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 Revised Universal Soil Loss Equation 1.06 - Description of RUSLE1.06c

Description of RUSLE1.06c

RUSLE1.06c uses an index method to estimate soil erosion. RUSLE1.06c uses factors that represent how climate, soil, topography, and land use affect rill and interrill (sheet and rill) soil erosion caused by raindrop impact and surface runoff. In general, erosion depends on the amount and intensity of rainfall and runoff, protective cover provided by land use, 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 RUSLE1.06c with the equation:

A = R K L S C P

where: A = average annual soil loss (tons/acre), K = soil erodibility factor, L = slope length factor, S = slope steepness factor, C = cover-management factor, and P = support practices factor. A soil loss (erosion rate) in tons per acre per year is computed by substituting values for each RUSLE1.06c factor to represent conditions at a specific site. RUSLE1.06c is based primarily on the analysis of a large mass of experimental data. RUSLE1.06c uses equations based on fundamental erosion processes so that RUSLE1.06c can be applied to situations where experimental data are inadequate to define RUSLE1.06c factor values.

The product R?K in RUSLE1.06c is an estimate of soil loss from unit plot conditions. These two factors have dimensions and units, whereas the other RUSLE1.06c factors 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, 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 in a seedbed like condition 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 storm's maximum 30-minute intensity. Erosivity ranges from less than 10 (US customary units) in the western US to greater than 600 in Louisiana. All other factors being the same, soil loss is about 100 times greater at New Orleans, Louisiana than at Las Vegas, Nevada.

New R-values have been recently developed from analysis of modern climate data. These values are available in RUSLE2 and should be used in RUSLE1.06c. The RUSLE1.06c database that is downloaded from this site contains the new erosivity values. See the Bulletin section of this Internet site for additional information.

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 unit plot measures soil erodibility without the influence of cover-management.

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 RUSLE1.06c definitions. The definition for K, and for all RUSLE1.06c 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. RUSLE1.06c estimates the total of rill and interrill erosion combined. 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 influence of slope length, which is represented by the L factor, is greater for those conditions where rill erosion is greater than interrill erosion.

Erosion increases with slope steepness, but in contrast to the L factor, RUSLE1.06c 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 the 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 support 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 RUSLE1.06c 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 an erosion control plan. For example, the C factor describes how vegetation, tillage systems, and addition of mulches affect soil loss.

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), soil 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 variables 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 of the soil loss ratio that has been weighted according to the variation of rainfall erosivity (EI distribution) over the year.

The long-term average 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 cover-management condition to soil loss from the unit plot at a given time. RUSLE1.06c - 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 support practice P factor describes how practices such as contouring, strip cropping, concave slopes, terraces, sediment basins, grass hedges, silt fences, straw bales, and subsurface drainage affect rill and interrill erosion. These practices are applied to support the basic cultural practices used to control erosion, such as vegetation, cover-management system, and mulch additions that are represented by the C factor.

Support practices typically affect erosion by redirecting runoff around the slope to reduce runoff erosivity or by slowing down the runoff to cause deposition such as on concave slopes or by barriers like vegetative strips and terraces. The major factors considered in estimating a P factor value include runoff rate as a function of geographical location, soil, and cover-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.