The document summarizes the Soil Vulnerability Index (SVI), which classifies soils into four risk categories (low, moderate, moderately high, high) for soil runoff and leaching potential without conservation practices. The SVI was developed using simulations and soil properties tested against thresholds. For runoff risk, thresholds were set at <2 tons/acre for low, 2-5 tons for moderate, 5-8 tons for moderately high, and >8 tons for high. For leaching risk, thresholds were set at <50mm, 50-125mm, 125-200mm, and >200mm of percolation respectively. Hydrologic groups, soil erodibility, slope, and drainage were also

1. Allen Thompson, University of Missouri
Claire Baffaut, USDA-ARS, CSWQ Columbia, MO
Sapana Lohani, University of Missouri
SVI: How does it work?
Soil Vulnerability Index for Risk
Classification

2. Background
SVI system:
• Similar classification to Land Capability Class
• For cultivated land only
o Includes managed hayland, pastureland and known CRP uses
o Original intent for use at field scale
• Locally relevant with measureable difference between Classes
o Actual distinction between Low compared to Moderate, etc.
• Assumes no conservation practice in place

3. Three Important Driving Factors
Related to Threshold Values
• Statistically tested against output from CEAP APEX simulations
o SSURGO/gSSURGO soil properties using soil map unit components for
NRI points in Ohio and Tennessee River basins
o At least a 75% match between output from APEX classification and soil
properties
o Hydrologic Soil Group (A, B, C, D)
o Soil Erodibility (RUSLE2 K-factor)
o Slope Categories

4. Risk Categories
• Classified into Four Risk Categories
oLow
oModerate
oModerately High
oHigh

5. Thresholds for Soil Runoff Risk
• Lower threshold limit: 2 tons/ac sediment loss
(without conservation practices)
o (based on researcher’s experience as acceptable limit)
• Sediment loss < 2 tons/ac
classified as Low
• Results from Highly Erodible Lands (HEL) indicated 90% of HEL points
had sediment loss > 8 tons/ac (based on NRCS technical guides)
• Sediment loss > 8 tons/ac
 classified as High
• Breakpoint for other two classes at mid point (5 tons/ac)

6. Summary Soil Runoff Risks
Risk Category Sediment loss threshold
Low < 2 tons/ac
Moderate 2 to 5 tons/ac
Moderately High 5 to 8 tons/ac
High > 8 tons/ac

7. Thresholds for Leaching
• Considered percolation volume (PRK)
• Considered NO3-N in percolation (PRKN)
• Considered in reverse order to runoff results
(i.e. low runoff infers high percolation)
• No Practice scenario generally applied > =1.8 times more
N than crop required
Settled on percolation volume PRK under no
practice scenario. Use of N was too variable.

8. Thresholds for Leaching (cont.)
• Established High threshold of 200 mm percolation
o Used Hyd group A, slopes ≤ 2% and coarse textured soil
o Verified that 75% of those soils had > 200 mm percolation
o Nearly half of the remaining 25% soils had hay in rotation
• (thus likely reduced leaching rates)
o 200 mm percolation ~ 25% of average annual precipitation for
Upper Mississippi region

9. Thresholds for Leaching (cont.)
• Established Low threshold of 50 mm percolation
o Used Hyd group D, moderately fine and fine textured soils
(>35% clay in surface layer)
• No restrictions due to slope
o (if flat and low leaching then definitely Low)
o 85% had < 50 mm leaching
o 75% had < 30 mm leaching
• Selected 50mm leaching value
o Represents 1/4th of High threshold as was used for runoff risk
• Breakpoint for other two classes at midpoint (125 mm)

10. Summary Leaching Risks
Risk Category Percolation Threshold
Low < 50 mm
Moderate 50 to 125 mm
Moderately High 125 to 200 mm
High > 200 mm

11. Additional Factors
• Used Recursive Partitioning procedure to develop:
o K-factor breaks (soil erodibility, RUSLE2)
o Slope factor breaks
o For leaching, if soil drained, risk class increased by two
categories (no distinction between surface or subsurface)

12. Slide 12
Soil Runoff Potential
Hydrologic Group Slope K-factor Risk Class
A All All Low
B <4 All Low
B 4 to 6 <0.32 Moderate
B 4 to 6 All Moderately High
B >6 All High
C <2 All Low
C 2 to 6 <0.28 Moderate
C 2 to 6 >0.28 Moderately High
C >6 All High
D <2 <0.28 Low
D <2 >0.28 Moderate
D 2 to 4 All Moderately High
D >4 All High

13. Slide 13
Hydrologic
Group
Slope K-factor Risk Class
A <=12 all High
A >12 all Moderately High
B <3 <0.24 High
B >=3 & <=12 <0.24 Moderately High
B All other B All other B Moderate
C Moderate
D Low
Soil Leaching Potential
Note: Hyd Group C & D High risk class
if Organic Soils

14. Sites used for SVI evaluation

15. Summary
• SVI originally designed for field use
• Cultivated land only
• Assumes no conservation practice in place
• Divided into four risk classes
• Classifications for Soil Runoff and Leaching

16. Funding provided for this project by
USDA-NRCS and USDA-ARS.