Appendix A
1
Appendix A: Estimating Soil Loss with the USLE
The primary method of estimating soil losses from rainfall and runoff is an empirical
equation 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 A newer version of the
USLE, 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 of
RUSLE 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 location
K = soil erodibility factor
L = slope length factor
S = slope steepness factor
C= 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 falling
raindrops 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 A2
can 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 the
length 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 slope
length 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
Appendix A
2
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 appropriate
selection 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 conditions
applicable 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
or short brush,
0.5 m (1.6 ft.) fall ht.
25
50
75
Appreciable brush
or bushes,
2 m 6.6 ft. fall ht.
25
50
75
Trees but no appreciable,
low brush ,
4 m (13.1 ft.) fall ht.
25
G
W
G
W
G
W
.36
.36
.26
.26
.17
.17
.17
.20
.13
.16
.10
.12
.09
.13
.07
.11
.06
.09
.038
.082
.035
.075
.031
.068
.012
.041
.012
.039
.011
.038
.003
.011
.003
.011
.003
.011
G
W
G
W
G
W
.40
.40
.34
.34
.28
.28
.18
.22
.16
.19
.14
.17
.09
.14
.085
.13
.08
.12
.040
.085
.038
.081
.036
.077
.013
.042
.012
.041
.012
.040
.003
.011
.003
.011
.003
.011
G
.42
.19
.10
.041
.013
.003
W
.42
.23
.14
.087
.042
.011
50
G
.39
.18
.09
.040
.013
.003
W
.39
.21
.14
.085
.042
.011
75
G
.36
.17
.09
.039
.012
.003
W
.36
.20
.13
.083
.041
.011
__________________________________________________________________________________________
1
All 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.
2
Average fall height of waterdrops from canopy to soil surface.
3
Portion of total-area surface that would be hidden from view by canopy in a vertical projection (a birds’s-eye view).
4
G: Cover at surface is grass, grasslike plants, decaying compacted duff, or litter at least 2 inches deep. W: Cover at
surface is mostly broadleaf herbaceous plants (as weeds with little lateral-root network near the surface, and/or
undecayed residue).
Appendix A
3
Figure A1. Average annual values of the rainfall erosion index.
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
4
Appendix A
5
Figure A3. Topographic LS factor
Appendix A
6
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 75100% 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 factors
and the corresponding calculations of erosion are summarized in Table A3.
Table A3 Calculation of average annual erosion on hypothetical watershed.
USLE Factor
Rainfall and runoff erosivity factor, R
Soil Erodibility factor, K
Toporgraphic factor, LS
Cover management factor, C
Conservation practice factor, P
Average annual erosion, A
(ton/acre/year)
Area of section, acres
Total gross erosion, tons
Section A
350
0.10
2.3
0.003
1.0
0.24
Section B
350
0.24
2.6
0.10
1.0
21.8
Section C
350
0.15
2.4
0.10
1.0
12.6
13
3
25
545
12
151
Appendix A
7
Figure A4. Hypothetical watershed
Appendix A
8
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 shields
the 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, improved
grasses. 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 for
sections 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 and
smoothing 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 reduce
the slope lengths and more precisely manage the flow of runoff from the site.