Modification of the Soil Vulnerability Index to account for increased erosion risk from winter precipitation in the southern United States

The Soil Vulnerability Index for Cultivated Cropland (SVI-cc) developed by the USDA Natural Resources Conservation Service uses Soil Survey Geographic (SSURGO) soil properties to classify cultivated cropland into four levels of vulnerability to sediment and nutrient losses: Low, Moderate, Moderately High, and High. SVI-cc includes multiple components, including the runoff component, which is based on the risk of sediment from cropland being discharged into streams. Prior evaluation across 13 watersheds showed that SVI worked well. However, there was interest in determining how precipitation characteristics may influence vulnerability. The objective of this research was to evaluate the effects of rainfall seasonal characteristics on the runoff component of SVI and propose improvements to the SVI vulnerability classification, if needed. The study simulated sediment yields using calibrated models developed with the Soil and Water Assessment Tool for three Conservation Effects Assessment Project (CEAP) watersheds in Missouri, Ohio, and Pennsylvania. Sediment yields from cropland were simulated using 1985 to 2014 precipitation data from 11 CEAP watersheds spanning Vermont in the North to Texas, Georgia, and Mississippi in the South. The relationships between sediment loss and rainfall erosivity were inconclusive. However, the relationships between sediment loss and rainfall amounts showed that the risks of sediment discharge into streams increase or decrease depending on winter precipitation amounts and that this effect is more pronounced for steeper land. We propose to modify the SVI classification ruleset by increasing vulnerability by one class in regions with similar winter and summer precipitation amounts, which is characterized by low correlation (r(2) < 0.5) between monthly precipitation and erosivity. Doing so would alert conservation managers and landowners to the need for conservation practices that protect against winter erosion such as cover crops, a winter cash crop, appropriate residue management, and careful winter grazing, in addition to practices that are useful all year round such as terraces, grass waterways, and conservation rotations.