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Appendix A

Appendix A: Estimating Soil Loss with the USLE

The primary method of estimating soil losses from rainfall and runoff is an empiricalequation called the Universal Soil Loss Equation (USLE). The USLE was developed by

statistical analyses of many plot-years of rainfall, runoff, and sediment loss data from many

small plots located around the country (Wischmeier and Smith, 1978). A newer version of theUSLE, called RUSLE (Revised Universal Soil Loss Equation) has been developed by Renard, et

al. (1991). RUSLE is more detailed than the USLE and, therefore, it is a computer program.

The Soil and Water Conservation Society, (SWCS, 1993) offers training courses in the use ofRUSLE at various locations around the country. For erosion control planning purposes, use of

the USLE or RUSLE will be adequate. For assistance in computed soil erosion for your location,

contact the local Natural Resources Conservation Service (NRCS).

The Universal Soil Loss Equation is:

A = RKLSCP

Where A = average annual soil loss in tons per acre per year

R = rainfall and runoff erosivity index for a given locationK = soil erodibility factor

L = slope length factor

S = slope steepness factorC= cover and management factor

P = conservation or support practice factor

The erosion index (EI) for a given storm is a product of the kinetic energy of the fallingraindrops and its maximum 30 minute intensity. The sum of these EI values over a year divided

by 100 give the annual R factor. The long-term average annual rainfall and runoff erosivity, R,factors to be used in calculations for soil loss are presented in Figure A1.

Soil erodibility is a measure of the susceptibility of a given soil to erosion by rainfall and

runoff. The properties of a soil that influence its erodibility are: soil texture, soil structure,organic matter content, and soil permeability. Soil erodibility (K) factors have been computed

by the Natural Resources Conservation Service. The soil types on a particular site can be

identified using the maps included in published soil surveys. Soil surveys are available from the

Natural Resources Conservation Service. If a soil survey for the area is not available, Figure A2can be used to obtain the K factor for the soil.

The topographic factors L and S are used to adjust the erosion rated based upon thelength and steepness of the slope. The erosivity of runoff increases with the velocity of the

runoff water. Steep slopes produce high runoff velocities. Soil loss increases with increasing

slope due to the greater volume of runoff accumulating on the longer slope lengths. The slopelength is the distance from the point of origin of the runoff to the point where the slope steepness

decreases sufficiently to cause deposition or to the point where runoff enters a well-defined

channel. Often the L and S factors are combined into a single topographic factor, LS. If the

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Appendix A

slope length and steepness are known, this combined LS factor can be determined from Figure

A3.

The cover and management factor, C, is the ratio of soil loss from land use under

specified conditions to that from continuously fallow and tilled land. The USLE was developed

for use on agricultural fields. It is adapted to use in nonagricultural conditions by appropriateselection of the C factor. This is often done by relating the land use conditions to some

agricultural situation. For example, a firing range with a grass cover might be assumed to be

similar to a pasture. Annual values of C for various cover and management conditionsapplicable to Army land uses are presented in Table A1.

Table A1 Cover management, C factors for permanent pasture, rangeland, and idle land.

__________________________________________________________________________Vegetal Canopy Cover That Contacts the Surface

Type and Height Canopy Percent Ground Cover

of Raised Canopy2 Covers3 % Type4 0 20 40 60 80 95-100

____________________________________________________________________________________________No appreciable canopy G .45 .20 .10 .042 .013 .003

W .45 .24 .15 .090 .043 .011

Canopy of tall weeds 25 G .36 .17 .09 .038 .012 .003

or short brush, W .36 .20 .13 .082 .041 .011

0.5 m (1.6 ft.) fall ht. 50 G .26 .13 .07 .035 .012 .003

W .26 .16 .11 .075 .039 .011

75 G .17 .10 .06 .031 .011 .003

W .17 .12 .09 .068 .038 .011

Appreciable brush 25 G .40 .18 .09 .040 .013 .003or bushes, W .40 .22 .14 .085 .042 .011

2 m 6.6 ft. fall ht. 50 G .34 .16 .085 .038 .012 .003W .34 .19 .13 .081 .041 .011

75 G .28 .14 .08 .036 .012 .003

W .28 .17 .12 .077 .040 .011

Trees but no appreciable, 25 G .42 .19 .10 .041 .013 .003

low brush , W .42 .23 .14 .087 .042 .011

4 m (13.1 ft.) fall ht. 50 G .39 .18 .09 .040 .013 .003

W .39 .21 .14 .085 .042 .01175 G .36 .17 .09 .039 .012 .003

W .36 .20 .13 .083 .041 .011

__________________________________________________________________________________________1All values shown assume: (1) random distribution of mulch or vegetation, and (2) mulch of appreciable depth

where it exists. Idle land refers to land with undisturbed profiles for at least a period of three consecutive years.2Average fall height of waterdrops from canopy to soil surface.3Portion of total-area surface that would be hidden from view by canopy in a vertical projection (a birdss-eye view).4G: Cover at surface is grass, grasslike plants, decaying compacted duff, or litter at least 2 inches deep. W: Cover atsurface is mostly broadleaf herbaceous plants (as weeds with little lateral-root network near the surface, and/or

undecayed residue).

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Appendix A

Figure A1. Average annual values of the rainfall erosion index.

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Appendix A

Figure A2. Soil-erodibility nomograph. Where the silt fraction does not exceed 70 percent, the

equation is 100 K = 2.1 M1.18 (104) (12 - a) + 3.25 (b - 2) + 2.5 (c - 3) where M = (percent si +vfs) (100-percent c), a = percent organic matter, b = structure code, and c = permeability class

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Appendix A

Figure A3. Topographic LS factor

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Appendix A

The conservation practice factor, P, is used to account for the positive impacts of such

agricultural management practices as planting on the contour, strip cropping, and use of terraces.Since Army lands generally not cropped, the primary conservation practice factors of interest

will be terraces. Terraces reduce the slope length, and sometimes the slope steepness that, in

turn, reduce the L and S factors in the USLE. Thus, the P factor is taken to be 1.0.

Example Application of the USLE

Consider a hypothetical watershed shown in Figure A4. Part of the watershed is in

woods and the rest of the area is part of a small arms training area. From field observations,

U.S.G.S. topographic maps, and the county soil survey the following information is obtained:

Section A: Mature forest with underbrush and undisturbed litter. Area = 13 acres. Slope length

= 540 feet. Slope steepness = 8%. Loamy sand soil.

Section B: Open range with sparse (approximately 40% surface coverage) grassy vegetation.

Area = 25 acres. Slope length = 740 feet. Slope steepness = 8%. Sandy loam soil.

Section C: Open range with sparse (approximately 40% surface coverage) grassy vegetation.

Area = 12 acres. Slope length = 610 feet. Slope steepness = 8%. Loamy coarse sand.

To compute the sediment loss under current land-use conditions. Values must be

obtained for each of the six factors in the USLE. The R factor for 350 has been selected for the

locale. From the soil survey we find the K factors for the soils to be 0.10, 0.24, and 0.15,respectively. Using Figure A5, the LS factor for Section A with a slope length of 540 feet and a

steepness of 8% is found to be 2.3. Similarly, the LS factors for Sections B and C are

determined to be 2.6 and 2.4, respectively. From Table A2 the C factor for woodland with 75-100% canopy with litter is 0.003. The C factor for grassy vegetation with no appreciable canopy

and 40% ground cover is 0.10. There are no conservation practice factors in place on these fields

presently. Therefore, the P factor for these conditions is 1.0. The soil loss for each section can

not be computed by multiplying the factors for each section. The values for the USLE factorsand the corresponding calculations of erosion are summarized in Table A3.

Table A3 Calculation of average annual erosion on hypothetical watershed.

USLE Factor Section A Section B Section C

Rainfall and runoff erosivity factor, R 350 350 350

Soil Erodibility factor, K 0.10 0.24 0.15

Toporgraphic factor, LS 2.3 2.6 2.4Cover management factor, C 0.003 0.10 0.10

Conservation practice factor, P 1.0 1.0 1.0Average annual erosion, A

(ton/acre/year)

0.24 21.8 12.6

Area of section, acres 13 25 12

Total gross erosion, tons 3 545 151

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Appendix A

Figure A4. Hypothetical watershed

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Appendix A

Virtually no erosion would be expected to occur in the mature forested area because the

canopy formed by the mature trees and understory, as well as the litter on the forest floor shieldsthe soil from the erosive energy of the falling raindrops. The soil losses from Sections B and C

are large. Some action needs to be taken to reduce the amount of the soil losses in Sections B

and C. A reasonable first step would be to replant these Sections with permanent, improvedgrasses. Assume that with improved management, i.e., fertilization and periodic mowing, a grass

density amounting to 80% surface coverage can be maintained. Then, the C factor is reduced

from 0.1 to 0.013 (Table A2). Using this value for the C factor, the annual average soil loss forsections B and C are reduced to 2.8 and 1.6 tons/acre/year. These losses are acceptable in terms

of their amount compared with the established soil loss tolerance for these soils.

Replanting these areas may not, however, completely solve erosion problems on this site.Given the long slope lengths, there exists considerable opportunity for surface runoff to

concentrate in many locations within these fields. This concentrated flow, if not carefully

managed, may undercut the vegetation and erode the soil underneath. Thus, a complete sediment

and erosion control plan for this hypothetical site might also include some land forming andsmoothing before planting, and installation of grassed waterways to carry the runoff from the

field at velocities that will not destroy the vegetation. Terraces might also be included to reducethe slope lengths and more precisely manage the flow of runoff from the site.

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