SECTION 1 GENERAL RESOURCE REFERENCES
4. Erosion Prediction - Revised Universal Soil Loss Equation (RUSLE)
The Revised Universal Soil Loss Equation (RUSLE) is an easily and widely used computer program that estimates rates of soil erosion caused by rainfall and associated overland flow.
RUSLE is used by numerous government agencies and private organizations and individuals to assess the degree of rill and inter-rill erosion, identify situations where erosion is serious, and guide development of conservation plans to control erosion. RUSLE has been applied to cropland, rangeland, disturbed forest lands, landfills, construction sites, mining sites, reclaimed lands, military training lands, parks, land disposal of waste, and other land uses where mineral soil material is exposed to the erosive forces of raindrop impact and overland flow.
RUSLE has been developed and is maintained by the USDA-Agricultural Research Service (ARS) in cooperation with the USDA-Natural Resources Conservation Service (NRCS), USDI-Office of Surface Mining, Reclamation, and Regulation, USDI-Bureau of Land Management, Soil and Water Conservation Society, University of Tennessee, Purdue University, and University of Minnesota. Other users include the Department of Defense, U.S. Environmental Protection Agency, U.S. Department of Energy, USDA Forest Service, state agencies regulating land fills, surface mine companies, commercial firms that develop and retail erosion control products, private consultants that develop conservation plans and teach erosion control technology, and university faculty who teach RUSLE in the classroom. RUSLE is used in numerous foreign countries as well.
In the United States, the NRCS is the principal user of RUSLE and has implemented RUSLE in most of its local field offices. The NRCS is the major source for data needed to apply RUSLE and is the leading authority on field application of RUSLE.
This web-site: http://www.sedlab.olemiss.edu/Rusle/ is the "official" USDA-Agricultural Research Service (ARS) site for RUSLE. A copy of the current version of RUSLE, version 1.06b (Jan. 19, 2001), and associated data files can be downloaded from this site. Information is also provided on how to access RUSLE documentation, application guides for RUSLE, and help contacts. The current 1.06b version contains a calculation change for the soil moisture (SM) sub-factor of the cover-management (C) factor. This change brings the SM calculation in line with that in the upcoming release of RUSLE 2.
RUSLE uses a particular set of definitions, partly because the disciplines involved in soil erosion have not developed a standard set of definitions. Observance of RUSLE definitions is critical to getting accurate results.
RUSLE estimates average annual soil loss, expressed as mass per unit area per year, which is defined as the amount of sediment delivered from the slope length assumed in the RUSLE computation. RUSLE uses U. S., customary units and computes soil loss in units of tons/acre/year, which is the sediment load at the end of the slope length divided by the slope length. In that context, RUSLE is a sediment yield equation that describes sediment yield at the end of the RUSLE slope length.
The RUSLE slope length is defined according to the problem being addressed. The typical application for RUSLE is development of a conservation plan to protect the eroding portion of a landscape from being excessively degraded by soil erosion, that is, to protect the soil as a resource. In this application, slope length is defined as the distance from the origin of overland flow along the flow path to the point where deposition begins to occur on concave slopes or to a concentrated flow channel. In some cases, the slope can flatten to cause deposition and then steepen so that erosion occurs on the lower portion of the slope. Slope length passes through the depositional area when soil loss is being estimated on the lower portion of this slope.
Another application of RUSLE is to estimate the amount of sediment leaving a landscape that may cause off-site damages such as sedimentation in a road ditch. In this case, the slope length is the distance from the origin of overland flow through depositional overland flow areas to the first "concentrated flow" area that collects the overland flow to the point that the runoff can no longer be considered overland flow. Consideration outside of RUSLE must be given to deposition that occurs in concentrated flow areas, except terrace and diversion channels that are considered by RUSLE, to fully estimate sediment yield from a landscape area.
RUSLE also computes soil loss for individual slope segments. These soil loss values represent net sediment production for those segments, which is the net between detachment and deposition within the segment.
Detachment is the removal of soil particles from the soil mass, which adds sediment to the sediment load being transported downslope. Deposition is the transfer of sediment from the sediment load back to the soil mass. Local deposition is the deposition of sediment very near to the point where the sediment was detached. Deposition of sediment eroded from soil clods in nearby depressions formed by the clods is an example of local deposition. Remote deposition is the deposition of sediment far from its point of origin such as deposition in a terrace channel or on the toe of a concave slope.
Sediment load is a measure of the amount of sediment being transported downslope. Sediment yield, as used by RUSLE, is the sediment load at the end of the slope length, at the outlet of terrace diversion channels, or sediment basins that are considered by RUSLE.
RUSLE is an index method having factors that represent how climate, soil, topography, and land use affect rill and interrill soil erosion caused by raindrop impact and surface runoff. In general, erosion depends on the amount and intensity of rainfall and runoff, protection provided to the soil by land use against the direct forces of raindrop impact and surface runoff, susceptibility of soil to erosion as a function of intrinsic soil properties and soil properties modified by land use, and the topography of the landscape as described by slope length, steepness, and shape.
These influences are described in RUSLE with the equation:
A = R K L S C P
where: A = average annual soil loss, K = soil erodibility factor, L = slope length factor, S = slope steepness factor, C = cover-management factor, and P = supporting practices factor. A soil loss (erosion rate) in tons per acre per year is computed by substituting values for each RUSLE factor to represent conditions at a specific site.
RUSLE is a "lumped" process-type model based on the analysis of a large mass of experimental data and equations based on fundamental erosion processes where experimental data are inadequate to define RUSLE factor values. Rather than explicitly representing the fundamental processes of detachment, deposition, and transport by rainfall and runoff, RUSLE represents the effects of these processes on soil loss.
The product RK in RUSLE is an estimate of soil loss from unit plot conditions. These two factors have dimensions and units, whereas the other RUSLE factor are dimensionless relative to unit plot conditions. A unit plot is 72.6 ft long on a 9 percent steepness, is maintained in continuous fallow, is tilled up and down hill according to a particular sequence of operations much like those used in clean-tilled row crops, and is cultivated periodically to break the crust that forms from rainfall and to control weeds. The soil surface is left relatively smooth and free of ridges after the last tillage operation in the sequence.
R factor:The R factor represents the erosivity of the climate at a particular location. An average annual value of R is determined from historical weather records and is the average annual sum of the erosivity of individual storms. The erosivity of an individual storm is computed as the product of the storm's total energy, which is closely related to storm amount, and the storms's maximum 30 minute intensity. Erosivity range from less than 8 (US customary units) in the western US to about 700 for New Orleans,. All other factors being the same, soil loss is 100 times greater at New Orleans, Louisiana than at Las Vegas, Nevada.
Maps of R values have been computed from historical weather records and have been plotted onto maps and placed in databases used by RUSLE.
K factor:The K factor is an empirical measure of soil erodibility as affected by intrinsic soil properties. Erosion measurements based on unit plot conditions are used to experimentally determine values for K.
The factor K is a measure of soil erodibility under this standard condition. Land use, such as incorporation of organic material into the soil, affects soil erodibility, but such effects are considered in the C factor. The K factor is influenced by the detachability of the soil, infiltration and runoff, and the transportability of the sediment eroded from the soil.
The main soil properties affecting K are soil texture, including the amount of fine sand in addition to the usual sand, silt, and clay percentage used to describe soil texture, organic matter, structure, and permeability of the soil profile. In general terms, clay soils have a low K value because theses soils are resistant to detachment. Sandy soils have low K values because these soils have high infiltration rates and reduced runoff, and sediment eroded from these soils is not easily transported. Silt loam soils have moderate to high K values because soil particles are moderate to easily detached, infiltration is moderate to low producing moderate to high runoff, and the sediment is moderate to easily transported. Silt soils have the highest K values because these soils readily crust producing high runoff rates and amounts. Also, soil particles are easily detached from these soils, and the resulting sediment is easily transported.
This mixture of effects illustrates that K is empirical. It is not a soil property, but is defined by RUSLE definitions. The definition for K, and for all RUSLE factors as well, must be carefully observed to achieve accurate results. For example, using K to account for reduced soil loss from incorporation of manure is not proper and produces incorrect results.
LS factor:The L and S factors jointly represent the effect of slope length, steepness, and shape on sediment production. RUSLE represents the combined effects of rill and interrill erosion. Rill erosion is primarily caused by surface runoff and increases in a downslope direction because runoff increases in a downslope direction. Interrill erosion is caused primarily by raindrop impact and is uniform along a slope. Therefore, the L factor is greater for those conditions where rill erosion tends to be greater than interrill erosion.
Erosion increases with slope steepness, but in contrast to the L factor for the effects of slope length, RUSLE makes no differentiation between rill and interrill erosion in the S factor that computes the effect of slope steepness on soil loss.
Slope shape is a variation of slope steepness along the slope. Slope steepness and position along the slope interact to greatly affect erosion. Soil loss is greatest for convex slopes that are steep near the end of the slope length where runoff rate is greatest and least for concave slopes where the steep section is at upper end of the slope where runoff rate is least.
The LS factor is a measure of sediment production. Deposition can occur on concave slopes where transport capacity of the runoff is reduced as the slope flattens. This deposition and its effect on sediment yield from the slope is considered in the supporting practices P factor.
C factor:The C factor for the effects of cover-management, along with the P factor, is one of the most important factors in RUSLE because it represents the effect of land use on erosion. It is the single factor most easily changed and is the factor most often considered in developing a conservation plan. For example, the C factor describes the effects of differences between vegetation communities, tillage systems, and addition of mulches.
The C factor is influenced by canopy (cover above but not in contact with the soil surface), ground cover (cover directly in contact with the soil surface), surface roughness, time since last mechanical disturbance, amount of live and dead roots in the soil, and organic material that has been incorporated into the soil. These variable change through the year as plants grow and senesce, the soil is disturbed, material is added to the soil surface, and plant material is removed. The C factor is an average annual value for soil loss ratio, weighted according to the variation of rainfall erosivity over the year.
The average annual distribution of erosivity during a year varies greatly with location. In the US, erosivity is nearly uniform throughout the year in the mid-south region, is concentrated in the late spring in the western cornbelt, and is concentrated in late fall and early winter in the Pacific coast region.
Soil loss ratio is the ratio of soil loss from a given land use to that from the unit plot at a given time. RUSLE computes soil loss ratio values as they change through time with each half month period using equations for subfactors related to canopy, ground cover, roughness of the soil surface, time since last mechanical disturbance, amount of live and dead roots in the upper soil layer, amount of organic material incorporated into the soil, and antecedent soil moisture in the Northwest Wheat and Range Region.
P factor:The supporting practice P factor describes the effects of practices such as contouring, strip cropping, concave slopes, terraces, sediment basins, grass hedges, silt fences, straw bales, and subsurface drainage. These practices are applied to support the basic cultural practices used to control erosion, such as vegetation, management system, and mulch additions that are represented by the C factor.
Supporting practices typically affect erosion by redirecting runoff around the slope so that it has less erosivity or slowing down the runoff to cause deposition such as concave slopes or barriers like vegetative strips and terraces. The major factors considered in estimating a P factor value include runoff rate as a function of location, soil, and management practice; erosivity and transport capacity of the runoff as affected by slope steepness and hydraulic roughness of the surface; and sediment size and density.
Version 1.06b is the current version of RUSLE being released by the USDA-Agricultural Research Service (ARS) for general use. It, along with data files, can be downloaded from this web site.
RUSLE was first released for widespread use in late 1992 as version 1.02. Improved versions of RUSLE were periodically released to correct errors and to give RUSLE increased capability. Previous versions of RUSLE were released for fee by the Soil and Water Conservation Society (SWCS) through a Cooperative Research and Development Agreement with ARS that gave the SWCS a copyright on RUSLE. That agreement expired in 1996. The last version of RUSLE covered by that agreement was RUSLE1.05. Version 1.06b is not covered by the copyright and can be freely downloaded and used without payment of a fee.
Version 1.04 was the last version of RUSLE distributed by the SWCS. Version 1.05 was released to the SWCS but was never sold to the public. Version 1.06b supersedes all previous versions of RUSLE. Should you need a copy of version 1.02, 1.03, or 1.04 contact the SWCS.
Version 1.04 included major improvements in the C factor for rangelands and no-till cropland. RUSLE1.05 improved the handling of residue for crops that senesced, the computation of the effects of mechanical disturbance on rangelands, and the effect of row grade on soil loss in low rainfall areas.
New features in version 1.06 include computation of deposition on concave slopes, in terrace channels, and in sediment basins as a function of sediment characteristics; computation of deposition in terrace channels as a function of the incoming sediment load and the transport capacity in the terrace channel; computation of b values for the effectiveness of ground cover based on land use, slope steepness and length, and the ratio of rill to inter-rill erodibility; computation of the slope length factor from an estimate of the ratio of rill to inter-rill erosion, slope steepness, and land use; and improved computation of the effectiveness of ground cover on steep slopes at construction sites. The current 1.06b version also contains a calculation change for the soil moisture (SM) sub-factor of the cover-management (C) factor.
The USDA-Natural Resources Conservation Service (NRCS) is implementing RUSLE throughout its system of field offices. Implementation of RUSLE1.04 was well underway when a problem was discovered in the way that RUSLE handle residue for crops like soybeans that senesce and drop leaves to the soil surface. That problem was corrected in an interim version of RUSLE, known as version 1.05d.
NRCS continued its implementation-using version 1.05d, although the southern and eastern states continued to use 1.04, but adjusted parameter values in databases to correct the senescence problem. In most states, NRCS released RUSLE in the form of hardcopy tables and figures placed in Field Office Technical Guides.
Another interim version, 1.05q, was adopted by NRCS in 1997 for use in its Field Office Computing Systems (FOCS) software. This UNIX-based model has been implemented in field offices in several states. The differences between this version and 1.05d were in computations for strip-cropping and for the time invariant C and P factors for rangelands.
The "official" version of RUSLE1.05 was released to SWCS in September 1996. A major revision of RUSLE, known as RUSLE2 is in progress. RUSLE2s object-oriented, Windows interface allows dramatic scientific and graphical advances. RUSLE2 is derived from previous DOS RUSLE software, and is based on the widely-used Revised Universal Soil Loss Equation (RUSLE). It will also be much easier to learn and use than the current version of RUSLE. RUSLE2 will be very flexible and customizable to particular user preferences. It will also allow a choice of units between the U.S. customary units and SI (metric) units. It is also a key part of the emerging ARS Modular Soil Erosion System (MOSES). Expected release date is 2002.
Getting Assistance - Documentation
The USDA-Agriculture Handbook (AH) 703 documents RUSLE in detail through version 1.04. AH703 provides information on the equations used in RUSLE, core values for data inputs, and instructions on building data files and using the computer program. Copies of AH703 are available from the US Government Printing Office and the National Technical Information Service. Single copies can be obtained by contacting Keith McGregor or Ken Renard.
The USDA AH703 is also available electronically in Acobat PDF form from the link below. Included in the electronic AH703 is an updated Fig. 6-2 and an errata. Also available are electronic copies of USDA AH 537 and AH 282, and their associated supplements and errata, describing the USLE and earlier erosion prediction technology efforts.
SECTION 1 GENERAL RESOURCE REFERENCES
4. Erosion Prediction - The Wind Erosion Equation (WEQ)
Using wind tunnels and field studies, the lateDr. W. S. Chepil and co-workers set out in the mid-1950's to develop the first wind erosion prediction equation which is now used by the Natural Resources Conservation Service (NRCS) and other action agencies throughout the country.
By 1954, Chepil and his coworkers began to publish results of their research in the form of wind erosion prediction equations. In 1959, Chepil released an equation:
E = IRKFBWD
E = quantity of erosion
I = soil cloddiness
R = residue
K = roughness
F = soil abradability
B = wind barrier
W = width of field
D = wind direction
Wind velocity at geographic locations was not addressed in this equation. In 1962, Chepils group released the equation:
E =¦ (ACKLV)
A =percentage of soil fractions greater than 0.84millimeter.
Factors C, K, L, and V were the same as in the present equation although they were not handled the same. A C-factor map for the western half of the United States was also published in 1962.
In 1963, the current form of the equation:
was first released. In 1965, the concept of preponderance in assessing wind erosion forces was introduced.
In 1968, monthly climatic factors were published. These are no longer used by NRCS. Instead, NRCS adopted a proposal for computing soil erosion by periods using wind energy distribution. In 1981, the Wind Erosion Research Unit provided NRCS with data on the distribution of erosive wind energy for the United States and in 1982 provided updated annual C factors.
Although the present equation has significant limitations, it is the best tool currently available for making reasonable estimates of wind erosion. Currently, research and development of improved procedures for estimating wind erosion are underway.
The present Wind Erosion Equation is expressed as:
E =¦ (IKCLV)
E =estimated average annual soil loss in tons per acre per year
=indicates relationships that are not straight-line mathematical calculations
I = soil erodibility index
K = soil surface roughness factor
C = climatic factor
L =the unsheltered distance
V =the vegetative cover factor
The I factor, expressed as the average annual soil loss in tons per acre per year from a field area, accounts for the inherent soil properties affecting erodibility. These properties include texture, organic matter, and calcium carbonate percentage. I is the potential annual wind erosion for a given soil under a given set of field conditions. The given set of field conditions for which I is referenced is that of an isolated, unsheltered, wide, bare, smooth, level, loose, and noncrusted soil surface, and at a location where the climatic factor (C) is equal to 100.
The K factor is a measure of the effect of ridges and cloddiness made by tillage and planting implements. It is expressed as a decimal from 0.1 to 1.0.
The C factor for any given locality characterizes climatic erosivity, specifically windspeed and surface soil moisture. This factor is expressed as a percentage of the C factor for Garden City, Kansas, which has a value of 100.
The L factor considers the unprotected distance along the prevailing erosive wind direction across the area to be evaluated and the preponderance of the prevailing erosive winds.
The V factor considers the kind, amount, and orientation of vegetation on the surface. The vegetative cover is expressed in pounds per acre of a flat small-grain residue equivalent.
Solving the equation involves five successive steps. Steps 1, 2 and 3 can be solved by multiplying the factor values. Determining the effects of L and V (steps 4 and 5) involves more complex functional relationships.
Step 1: E1 = I
Factor I is established for the specific soil. I may be increased for knolls less than 500 feet long facing into the prevailing wind, or decreased to account for surface soil crusting, and irrigation.
Step 2: E2 = IK
Factor K adjusts E1 for tillage-induced oriented roughness, Krd (ridges) and random roughness, Krr (cloddiness). The value of K is calculated by multiplying Krd times Krr. (K = Krd x Krr).
Step 3: E3 = IKC
Factor C adjusts E2 for the local climatic factor.
Step 4: E4 = IKCL
Factor L adjusts E3 for unsheltered distance.
Step 5: E5 = IKCLV
Factor V adjusts E4 for vegetative cover.
Limitations of the equation
When the unsheltered distance, L, is sufficiently long, the transport capacity of the wind for saltation and creep is reached. If the wind is moving all the soil it can carry across a given surface, the inflow into a downwind area of the field is equal to the outflow from that same area of the field, for saltation and creep. The net soil loss from this specific area of the field is then only the suspension component. This does not imply a reduced soil erosion problem because, theoretically, there is still the estimated amount of soil loss in creep, saltation, and suspension leaving the downwind edge of the field.
The equation does not account for snow cover or seasonal changes in soil erodibility. The equation does not estimate erosion from single storm events, and surface armoring by non-erodible gravel is not usually addressed in the I factor.
Alternative procedures for using the WEQ
The WEQ Critical Period Procedure is based on use of the Wind Erosion Equation as described by Woodruff and Siddoway in 1965. The conditions during the critical wind erosion period are used to derive the estimate of annual wind erosion.
The Critical Wind Erosion Period is described as the period of the year when the greatest amount of wind erosion can be expected to occur from a field under an identified management system. It is the period when vegetative cover, soil surface conditions, and expected erosive winds result in the greatest potential for wind erosion.
Erosion estimates developed using the critical period procedure are made using a single set of factor values (IKCL & V) in the equation to describe the critical wind erosion period conditions.
The critical period procedure is currently used for resource inventories. NRCS usually provides specific instructions on developing wind erosion estimates for resource inventories.
The WEQ Management Period Procedure was published by Bondy, Lyles, and Hayes in 1980. It solves the equation for situations where site conditions have significant variation during the year or planning period where the soil is exposed to soil erosion for short periods, and where crop damage is the foremost conservation concern, rather than the extent of soil loss. The management period procedure is described as being more responsive to changing conditions throughout the cropping year but is not considered more accurate than the critical period procedure.
Comparisons should not be made between the soil erosion predictions made by the management period procedure and the critical period procedure. In other words, where a conservation system has been determined to be acceptable by the management period procedure and placed in a conservation plan or the FOTG, then only the management period procedure will be used to determine if other conservation systems, planned or applied, provide equivalent treatment.
Adjustments to the WEQ soil erodibility factor, I, can be made for temporary conditions that include irrigation or crusts, but such adjustments are to be used only with the management period procedure. The use of monthly preponderance data to determine equivalent field width is also applicable only to the management period procedure.
States will use critical period or the management period procedure, within published guidelines, for conservation planning. The management period procedure will not be used for resource inventories unless specifically stated in instructions. Refer to individual program manuals for more specific instructions pertaining to the use of the Wind Erosion Equation.
In the late 1980s a computer version of WEQ was developed that allowed management period calculations. In 1997 Circular No 2 (amendment to the National Agronomy Manual) added an adjustment for irrigated fields, an adjustment for random roughness, and a way to interpolate the climate factors. Use of the management period procedure can be simplified through the use of worksheets on which information for each management period is documented. An acceptable WEQ calculator has been developed in Microsoft Excel, and is being adapted for use in many states. The most current worksheet version of the WEQ Management Period Procedure and be found at