USLE Universal Soil Loss Equation

1. USLEUniversal Soil Loss Equation • To guide methodical decision making in conservation planning on a site basis • To predict longtime average soil losses and runoff from specific areas in specified cropping and management systems.

2. USLEUniversal Soil Loss Equation • To enable planners to project limited erosion data to many locations and conditions not directly represented by research • For estimating average annual soil loss from sheet and rill erosion only.

3. USLEUniversal Soil Loss Equation • A = R K LS C P • A is the average annual soil loss in tons per acre • The equation can be used to estimate A or to determine the value of other parameters to meet a desired value for A

4. USLEUniversal Soil Loss Equation • A = R K LS C P • R is the rainfall factor • R = SEI • E is the Energy in the Rainfall • I is the maximum half-hour rainfall intensity for the storm. • R varies with the climate at a particular location. (See Fig. 6.4, p.94)

5. Average Annual Rainfall Factor (R)

6. Iowa NRCS 1999 USLE R-Factor 120 130 135 135 145 145 150 155 160 160 145 130 135 140 145 145 150 155 155 160 160 155 130 135 140 145 145 145 150 155 145 160 165 160 145 155 160 140 145 145 150 155 160 140 160 160 165 165 145 145 150 150 155 155 160 145 145 150 165 155 160 160 165 165 165 165 165 165 170 155 160 160 170 170 170 170 170 170 160 165 170 170 175 175 175 175 175 175 175 165 170 180 180 180 180 180 180 180 175

7. USLEUniversal Soil Loss Equation • A = R K LS C P • K is the soil erodibility factor • (tons/acre/unit of R) • K depends on the type of soil • Texture • Clay and Organic Matter Content • Structure, Permeability, Drainage

8. USLEUniversal Soil Loss Equation • A = R K LS C P • LS is the field topography factor • L is the slope length factor • S is the slope degree factor • L = 1 for a field length of 72.6 feet • S = 1 for a field slope of 9% • LS is a ratio of erosion for the given condition to erosion for the standard

10. USLEUniversal Soil Loss Equation • A = R K LS C P • C is the Cropping and management factor • C is a ratio of the erosion rate for the given condition to the erosion rate for the standard condition • The standard condition is a bare soil • All other conditions will have C<1 • C also depends on rainfall timing

11. USLE C-Factor • Continuous Fallow 1.00 • Fresh Clean-Tilled Seedbed 0.80 • Corn at Full Canopy 0.25 • Established Thick Meadow 0.004 • Established Meadow Poor Cover 0.1 • Typical Rowcrop Annual Value 0.40

12. Crop Residue C-factor

13. USLE: C-factors for CornSpring Plow (residue left)from table 6.2, page 98 • Fallow (rough plow) 0.36 • SB to 10% cover 0.60 • To 50% cover 0.52 • To 75% cover 0.41 • To harvest (90% cover) 0.24 • Harvest to Plowing (RdL) 0.30 • average 0.405

16. USLEUniversal Soil Loss Equation • A = R K LS C P • P is the factor for supporting conservation practices. • The standard condition for P is direct up-and-down the slope cultivation. • P will be less than one for all other conditions. • P depends on field slope

17. CONSERVATION PRACTICE FACTOR: P • The P-factor is the ratio of soil loss under the given condition to soil loss from up-and-down-slope farming. Therefore it is a value between 0 and 1

18. USLE P-factor

19. CONSERVATION PRACTICE FACTOR: P • On nearly level land contouring has little effect, so the ratio is 1.0 • On very steep land contouring has little effect, so the ratio is 1.0 • The greatest effect of contouring on erosion is on slopes between 3 and 8 percent.

20. USLE WHEN TERRACES ARE USED • Farming will be on the contour, so use the P-factor for contouring. • The LS-factor will reflect the terrace spacing as the length of slope. • If the concern is with off-field damages from sediment use the Sediment Delivery Factor of 0.2 as well as the contouring P-factor

21. USLE with Strip-Cropping • Use the contouring P-factor as well as the Strip-Cropping Factor

22. USLE Equation Terms

23. USLE: Example 1 • Estimate the average annual soil loss for a field located in central Iowa with a Silt Loam soil containing 2% organic matter and a 7% field slope for a 300-ft. slope length if the annual crop management factor is 0.42 and farming is parallel to the field boundaries.

24. Example 1 Solution • Solution: Use the Universal Soil Loss Equation: A = RKLSCP • From Fig. 6.4, page 94 in the text, for Central Iowa, R = 170 • From Table 6.1, page 95 in the text, for Silt Loam Soil with 2%OM, K = 0.42 t/a

25. Example 1 Solution • From Fig. 6.5, page 95, for 7% slope with 300-ft length, LS = 1.3 • Given C = 0.42 • From Table 7.1, page 108 in the text, for a 7% slope with contouring P = 0.5 and for farming up-and-down the slope P = 1.0. Parallel to field boundaries will be a mix. Use an average value of 0.8. • A = 170 x 0.42 x 1.3 x 0.42 x 0.8 = 31.19 t/a/yr.

26. USLE: Example 2 • If a change was made in Example 1 to farming on the contour, what would be the annual soil loss rate? • Solution: This would only change P in the above solution. Use the ration A2/A1 = P2/P1 • As shown above, the value of P for this condition is 0.5. Thus : • A2 = 31.19 (0.5/0.8) = 19.49 t/a/yr.

27. USLE: Example 3 • If the field in example 1 was altered by installing terraces at a 150-foot spacing, what would be the annual soil loss rate? • Solution: With terraces, the farming would be on the contour, so this solution just involves changing the topography factor, LS, from Example 2. Use the ratio A2/A1 = LS2/LS1. From Fig. 6.5, page 95 in the text, the new factor, LS2 for 7% slope and 150-foot slope length is 1.01. From Example 1, the old factor, LS1 is 1.3. Thus, the new estimate for the annual soil loss rate is: • A2 = 19.49 (1.01/1.3) = 15.14 t/a/yr.

28. USLE: Example 4 • With terraces installed some of the sediment removed from the soil surface by the erosion process will be deposited in the terrace channel. From example 3, above, estimate how much will be delivered off the field if the terraces have graded channel outlets. • Solution: . From Table 7.1, page 108 in the text, for terraces with graded channel outlets, about 80% of the eroded sediment will be trapped in the terrace channel and 20% will be delivered to the outlet ditch or stream. • Total off-site delivery = 0.2 x 15.14 = 3.03 t/a/yr.