**4.10 Universal soil loss equation for water erosion**

The universal soil loss equation (USLE) was given by Wischmeier and Smith (1978) based on the soil erosion causing factors [24].

$$\mathbf{A} = \text{RKL SCP} \tag{1}$$

where A, mean annual soil loss (metric tons hectare<sup>−</sup><sup>1</sup> year<sup>−</sup><sup>1</sup> );

*R*, rainfall erosivity factor;

*K*, soil erodibility factor

*L*, slope-length factor

*S*, slope-steepness factor;

*C*, cover and management factor;

*P*, support practice factor.

Among the above-listed factors, vegetation and to some extent soil can be managed to reduce the rate of the soil erosion but the climatic and topographic factors, except slope length, are not manageable. Primarily, soil loss through erosion is a function of erosivity of raindrops and erodibility of the soil which can be mathematically expressed as follows:

$$\text{Erosion} = f \left( \text{Erosivity}, \text{Eroability} \right) \tag{2}$$

where Erosivity is the potential of rainfall to cause erosion under given soil type and climatic condition; Erodibility is the vulnerability or susceptibility of the soil to erosion which depends on soil bio-physico-chemical properties, and land use and crop management practice. Sandy soils can be easily detached while well aggregated clayey soils are more resistant to erosion than sandy soils. When clay particles detached they can be easily removed by runoff due to their smaller size. Silt soils are the most erodible type of soil [9].
