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. water ponded on the soil surface than on steep slopes. Higher rainfall The numerical value used for R in intensities that are correlated with RUSLE must quantify the effect of higher R factors also tend to increase raindrop impact and must also reflect the depth of ponded surface water, the amount and rate of runoff likely to which in turn protects the soil from be associated with the rain. The rainfall impact. To account for this erosion index (R) meets these soil protection by a ponded water requirements better than any of the layer on low slopes under high rainfall many other rainfall parameters and rates, the R factor should be adjusted using the RUSLE computer program.

West Virginia March 2001 SOIL ERODIBILITY FACTOR (K) with the cover-management factor (C) are primarily due to the effect of Soil erodibility is a complex property organic matter or organic carbon on and is thought of as the ease with soil loss. The organic-carbon content which soil is detached by splash of soils depends on the annual during rainfall or by surface flow or additions of surface and subsurface both. From a fundamental crop residue and manure and on their standpoint, however, soil erodibility decomposition rate. No sharp should be viewed as the change in the delineation can be made where the soil per unit of applied external force effects of crop residue cease to be part or energy. RUSLE uses a restrictive of a C factor and instead become part and applied definition of soil of the K factor. erodibility. Soil erodibility is related to the integrated effect of rainfall, SOILS WITH ROCK FRAGMENTS runoff, and infiltration on soil loss and is commonly called the soil- Rock fragments, when present on the erodibility factor (K). The soil- soil surface, significantly reduce soil erodibility factor (K) in RUSLE detachment by rainfall. When present accounts for the influence of soil in a coarse-textured-soil profile properties on soil loss during storm (having sand and loamy sand events on upland areas. textures), the fragments can appreciably reduce infiltration. In practical terms, the soil-erodibility factor is the average long-term soil Surface cover by rock fragments and soil-profile response to the varies from site to site on otherwise erosive powers of rainstorms; that is, identical soils. The fragments act as a the soil-erodibility factor is a lumped surface mulch by protecting the soil parameter that represents an surface from raindrop impact in a integrated average annual value of the manner similar to that of surface total soil and soil profile reaction to a mulches of straw and chopped stalks. large number of erosion and Rock fragments are usually not hydrologic processes. These moved by water from interrill areas processes consist of soil detachment but remain behind on the soil surface and transport by raindrop impact and and act as an “armor”. surface flow, localized deposition due to topography and tillage-induced Subsurface rock fragments affect roughness, and rainwater infiltration infiltration and thus runoff in a into the soil profile. manner similar to that of subsurface residue by reducing the soil void The soil-erodibility factor (K) space and soil hydraulic conductivity represents the effect of soil properties in coarse-textured soils. Moreover, and soil profile characteristics on soil because soil-mechanical-analysis loss. Some interdependency exists procedures are based on particle-size between the K factor and other fractions smaller than 2mm, rock RUSLE factors. For instance, the fragments larger than 2 mm are traditional topographic relationships usually excluded when estimating K- for slope length and steepness factors factor values. However, rock derived from soil-loss measurements fragments are part of a continuum of on mostly medium-textured, poorly particle sizes in the mineral phase of aggregated surface soils. Interactions the soil and therefore can be

West Virginia March 2001 considered as part of the soil- 2-ft. contour intervals if considerable erodibility factor. (See Appendix 2) care is used.

Both slope length and steepness TOPOGRAPHIC FACTOR (LS) substantially affect sheet and rill erosion estimated by RUSLE. The The effect of topography on erosion in effects of these factors have been RUSLE is accounted for by the LS evaluated separately in research factor. Erosion increases as slope using uniform-gradient plots. length increases, and is considered by However, in erosion prediction, the the slope length factor (L). Slope factors L and S are usually evaluated length is defined as the horizontal together. distance from the origin of overland flow to the point where either (1) the Consolidation of soils affect the LS slope gradient decreases enough that values. Pasture soil have a low rate of deposition begins or (2) runoff rill to interrill erosion; cropland has a becomes concentrated in a defined medium ratio; and construction and channel. It is important to remember mining sites has a high ratio that change of vegetative cover type (Appendix 2-Tables LS 1 thru 3). The does not affect length of slope. effect is that a high ratio will result in Surface runoff will usually higher erosion levels. concentrate in less than 250 feet in West Virginia under natural UNIFORM SLOPES conditions. As the slope gets steeper, the length of slope is shorter. For The combined LS factor in RUSLE most cropland, slope of 125-150 ft. represents the ratio of soil loss on a are common unless the surface has given slope length and steepness to been carefully graded into ridges and soil loss from a slope that has a furrows that maintain flow for long length of 72.6 ft. and a steepness of distances. Few slope lengths as long 9%, where all other conditions are the as 250 ft. should be used in RUSLE. same. LS values are not absolute Slope length is best determined by values but are referenced to a value of pacing or measuring in the field. For 1.0 at a 72.6-ft. slope length and 9% steep slopes, these lengths should be steepness. converted to horizontal distance for use in RUSLE. Slope lengths Procedures are developed in the estimated from contour maps are RUSLE computer program for usually too long because most maps predicting soil loss on uniform slopes, do not have the detail to indicate all where steepness is the same over concentrated flow areas that end their entire length. RUSLE slope lengths. (Figure 1, Appendix 2) illustrates some typical slope lengths. IRREGULAR AND SEGMENTED SLOPES The slope steepness factor (S) reflect the influence of slope gradient on The shape of a slope affects the erosion. Slope is estimated in the average soil loss and the soil loss field by use of an inclinometer, Abney along the slope. For example, the level or similar device. Slope may be average soil loss from a convex slope estimated from contour maps having can easily be 30% greater than that

West Virginia March 2001 for a uniform slope with the same contour, until the origin of overland steepness as the average steepness of flow is reached. Often this point is the convex slopes. The average not at the top of the hill but at a erosion on a concave slope that does divide down the nose of a ridge. not flatten enough to cause deposition is less than that on a uniform slope The lower end of the slope length is that is equivalent to the average located by walking downslope concave-slope steepness. Maximum perpendicular to the contour until a erosion along a concave slope, which broad area of deposition or a natural occurs about one-third of the way or constructed waterway is reached. along the slope, may nearly equal the These waterways are not necessarily maximum erosion on a uniform slope. eroded or incised channels and this Therefore, when the slope shape is lack of channels can make it difficult significantly curved, use of the to determine the end of slope. One procedure for an irregularly shaped aid is to visualize the locations on the slope should be used by breaking a landscape where eroded channels or non-uniform slope into a number. gullies would naturally form.

The procedure for irregular slopes can If a slope flattens enough near its end, include the evaluation of changes in deposition may occur. When erosion soil type along a slope. The RUSLE and deposition rates are low and computer program allows a planner erosion has not recently occurred, the ability to evaluate a combination deposition begins at the point where of slope segments. slope has decreased to about 5%. Deposition does not necessarily occur GUIDES FOR CHOOSING SLOPE everywhere a slope flattens. The best LENGTHS time to train yourself on slope lengths is during early spring runoff when In training sessions, more questions you can visually track runoff, find the are asked about slope length than contour, and locate point of about any other RUSLE factor. Slope deposition or concentrated flow (See length is the factor that involves the Figures 1 & 2 for illustration to most judgement, and length determine slope length). determinations made by users vary greatly. Figure 1 illustrates the major slope-length situations that are found COVER-MANAGEMENT FACTOR (C) in the field. The C factor is used in the Revised Actually, an infinite number of slope USLE (RUSLE) to reflect the effect of lengths exist in the field. To apply cropping and management practices RUSLE, erosion can be calculated for on erosion rates, and is the factor several of them and the results used most often to compare the averaged according to the area relative impacts of management represented by each slope length. options on conservation plans. Sometimes a particular position on The C factor indicates how the the landscape is chosen as the conservation plan will affect the location for a slope length. To average annual soil loss and how that establish the ends of the slope length, soil-loss potential will be distributed the user walks upslope from that in time during construction activities, position, moving perpendicular to the

West Virginia March 2001 crop rotations, or other management time, so calculated SLR values will schemes. also change little. It is adequate to calculate a C factor based on a single As with most other factors within average SLR representing the entire RUSLE, the C factor is based on the year. concept of deviation from a standard, in this case an area under clean-tilled In almost all cropland scenarios and continuous-fallow conditions. The in many cases where pasture is being soil loss ratio (SLR) is then an managed, the crop and soil estimate of the ratio of soil loss under parameters change with time due to actual conditions to losses either specific management practices experienced under the reference or natural cyclic effects such as conditions. Studies indicate that the winter knockdown and spring growth. general impact of cropping and This demands that the SLR values be management on soil losses can be calculated frequently enough over the divided into a series of subfactors. In course of a year or a crop rotation to this approach the important provide an adequate measure of how parameters are the impacts of they change. This is especially previous cropping and management, important because the erosion the protection offered the soil surface potential is also changing with time. by the vegetative canopy, the The calculated average annual soil reduction in erosion due to surface loss should be high if a cropping or cover and surface roughness, and in management scheme happens to some cases the impact of low soil leave the soil susceptible to erosion at moisture on reduction of runoff from a time of high rainfall erosivity. low-intensity rainfall. As used in RUSLE incorporated this effect into RUSLE, each of these parameters is calculations of SLR values based on assigned a subfactor value, and these crop-growth stages. These values values are multiplied together to yield were based on tillage type, elapsed a SLR. time since a tillage operation, canopy development, and date of harvest. An individual SLR value is thus RUSLE calculations are based on a calculated for each time period over 15-day time step. This means that which the important parameters can SLR values are calculated every 15 be assumed to remain constant. days throughout the year, and that Each of these SLR values is then the important parameters are weighted by the fraction of rainfall assumed to remain constant during and runoff erosivity (EI) associated those 15 days. with the corresponding time period, and these weighted values are If a management operation occurs combined into an overall C factor within the period, the parameters can value. (See Appendix 2) no longer be assumed constant; the half-month period is then broken into USE OF TIME-VARYING OR two segments and two SLR values are AVERAGE ANNUAL VALUES calculated.

For areas such as pasture that have Calculations of the SLR for the reached a relative equilibrium, the average annual and time-varying parameters used in computing SLR approaches are the same and require values may change very slowly with the same input parameters; the

West Virginia March 2001 difference lies in how often the RUSLE contains another database to calculation is performed. In the time- store user-supplied information varying approach (cropland), the SLR defining the impacts of management value is then weighted by the operations on the soil, vegetation, and percentage of EI associated with that residues, and uses that information to segment. In the average annual modify the variables accordingly. approach, everything is assumed constant (pasture), so the calculation The RUSLE program contains a third is made once. database that represents the climate for the area of interest. This is COMPUTATION OF SOIL-LOSS important to the C-factor calculations RATIOS in two ways: first, the EI distribution within the database set is used to Based on new descriptions of weight each SLR value in calculating cropping and management practices the overall C-factor value. Second, and their influence on soil loss, soil- the set also contains temperature and loss ratios are computed as rainfall data, which are needed to SLR = PLU • CC • SC • SR • SM calculate the rate of residue Where SLR is the soil-loss ratio for decomposition. the given conditions, PLU is the prior- land-use subfactor, CC is the canopy- cover subfactor, SC is the surface- SUPPORT PRACTICE FACTOR (P) cover subfactor, SR is the surface- roughness subfactor, and SM is the By definition, the support practice soil-moisture subfactor. factor (P) in RUSLE is the ratio of soil loss with a specific support practice to Each subfactor contains cropping and the corresponding loss with upslope management variables that affect soil and downslope tillage. These erosion. Individual subfactors are practices principally affect erosion by expressed as functions of one or more modifying the flow pattern, grade, or variables, including residue cover, direction of surface runoff and by canopy cover, canopy height, surface reducing the amount and rate of roughness, below-ground biomass runoff. For cultivated land, the (root mass plus incorporated residue), support practices considered include prior cropping coil moisture, and contouring (tillage and planting on or time. near the contour), stripcropping, terracing, and subsurface drainage. RUSLE uses a CROP database to store the values required to calculate P does not consider improved tillage the impact on soil loss of any practices such as no-till and other vegetation within the management conservation tillage systems, sod- plan. These user-defined sets of based crop rotations, fertility values specify the growth treatments, and crop-residue characteristics of the vegetation, the management. Such erosion-control amount of residue the vegetation will practices are considered in the C produce, and the characteristics of factor. that residue. The program uses that information to calculate the change An overall P-factor value is computed with time of the variables listed above as a product of P subfactors for and their impact on the subfactors.

West Virginia March 2001 individual support practices, which moderate, high, and very high and are typically used in combination. where the ridges follow the contour so closely that runoff spills over the CONTOUR TILLAGE ridges uniformly along their length. Data showing the greatest The effect of contour tillage on soil effectiveness of contouring were erosion by water is described by the generally from plots having high countour P factor in the Revised ridges. Universal Soil Loss Equation (RUSLE). If erosion by flow occurs, a network of EFFECT OF STORM SEVERITY small-eroded channels or rills develops in the areas of deepest flow. Data from field studies indicate that On relatively smooth soil surfaces, the contouring is less effective for large flow pattern is determined by random storms than for small storms. The natural microtopography. When reduced effectiveness depends on tillage leaves high ridges, runoff stays both amount of runoff and peak rate within the furrows between the ridges, runoff. These runoff variables are and the flow pattern is completely directly related to rainfall amount and determined by the tillage marks. High intensity, which are the principal ridges from tillage on the contour variables that determine EI (storm cause runoff to flow around the slope, energy times maximum 30-min significantly reducing the grade along intensity), the erosivity factor in the flow path and reducing the flow’s RUSLE. Therefore, values for the detachment and transport capacity contouring subfactor should be near compared to runoff directly 1 (little effectiveness) when EI is high downslope. and infiltration into the soil is low, and should be small (greater When grade is sufficiently flat along effectiveness) when EI is low and the tillage marks, much of the infiltration is high. Loss of contouring sediment eroded from the ridges effectiveness is likely to occur from a separating the furrows is deposited in few major storms. the furrows. However, tillage is seldom exactly on the contour. EFFECT OF OFF-GRADE Runoff collects in the low areas on the CONTOURING landscape and if accumulated water overtops the ridges, then rill and Contouring alone is often inadequate concentrated flow erosion usually for effective erosion control. Runoff occur, especially in recently tilled frequently flows along the furrows to fields. Runoff from contoured fields is low areas on the landscape, where often less that that from fields tilled breakovers occur. Grassed waterways upslope-downslope. Contour tillage are needed in conjunction with reduces erosion by reducing both the contouring to safely dispose of the runoff and the grade along the flow runoff that collects in natural path. waterways at the breakovers.

EFFECT OF RIDGE HEIGHT

Table 2 Appendix 1 represents the effectiveness of contouring where ridge heights are very low, low,

West Virginia March 2001 SUPPORT PRACTICE FACTOR (P) sediment reaching offsite water FOR CROSS-SLOPE bodies. Neither practice traps eroded STRIPCROPPING, BUFFER STRIPS, sediment on the hillslope and AND FILTER STRIPS therefore has minimal benefit as a P factor. Stripcropping is a support practice where strips of clean-tilled or nearly Densely vegetated strips that induce clean-tilled crops are alternated with deposition of eroded sediment are strips of closely growing vegetation assigned a P-factor value. Deposition such as grasses and legumes. must occur on the hillslope in areas Stripcropping for the control of water where crops are routinely grown to erosion is variously described as deserve a low P factor indicative of contour stripcropping, cross-slope greatest value to soil conservation. stripcropping, and field stripcropping. Therefore, P-factor values for Each of these practices has the maintenance of soil productivity are common characteristic of crops in lowest for cross-slope stripcropping, rotation forming strips of nearly equal moderate for buffer strips, and width. The difference between the highest for filter strips. A P value of practices is the degree of deviation 1.0 is assigned to filter strips because from the contour. All of them, they provide little protection to the including contour stripcropping, majority of the field where crops are involve some degree of off-grade grown. contouring. The effectiveness of all of them can be determined with the A major advantage of stripcropping is same equations in the RUSLE the rotation of crops amoung the computer model. strips. By rotating crops among strips, each clean-tilled crop receives Buffer strips, located at intervals up benefit from the sediment deposited the slope, are resident strips of in a previous year by the closely perennial vegetation laid out across growing crop or the rough strip. the slope. Like the strips in cross- Stripcropping significantly reduces slope stripcropping, they may or may the rate of sediment moving down the not be on the contour. These strips, slope. Because filter strips are predominantly composed of grass located at the base of slopes, the species, are not in the crop rotation, strips do not greatly affect this rate. are usually much narrower than the In general, the benefit of deposition adjacent strips of clean-tilled crops, depends on the amount of deposition and may be left in place for several and its location. Sediment deposited years or permanently. The far down the slope provides less effectiveness of buffer strips in benefit than does sediment deposited trapping sediment and reducing on the upper parts of the slope. With erosion can also be evaluated by the buffer strips, the sediment is trapped RUSLE model. and remains on small areas of the slope, such as terraces; thus the Vegetated filter strips are bands of entire slope does not benefit as much vegetation at the base of a slope. as it does in stripcropping. Riparian filter strips are located along stream channels or bodies of water. These conservation practices are designed to reduce the amount of

West Virginia March 2001 SOIL LOSS TOLERANCE (T) losses, reduction of soil organic matter, and loss of plant nutrients. A major purpose of the soil-loss equation is to guide the making of A deep, medium-textured, moderately methodical decisions in conservation permeable soil that has subsoil planning. The equation enables the characteristics favorable for plant planner to predict the average rate of growth has a greater tolerable soil- soil erosion for each of various loss rate than do soils with shallow alternative combinations of cropping root zones or high percentages of systems, management techniques, shale at the surface. Widespread and erosion-control practices on any experience has shown that the particular site. The term “soil-loss concept of soil-loss tolerance may be tolerance” (T) denotes the maximum feasible and generally adequate for rate of soil erosion that can occur and indefinitely sustaining productivity still permit crop productivity to be levels. sustained economically. The term considers the loss of productivity due Soil-loss limits are sometimes to erosion but also considers rate of established to prevent or reduce soil formation from parent material, damage to offsite water quality. The role of topsoil formation, loss of criteria for defining the tolerance nutrients and the cost to replace limits of field soil-loss tolerance limits them, erosion rate at which gully for this purpose are not the same as erosion might be expected to begin, those for tolerances designed to and erosion-control practices that preserve cropland productivity. Soil farmers might reasonably be able to depth is not relevant for offsite implement. When predicted soil sediment control, and uniform limits losses are compared with the value for on erosion rates still allow a range in soil-loss tolerance at that site, RUSLE the amount of sediment per unit area provides specific guidelines for that is delivered to a stream. Soil bringing about erosion control within material eroded from a field slope may the specified limits. Any combination be deposited along field boundaries, of cropping, grazing and management in terrace channels, in depression for which the predicted erosion rate is areas, or on flat or vegetated areas less than the rate for soil-loss traversed by overland flow before it tolerance may be expected to provide reaches a watercourse. Erosion satisfactory control of erosion. Of the damages the cropland on which it satisfactory alternatives offered by occurs, but sediment deposited near this procedure, the alternative(s) best its place of origin does not directly suited to a particular farm or other affect water quality. enterprise may then be selected.

Values of soil-loss tolerance ranging from 2 to 5 ton acre yr. for the soils in West Virginia. Factors considered in defining these limits include soil depth, physical properties and other characteristics affecting root development, gully prevention, on- field sediment problems, seeding

West Virginia March 2001 APPENDIX 2

Erosion Prediction

Single Year “C” Factors for Cropland {January 1997}

Table 1 Cover/Management Condition and Code

Table 2 Guidelines for Selecting Ridge Heights for Contouring with RUSLE Single Year “C” Factors for Cropland

Soil Cover Modification Using Rock Fragments

Ten-Year Frequency Single-Storm Erosion Index Values for West Virginia

Grassland RUSLE Field Worksheet

Cropland RUSLE Field Worksheet

Table LS-1 Pasture, Hayland and Continuous No-Till

Table LS-2 Cropland with Tillage

Table LS-3 Construction and Mining Sites

Figure 1 Examples of Principles Used in Determining Slope Length

Figure 2 Slope Lengths on Row Crop Fields

West Virginia March 2001