This document aims to validate the Soil Vulnerability Index (SVI) for identifying Critical Source Areas (CSAs) of pollution in two watersheds in the Chesapeake Bay region. The SVI ranks soil vulnerability based on characteristics like drainage and slope. It was applied to the Tuckahoe Creek Watershed and Greensboro Watershed, which have different soil types. Hydrologic modeling outputs from SWAT were used as a reference to validate the accuracy of the SVI classifications in each watershed. The results suggest the SVI performs differently depending on landscape characteristics, so multiple approaches should be used to identify CSAs to inform conservation practices.

1. Validation of the SVI for the Choptank
River Watershed
Sangchul Lee
Gregory McCarty
Ali M. Sadeghi
Collaborators: Claire Baffaut &
Sapana Lohani (Columbia MO)

2. Background
• Identifying Critical Source Areas (CSAs) is important to effectively
reduce pollution.
• Hydrologic models have been widely used for CSA identification,
but their use is limited depending on data availability.
• So, the Soil Vulnerability Index (SVI) was developed to rank the
soil vulnerability to surface runoff or leaching based on soil and
topographic characteristics (e.g., soil drainage, K-factor, and
slope) where modeling is limited?
Surface runoff
Potential
Hydrologic Soil Group (HSG)
A B C D
Low All acres Slope < 4 Slope < 2
Slope < 2
K-factor < 0.28
Moderate None
4 ≤ Slope ≤ 6
K-factor < 0.32
2 ≤ Slope ≤ 6
K-factor < 0.28
Slope < 2
K-factor ≥ 0.28
Moderately
High
None
4 ≤ Slope ≤ 6
K-factor ≥ 0.32
2 ≤ Slope ≤ 6
K-factor ≥ 0.28
2 ≤ Slope ≤4
High None Slope > 6 Slope > 6 Slope > 4

3. • Although the SVI is easily applicable for CSA identification, a need
exist for a more thorough validation on different landscape
characteristics to demonstrate its usability.
• The goal was to validate the accuracy of the SVI on two adjacent
watersheds with contrasting soil characteristics within the Coastal
Plain of the Chesapeake Bay Watershed (CBW).
• The SWAT outputs were used, as reference data, for SVI validation.
Objective

4. Study sites
TCW and GW dominated by well- and poorly-drained soils, respectively

5. • Was developed to predict land management impacts on hydrology
and nutrient cycle.
• Hydrologic Response Unit (HRU) is a basic modeling unit (a unique
combination of land use, soil, and slope characteristics).
 A total of 542 and 760 HRUs were delineated, and 312 and 431 HRUs
were classified as croplands (cropland HRUs) for the TCW and GW,
respectively
• 14-year annual average of nitrate fluxes transported by surface runoff
was calculated for cropland HRUs.
 All cropland HRUs were assumed to have the same crop rotations to
disregard spatial variability of nitrate fluxes by fertilization
Soil and Water Assessment Tool (SWAT)

6. Classification results for Tuckahoe Creek Watershed (TCW)

7. Classification results for Greensboro Watershed (GW)

8. • CSAs could vary by classification methods and local
hydrologic characteristics.
• So, it is imperative to apply multiple approaches for a
more reliable CSAs identification in the process of
conservation practices implementations.
Summary