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Erosion Prediction WV-NRCS Technical Guide Section I EROSION PREDICTION Revised Universal Soil Loss Equation (RUSLE) GENERAL are needed for the selection of the control practices best suited to the The Revised Universal Soil Loss particular needs of each site. Equation (RUSLE) is an erosion model predicting longtime average annual Such guidelines are provided by the soil loss (A) resulting from raindrop procedure for soil-loss prediction splash and runoff from specific field using RUSLE. The procedure slopes in specified cropping and methodically combines research management systems and from information from many sources to pastureland. Widespread use has develop design data for each substantiated the RUSLE’s usefulness conservation plan. Widespread field and validity. RUSLE retains the six experience for more than four decades factors of Agriculture Handbook No. has proved that this technology is 537 to calculate A from a hillslope. valuable as a conservation-planning Technology for evaluating these factor guide. The procedure is founded on values has been changed and new the empirical Universal Soil Loss data added. The technology has been Equation (USLE) that is believed to be computerized to assist calculation. applicable wherever numerical values of its factors are available. Research has supplied information from which BACKGROUND at least approximate values of the equation’s factors can be obtained for Scientific planning for soil specific farm or ranch fields or other conservation and water management small land areas throughout most of requires knowledge of the relations the United States. The personal- among those factors that cause loss of computer program makes information soil and water and those that help to readily available for field use. reduce such losses. Controlled studies on field plots and small The Revised Universal Soil Loss watersheds have supplied much Equation (RUSLE) includes analyses valuable information on these of data not available when the complex interrelations of factors. But previous handbooks were prepared. the maximum benefits from such The analyses are documented so that research can be realized only when users can review, evaluate, and repeat the findings are applied as sound them in the process of making local practices on the farms, ranches, and analyses. other erosion-prone areas throughout the United States. Specific guidelines West Virginia March 2001 Furthermore, the technology was RUSLE computes the average annual revised to permit the addressing of erosion expected on field slopes as A problems not included or = R • K •L • S • C • P inadequately addressed in earlier versions of USLE. The current Where: revision is intended to provide the A = average soil loss expressed in most accurate estimates of soil loss ton/acre/yr. without regard to how the new values R = rainfall-runoff erosivity factor compare with the old values. the rainfall erosion index plus a factor for any significant runoff from snowmelt. SOIL-LOSS EQUATION K = soil erodibility factor the soil loss rate per erosion index unit for a The erosion rate for a given site specified soil as measured on a results from the combination of many standard plot, which is defined as a physical and management variables. 72.6-ft (22.1-m) length of uniform 9% Actual measurements of soil loss slope in continuous clean-tilled would not be feasible for each level of fallow. these factors that occurs under field L = slope length factor the ratio of conditions. Soil-loss equations were soil loss from the field slope length to developed to enable conservation soil loss from a 72.6-ft length under planners, environmental scientists, identical conditions. and others concerned with soil S = slope steepness factor the ratio erosion to extrapolate limited erosion of soil loss from the field slope data to the many localities and gradient to soil loss from a 9% slope conditions that have not been directly under otherwise identical conditions. represented in the research. C = cover-management factor the ratio of soil loss from an area with Erosion and sedimentation by water specified cover and management to involve the processes of detachment, soil loss from an identical area in transport, and deposition of soil tilled continuous fallow. (See particles. The major forces are from Appendix 1) the impact of raindrops and from P = support practice factor the water flowing over the land surface. ratio of soil loss with a support Erosion may be unnoticed on exposed practice like contouring, soil surfaces even though raindrops stripcropping, or terracing to soil loss are eroding large quantities of with straight-row farming up and sediment, but erosion can be down the slope. dramatic where concentrated flow creates extensive rill and gully Field worksheets are to be used to systems. document field conditions (Appendix 2) when estimating soil Sediment yield should not be loss. confused with erosion; the terms are not interchangeable. Sediment yield While not part of the equation, soil is the amount of eroded soil that is loss tolerance (T) denotes the delivered to a point in the watershed maximum rate of erosion that can that is remote from the origin of the occur and still permit crop detached soil particles. RUSLE does not estimate sediment yield. West Virginia March 2001 productivity to be sustained to A Guide for Predicting Sheet and economically. Rill Erosion on Forest Land, USDA- Forest Service. RUSLE users need to be aware that “A” (in addition to being a longtime Some recent research addresses the average annual soil loss) is the application of USLE technology to average soil loss over a field slope and mine spoils reconstructed topsoil and that the losses at various points on land disturbed by construction. The the slope may differ greatly from one effects of compaction on erosion are another. On a long uniform slope, the significant in such instances and are loss from the top part of the slope is treated as an integral part of the much lower than the slope average, subfactor for calculating C. Soil and the loss near the bottom of the consolidation also influences the L/S slope is considerably higher. This factor for construction and mining suggests that even if a field soil loss is sites. (Appendix 2-Table LS-3). Other held to “T,” soil loss on some portion RUSLE values are not affected by of the slope may exceed T, even when these land disturbance activities. the ephemeral gully and other types of erosion that are not estimated by For most conditions in the United RUSLE are ignored. These higher- States, the approximate values of the than-average rates generally occur at six factors for any particular site may the same locations year after year, so be obtained by use of the computer excessive erosion on any part of the program developed to assist with the field may be damaging the soil RUSLE evaluation. Further resource. description of these factors, their use and affects on soil loss is provided in With appropriate selection of its factor Appendix I. values, RUSLE will compute the average soil loss for a multicrop system, for a particular crop year in a REFERENCES rotation, or for a particular crop stage period within a crop year. Erosion Dissmeyer, George E. and Foster, variables change considerably from George R., 1980. A Guide for storm to storm about their means. Predicting Sheet and Rill Erosion on But the effects of the random Forest Land. USDA-US Forest fluctuations such as those associated Service. Atlanta, GA. with annual or storm variability in R and the seasonal variability of the C Renard, K.G. (et.al.) 1997. Predicting tend to average out over extended Soil Erosion by Water: A Guide to periods. Because of the unpredictable Conservation Planning with the short-time fluctuations in the levels of Revised Universal Soil Loss Equation influential variables, however, present – Agricultural Handbook 703. USDA. soil-loss equations are substantially Washington, DC. less accurate for the prediction of specific events than for the prediction of longtime averages. RUSLE does not estimate soil loss from disturbed forested conditions. Users of such technology are referred West Virginia March 2001 WV-NRCS Technical Guide Section I APPENDIX I RAINFALL AND RUNOFF groups of parameters tested against FACTOR (R) the plot data. The rainfall and runoff factor (R) of The energy of a rainstorm is a the Universal Soil Loss Equation function of the amount of rain and of (USLE) was derived from research all the storm’s component intensities. data from many sources. The data The median raindrop size generally indicate that when factors other than increases with greater rain intensity, rainfall are held constant, soil losses and the terminal velocities of free- from cultivated fields are directly falling waterdrops increase with larger proportional to a rainstorm drop size. Since the energy of a given parameter: the total storm energy (E) mass in motion is proportional to times the maximum 30-min intensity velocity squared, rainfall energy is (I30). directly related to rain intensity. Rills and sediment deposits observed R VALUES LOCATION after an unusually intense storm have sometimes led to the conclusion that Local values of the rainfall erosion significant erosion is associated with index are taken directly from the CITY only a few severe storms—that database in the RUSLE computer significant erosion is solely a function program. of peak intensities. However, more than 30 years of measurements in R VALUES FOR FLAT SLOPES many states have shown that this is not the case. The data shows that a Although the R factor is assumed to rainfall factor used to estimate be independent of slope in the average annual soil loss must include structure of RUSLE, splash erosion is the cumulative effects of the many less on low slopes. On flat surfaces, moderate-sized storms as well as the raindrops tend to be more buffered by effects of the occasional severe ones.
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