Validating the soil vulnerability index for a claypan watershed

Authors: CHAN, ROBERT - University Of Missouri; Baffaut, Claire; THOMPSON, ALLEN - University Of Missouri; Sadler, Edward

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2016
Publication Date: 4/3/2016
Citation: Chan, R., Baffaut, C., Thompson, A., Sadler, E.J. 2016. Validating the soil vulnerability index for a claypan watershed. Catena. 148(2):185-194. doi: 10.1016/j.catena.2016.03.009.

Interpretive Summary: Tools that indicate inherent soil vulnerability to agriculture can be used by producers, land managers, or resource managers to decide where to implement management practices or, more generally, where careful management is required for sustainable crop production. Complex computer simulation models can be used to identify these areas, but they often require additional resources to ensure correct use and interpretation of the results. The Soil Vulnerability index (SVI) was proposed by the USDA’s Natural Resources and Conservation Service to evaluate the vulnerability of soils and the risk to water quality when used for row crop agriculture. In this study, the SVI was evaluated in the Goodwater Creek Experimental Watershed (GCEW), a watershed in Missouri characterized by soils with a restrictive clay layer that increases runoff potential. Three methods were used: professional judgement, comparison to the Conductivity Claypan Index (CCI) developed specifically for these soils, and comparison to model results. Factors affecting the vulnerable areas were assessed and classified areas were compared. While the original definition of SVI included the use of a single representative slope for each soil type from the NRCS SSURGO database, slope values provided by a Digital Elevation Model (DEM) improved the usefulness of the classification. Overall, SVI, CCI and model results were consistent when all used DEM slopes. Additional testing of SVI is recommended in areas characterized by soils of different permeability and under different climates. Soil conservationists, resource conservation managers, and producers benefit from this research, which contributes to validated tools they can use.

Technical Abstract: Assessment studies of conservation efforts have shown that best management practices were not always implemented in the most vulnerable areas where they are most needed. While complex computer simulation models can be used to identify these areas, resources needed for using such models are beyond reach for most water resources managers. Soil and water conservationists need simple, spatially explicit tools such as the USDA-NRCS’s Soil Vulnerability Index (SVI) to evaluate the inherent vulnerability of soils and the risk they pose to water quality when used for row crop agriculture. In this study, the SVI was evaluated in the Goodwater Creek Experimental Watershed (GCEW), a claypan watershed in Missouri, using three methods: professional judgement, comparison to the Conductivity Claypan Index (CCI) developed specifically for claypan soils, and comparison to model results. Factors affecting the critical areas identified by each method were assessed and classified areas were compared. Slope and depth to claypan had the most variability in GCEW and were found to be influential in determining area classification by each index. While the original definition of SVI included the soil type representative slope from the USDA SSURGO database, slope values provided by a Digital Elevation Model (DEM) improved the index usefulness by classifying visibly degraded and non-degraded areas in different categories. High and moderately high vulnerability areas identified with SVI, CCI and model results with DEM slopes were consistent and matched professional judgement. Additional testing of SVI is recommended in areas characterized by soils of different permeability and under different climates.