The Universal Soil Loss Equation (USLE)

USLE has been designed to predict soil loss from agricultural land under various agroclimatic and topographical situations. Since soil erosion is influenced by many different variables, the essence of USLE is to isolate each variable and reduce its effect to a number, so that when these numbers are multiplied together, the answer is the amount of soil loss. The USLE is an erosion prediction model for estimating long-term averages of soil erosion from sheet and rill erosions from a specified land under specified conditions (Wischmeier and Smith, 1978). The equation (USLE) is presented as below.

A = R × K × L × S × C × P

where, A = soil loss per unit area in unit time, t ha-1 yr -1, R = rainfall erosivity factor which is the number of rainfall erosion index units for a particular location, K = soil erodibility factor - a number which reflects the susceptibility of a soil type to erosion, i.e., it is the reciprocal of soil resistance to erosion, L = slope length factor, a ratio which compares the soil loss with that from a field of a specified length of 22 meters, S = slope steepness factor, a ratio which compares the soil loss with that from a field of the specified slope of 9%, C = cover management factor - a ratio which compares the soil loss with that from a field under a standard treatment of cultivated bare fallow. The cover and management factor to account for the effects of vegetative cover and management techniques for reduction of the soil loss would be equal to 1.0 in the worst case. In an ideal case when there is no sediment loss, C would be zero, and P = support practice factor - a ratio of soil loss with support practice like contouring, strip cropping, or terracing to that with straight row farming up and down the slope.

The factors L, S, C, and P are each dimensionless ratios which allows comparison of the site for which soil loss is being estimated with the standard conditions of the database. Knowing the values of rainfall erosivity, soil erodibility and slope one can calculate the effectiveness of various erosion control measures with the purpose of introducing a cultivation system in an area with soil loss limited to the acceptable value.

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