For artificially drained cropland, SVI vulnerability to runoff remains unchanged but vulnerability to leaching is raised by two classes out of four to reflect the increased risk of nitrate (NO₃) transport.The SVI was reviewed within different contexts, but there is a need for SVI evaluation when artificial drainage is present.Thus, the objectives of this evaluation were to (1) evaluate SVI vulnerability to runoff and leaching for artificially drained cropland, and (2) propose changes to the SVI ruleset based on the findings of Objective 1. The SVI was evaluated for eight sites with artificial drainage located in regions ranging from Idaho to Maryland. Seven sites were watersheds ranging in size from 600 to 113,600 ha, with 44% to 84% cropland consisting of row crops or small grains.The eighth site consisted of six fields ranging from 7 to 30 ha in size. Consistency between SVI vulnerability, hydrologic processes that take place on the landscape, and outcomes such as crops grown were examined, using the accumulated experience and knowledge of the coauthors of this paper. Overall, SVI vulnerability to runoff and leaching was consistent with earlier research for sites with artificial subsurface drainage unless rainfall intensities were greater than they are in the Upper Mississippi and Ohio-Tennessee River basins. SVI vulnerability to leaching was greater than expected in case of surface drainage. In addition, complex soil map units can cause incorrect vulnerability classification at field scale. At the watershed or regional scale, the leaching component should be considered both with and without artificial drainage so that the causes of the vulnerability (permeable soils or artificial drainage) can be distinguished.