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Field Office Technical Guide (FOTG) THE REVISED UNIVERSAL SOIL LOSS EQUATION RUSLE RUSLE is an updated
version of the Universal Soil Loss Equation (USLE) and Agricultural
Handbook 537. The original USLE has been retained in RUSLE, however
it has been put into a computer program to facilitate calculations,
and the technology for factor evaluation has been altered and
new data has been introduced to evaluate each factor under more
specific conditions. A = Predicted
Average Annual Soil Loss(Tons/Acre/Year) Although not a factor in the RUSLE formula "Soil Loss Tolerance" (T), expressed in tons/acre/year, is an important criteria when we begin our management to control soil loss. "T" - Soil Loss Tolerance - is the maximum amount of soil loss, in tons/acre/year, that a given soil type can tolerate and still permit a high level of crop production to be sustained economically and indefinitely. "T" is often substituted for "A" in the RUSLE equation to establish a "Maximum C*P Value" for a given site. RUSLE Is a tool to predict long term average annual soil loss in ton/acre/year from specific field conditions using specific management systems. RUSLE cannot be used to estimate or predict soil loss from individual storms nor from a particular year of weather and related factors. The factors used in the RUSLE are based on long-term averages. RUSLE is only
to be used to predict sheet and rill erosion on cropland, pastureland,
and construction sites. RUSLE is not applicable to woodland and
is not to be used to predict soil loss on woodland sites. (R) THE RAINFALL-RUNOFF INTENSITY EROSIVITY INDEX FACTOR To understand the "R" value used In RUSLE one must first understand how the erosive potential of rainfall effects the soil erosion process. Raindrop erosion increases with the intensity of the rain. A long slow rain may have the same total energy as a short rain that is more intense. Total energy of the rainfall alone is not a good indicator of erosive potential. However, when energy is combined with rainfall intensity the result (EI-Energy/lntensity) is a good predictor of erosive potential. EI is the value of the product of total storm energy times the maximum 30 minute intensity. Technically, the term indicates how particle detachment is combined with transport capacity (the soil erosion process). The relation of soil loss to the EI parameter is considered linear, and the parameter's individual storm values are directly additive. The sum of an average years EI's for a particular locality is the "Rainfall Erosion Index - R" for that location. In the development of RUSLE these values where updated and as a result new "R" factors are available for each county. The "R" values for Washington State vary from 140 in northeastern Washington State to 245 in southern Washington State. The higher the "R" value the higher the erosion potential. (K) THE SOIL ERODIBILITY FACTOR Soil erodibility is a complex property and is thought of as the ease with which soil is detached by splash during rainfall and/or by surface flow. Soil erodibility is related to the integrated effect of rainfall, runoff, and infiltration. The soil erodibility
factor (K) is the soil loss rate per erosion index unit for a
specified soil as measured on a unit plot. A unit plot is defined
as 72.6 feet long with a uniform slope of 9% in continuously clean-tilled
fallow. The "K" represents both the susceptibility of
the soil to erode and the rate of runoff. The "K" factor represents the effect of soil properties and the soil profile characteristics on soil loss. The "K" values are expressed as average annual values. "K" values are assigned using a "Soil Erodibility Nomograph" that combines the effects of soil particle size, percent organic matter, soil structure code, and the profile permeability class. RUSLE has taken the process one step further and adjusted the "K" factor based on seasonal variability related to freeze/thaw and soil moisture during the year. RUSLE recomputes the "K" value bimonthly (24 times during the year). The RUSLE "K" values, found in the tables and charts, reflect the average annual adjusted "K" value for your location. (LS) THE LENGTH AND SLOPE FACTOR The length and slope factors used in RUSLE account for the effect of topography on erosion. Erosion increases as the slope length increases, and is considered the slope-length factor (L). Slope length is defined as the horizontal distance from the origin of flow to the point where either (1) the slope gradient decreases enough that deposition begins or (2) runoff becomes concentrated in a defined channel. Slope lengths will rarely exceed 400 feet in length unless grading has been done. Deposition usually begins to occur along a slope gradient at the point where the slope decreases by about 5%. Slope length is best determined by pacing or measuring in the field. The slope steepness factor (S) reflects the influence of slope gradient on erosion. Erosion potential increases with the steepness of the slope. Slope is measured in the field by use of a clinometer, Abney level, or similar device. Contour maps, unless down to a two-foot contour interval, should not be used to measure slope nor length of slope. Slope and length of slope are measured perpendicular to the contour lines. When measuring in the field it is important to visualize the contour lines and measure perpendicular to those lines. The combined LS factor in RUSLE represents the ratio of soil loss on a given slope length and steepness to the soil loss from a unit slope that has a length of 72.6 feet and a steepness of 9%, where all other conditions are the same. LS values are not absolute values but are referenced to a value of 1.0 at a 72.6 foot slope length and a 9% steepness. LS values less than 1.0 represent site conditions that erode less than the referenced condition of 72.6 ft. and 9% slope; and LS values more than 1.0 represent conditions more erosive than the reference condition. It is important to consider the shape and makeup of a slope when determining its LS value. Uniform slopes are slopes where the slope is generally uniform over the entire length. Irregular or complex slopes have slope changes along the measured slope length. Irregular or complex slopes should have the LS value calculated to obtain a more accurate soil loss prediction. Slopes that are convex (slopes tend to increase downslope) are more erosive than concave slopes (slopes tend to decrease downslope). This situation will be reflected in the LS value calculated for an irregular slope. Until users obtain FOCS-RUSLE to calculate LS values, charts are available to obtain LS values for uniform slopes. Occasionally situations arise where an LS value is needed for a complex slope (slopes change one or more times in length). When FOCS-RUSLE becomes available, users will be able to calculate LS values for complex slopes. RUSLE LS values
vary from the USLE values. RUSLE calculated LS values differently
depending on the site susceptibility to rill or interill erosion.
RUSLE will adjust LS values for the four (4) different situations. (C) THE COVER
MANAGEMENT FACTOR The "C" Factor is used within both the USLE and RUSLE to reflect the effect of cropping and management practices on erosion rates. The "C" Factor measures how soil loss potential will be distributed in time during construction activities, crop rotations, or other management schemes. As with most of the other factors within RUSLE, the "C" Factor is based on the concept of deviation from the standard. In this case the standard is an area under clean-tilled continuous fallow conditions. A Soil Loss Ratio (SLR) is then used to estimate soil loss under actual site conditions compared to losses experienced under the standard conditions (continuous fallow). RUSLE developed values for "C" by looking at conditions during specific crop stages (fallow, seedbed preparation. crop establishment, crop development, crop maturing, and harvest residue). USLE used average values for surface roughness, canopy cover, surface cover, and EI during each crop stage. RUSLE takes a much more thorough approach to calculating "C"Factors. RUSLE looks at the impact of cropping and management on several subfactors. It looks at the impacts from previous cropping and management (prior land use, PLU), the protection offered the soil surface by vegetative canopy (canopy cover, CC), the reduction in erosion due to surface cover and surface roughness (surface cover, SC; surface roughness, SR), and in some cases the impact of low soil moisture (SM)on reduction of runoff from low-intensity rainfall. RUSLE assigns a subfactor value to each of these parameters during each semi-monthly time period, and calculates a Soil Loss Ratio (SLR) for each time period. The SLR for each period is weighted by the fraction of rainfall and runoff erosivity (EI) for the corresponding period. The weighted values are then combined into an overall "C" Factor. The following
is a brief description of the subfactors impacting the RUSLE "C"
Factors. Canopy Cover (CC) Subfactor - expresses the effectiveness of vegetative canopy in reducing the energy of rainfall striking the soil surface. Although most of the rainfall eventually reaches the soil surface, the rainfall intercepted by the canopy reaches the soil surface with less energy. RUSLE using crop databases constantly tracks the growth of a crop to calculate percent of canopy cover and average fall height of the raindrop from the crop leaf surface. The taller canopy cover, the less effective is canopy cover because the raindrop gains more velocity before reaching the soil surface. Surface Cover (SC) Subfactor - affects erosion by reducing the transport capacity of runoff water, by causing deposition in ponded areas, and by decreasing the surface area susceptible to raindrop impact. This is measured by the amount of crop residue cover on the soil surface. RUSLE continuously tracks residue from harvest until it is decomposed. RUSLE assigns specific decomposition rates to residue based in the carbon:nitrogen ratio for the residue. RUSLE also tracks how much residue is buried by each type of tillage operation and then adjusts the decomposition rate for above and below ground residue. RUSLE recalculates these figures semi-monthly along with all the other subfactors. This is perhaps the single most important factor determining SLR's. The RUSLE SC subfactor does allow the measurement of rocks on the surface as apart of surface cover, whereas USLE did not. Surface Roughness (SR) Subfactor - surface roughness directly effects soil erosion. A rough surface has many depressions and barriers. During a rainfall event, these trap water and sediment causing rough surfaces to erode at lower rates than do smooth surfaces under similar conditions. Roughness also effects the degree and the rate of soil sealing by raindrop impact. Rougher soils generally have higher infiltration rates. The SR is defined by a baseline condition for a unit plot that is in clean cultivation, smooth, and exposed to rainfall of moderate intensity. RUSLE tracks SR throughout the year based on the time and type of field operation performed and the corresponding rainfall, temperature, and biomass decay rate. Soil Moisture
(SM) Subfactor -
antecedent soil moisture has a substantial influence on infiltration
and runoff and hence on soil erosion. Soil moisture is usually
high during susceptible crop stages in spring and early summer
when much of the erosion occurs. This situation closely parallels
the unit plot continuous fallow plots. This is true for most of
the continental United States. Where this situation is true, no
adjustments are needed for soil moisture. Only the Pacific Northwest
and Range Region adjust for soil moisture. (P) THE SUPPORT
PRACTICE FACTOR The "P" does not consider improved tillage such as notill and other conservation tillage systems, sod-based crop rotations, fertility treatment, and crop residue management. These erosion control measures are included in the "C" Factor. An overall "P" Factor value is computed as a product of "P" subfactors for individual support practices, which are typically used in combination. For example contouring is almost always used in stripcropping or terraces. RUSLE calculates the "P" Factor based on percent slopes, length of slope, roughness and ridge height, EI distribution, hydrologic soil group, and the effect of off grade contouring. Contouring is most effective on slopes of 2-12%. As slopes get steeper than 12% the effectiveness of contouring begins to taper off. Contouring has almost no effect on slopes exceeding 25%. Adding ridge height to contouring adds to the effectiveness of contouring. Ridge height refers to the amount of roughness left with tillage and planting operations. Data from field
studies indicate that contouring is less effective for large storms
than for small storms. The reduced effectiveness depends on both
the amount of runoff and the peak rate of runoff. These runoff
variables are directly related to rainfall amount and intensity
which are the principal variables that determine EI. RUSLE uses
a 10 year EI to calculate the effectiveness of contouring. Each
county is assigned an EI number that corresponds to the 10 Year
EI calculated from a local weather station. RUSLE SUMMARY
RUSLE uses three primary databases to evaluate RUSLE factor values: CROPLIST, CITYLIST, and OPLIST databases. The CROPLIST database identifies the crop being grown; how that crop develops and or decays on a semimonthly basis based on its expected yield; weight of residue at 30%, 60%. and 90% cover; the amount of root biomass in the upper 4 inches of soil; percent of canopy cover; and canopy fall height. Each one of the items is recalculated semi-monthly against all other RUSLE parameters. The CITYLIST database identifies the mean semi-monthly precipitation and temperature for each specific weather station used; the average annual R-Factor; EI; and the number of frost free days each year. Again each one of the factors is recalculated semi-monthly against all the other RUSLE parameters. The OPLIST database tracks the soil disturbing and other field operations. The database identified specific effects created by each field operation. It identifies such effects as the amount of surface disturbed; depth of disturbance; whether a crop is killed or begins growth; whether residue is added or taken away; and the amount of residue retained or buried by each operation. RUSLE is the present state of the art in sheet and rill soil loss prediction. RUSLE is enhanced through the use of the computer program to accurately describe and evaluate your specific site conditions. References:
RUSLE VERSION Note: The "RUSLE
Sheet and Rill Erosion Prediction Worksheet" can be reproduced
and used to train yourself or others on the procedure to calculate
soil loss with RUSLE. There is an "Example RUSLE Sheet and
Rill Erosion Prediction Worksheet" completed to use as a
reference.
1. County ________________________
"R" Factor ________"C" and "K" Factor
Zone___________
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December 20, 2001 |