Universal Soil Loss Equation (USLE)

The Universal Soil Loss Equation (USLE) predicts the long-term average annual rate of erosion on a field slope based on rainfall pattern, soil type, topography, crop system and management practices. USLE only predicts the amount of soil loss that results from sheet or rill erosion on a single slope and does not account for additional soil losses that might occur from gully, wind or tillage erosion. This erosion model was created for use in selected cropping and management systems, but is also applicable to non-agricultural conditions such as construction sites. The USLE can be used to compare soil losses from a particular field with a specific crop and management system to "tolerable soil loss" rates. Alternative management and crop systems may also be evaluated to determine the adequacy of conservation measures in farm planning.

The USLE model comprises five parameters: rainfall erosivity factor, soil erodibility factor, slope length factor, slope gradient factor, vegetation cover and management factor, and support practice management factor. The accuracy of USLE estimation is dependent on the spatial resolution of the input data. The USLE model is expressed by Equation:

A = R x K x LS x C x P  

þ  A represents the potential long-term average annual soil loss in tonnes per hectare (tons per acre) per year. This is the amount, which is compared to the "tolerable soil loss" limits.

þ  R is the rainfall and runoff factor by geographic location. The greater the intensity and duration of the rain storm, the higher the erosion potential. R factor can be calculated by the formula of Lo et al. (1985) for application:

R = 38.46+3.489P

where R is the annual mean rainfall erosivity (N h^-1 yr^-1) and P is the average annual precipitation (cm).

This equation is generalized by Kenneth & Jeremy (1994). After changing the unit of mean annual precipitation to mm yr-1 and the unit of R by multiplying by 10 to obtain SI units (MJ mm ha-1 h-1yr-1), Eq. (2) is simplified as follows:

R = 38.46+0.35P

where R is the rainfall erosivity (10 MJ mm ha-1 h-1yr-1) and P is the mean annual precipitation (mm yr-1). This equation is considered an appropriate estimator of rainfall erosion in tropical or subtropical climate regions (Eiumnoh, 2000).

þ K is the soil erodibility factor. It is the average soil loss in tonnes/hectare (tons/acre) for a particular soil in cultivated, continuous fallow with an arbitrarily selected slope length of 22.13 m (72.6 ft) and slope steepness of 9%. K is a measure of the susceptibility of soil particles to detachment and transport by rainfall and runoff. Texture is the principal factor affecting K, but structure, organic matter and permeability also contribute.

þ  LS is the slope length-gradient factor. The LS factor gives the effects of topography, such as the length and steepness of slopes, which are closely related to the amount of soil erosion. It has been shown that the steeper slope is, the higher the velocity of overland flow, which increases soil loss. is used to calculate the LS factor (Mitasova & Mitas, 1999) :

where LS is the slope-length and steepness factor (no unit), θ is the slope angle (degree), Flow Accumulation is used to integrate flow direction in the calculated LS, and Cellsize is the DEM resolution. The following picture is an example of Annual Average Precipitation and DEM in Mekong River Basin.

 þ  C is the crop/vegetation and management factor. It is used to determine the relative effectiveness of soil and crop management systems in terms of preventing soil loss. The C factor is a ratio comparing the soil loss from land under a specific crop and management system to the corresponding loss from continuously fallow and tilled land. The C factor resulting from this calculation is a generalized C factor value for a specific crop that does not account for crop rotations or climate and annual rainfall distribution for the different agricultural regions of the country. This generalized C factor, however, provides relative numbers for the different cropping and tillage systems, thereby helping you weigh the merits of each system.

þ  P is the support practice factor. It reflects the effects of practices that will reduce the amount and rate of the water runoff and thus reduce the amount of erosion. The P factor represents the ratio of soil loss by a support practice to that of straight-row farming up and down the slope. The most commonly used supporting cropland practices are cross-slope cultivation, contour farming and strip cropping

Support Practice	P Factor
Up & down slope	1.0
Cross slope	0.75
Contour farming	0.50
Strip cropping, cross slope	0.37
Strip cropping, contour	0.25

This figure shows an example of result of soil erosion in Mekong River Basin. 

The result mentioned that the 3S sub-basin has high soil erosion risk.

by Sophal Try
Please visit our Facebook page: Water Resources and Disaster Management
Credit to Hoang Thu Thuy for figures in case study of Mekong River Basin

Reference:

Eiumnoh, A (2000), “Integration of Geographic Information Systems (GIS) and Satellite Remote Sensing (SRS) for Watershed Management”, Technical Bulletin 150. Food & Fertilizer Technology Center, Taiwan.

FAO Proceedings of the validation forum on the Global Cassava development strategy (2000). “Strategic environmental assessment : An assessment of the impact of cassava production and processing on the environment and biodiversity”, Vol. 5, Food and Agriculture Organization of United Nations, International Fund for Agriculture Development.

Kenneth G. Renard, and Jeremy R. Freimund (1994). “Using Monthly Precipitation Data to Estimate the R-Factor in the Revised USLE”, Journal of Hydrology, Vol.157, pp. 287-306.

Lo A, El-Swaify S.A, Dangler E.W, and Shinshiro L (1985). “Effectiveness of Ei30 as an Erosivity Index in Hawaii”, Soil Erosion and Conservation, El-Swaify S.A., Moldenhauer W.C. & Lo A. (eds), Soil Conservation Society of America, Ankeny, Iowa, pp. 384-392.

Mitasova,H and Mitas.L (1999). “Modeling Soil Detachment with Rusle 3d Using GIS”, University of Illinois at Urbana-Champaign