Universal Soil Loss Equation
The Universal Soil Loss Equation (USLE) is a powerful tool that is widely used by soil conservationists in the United States and many other countries.
The USLE was developed by W. H. Wischmeier, D. D. Smith, and others with the U.S. Department of Agriculture (USDA), Agricultural Research Service (ARS), Soil Conservation Service (SCS), and Purdue University in the late 1950s. Its field use was institutionalized in the form of USDA Agricultural Handbook 282 in 1965, and later updated as USDA Agricultural Handbook 537 in 1978.
The USLE estimates average annual soil loss by sheet and rill erosion on those portions of landscape profiles where erosion, but not deposition, is occurring. It does not estimate deposition like that at the toe of concave slopes, and it does not estimate sediment yield at a downstream location. Also, it does not include ephemeral gully erosion.
The USLE is an empirically based equation, derived from a large mass of field data, especially erosion plots and rainfall simulator experiments, and computes sheet and rill erosion as follows: A=RKLSCP where A is computed soil loss, R is the rainfall-runoff erosivity factor, K is a soil erodibility factor, L is the slope length factor, S is the slope steepness factor, C is a cover management factor, and P is a supporting practices factor.
The R-factor is calculated as a product of the kinetic energy of a rainfall times its maximum 30-minute intensity of fall. As durations and intensities of rainstorms vary considerably from year to year, it is essential to consider a sufficiently long time period to obtain a representative R-value.
The K-factor represents the ease with which a soil can be eroded and is influenced by the soil texture, organic matter content, soil structural strength and permeability. K-values therefore need to be determined for individual soil types and can be estimated for known soils using the USDA erodibility nomograph
The L and S factors express the influence of the landscape on soil erosion. The LS combination is the ratio of soil loss from a particular slope to a ‘reference slope (22.1 m length and uniform 9% grade).
The C-factor is the ratio of erosion under a specified cover and management to the amount of erosion under a continuous bare fallow. It considers the type and density of vegetative cover on the soil as well as all related management practices, such as time between operations, weed control, tillage, watering, fertilization, crop residues etc. This factor is obviously very complicated and can only be assessed with confidence for research-verified crop/land management combinations.
The P-factor is the ratio of the erosion resulting from the described practice to that which would occur with up-and-down slope cultivation. It recognizes the influence of conservation practices, such as contour planting, strip cropping, terracing and combinations.