Geographic Variation of USLE/RUSLE Erosivity and Erodibility Factors

There is a great reliance on the Universal Soil Loss Equation (USLE) and the Revised Universal Soil Loss Equation (RUSLE) to predict long-term rainfall erosion in relation to land management in both the United States and the rest of the world. In the USLE/RUSLE modeling approach, geographic variations in soil loss associated with climate and soil are taken into account through variations in the rainfall-runoff erosivity factor ($R$) and the soil erodibility factor ($K$) and equations developed in the United States to predict the values for these factors are often used outside the United States without validation in the area that they are applied. Data on rainfall kinetic energy required to calculate the event erosivity index, $E{I}_{30}$, are seldom directly available and consequently, $E{I}_{30}$ values are often calculated using rainfall intensity-kinetic energy relationships that may produce erroneous results because the rainforming systems at a location differ to a great extent from those occurring at the location where the drop-size data were obtained. Also, failure to consider runoff as an independent factor in producing rainfall erosion in the USLE/RUSLE leads to spatial variations in $K$ that cannot be determined simply from measured soil properties. Erosivity indices that include runoff as a factor may lead to soil erodibility factors that are better related to measured soil properties but that potential has yet be realized. Meanwhile, models like RUSLE2 take account of the runoff effect on soil loss through temporal variations in $K$ that are empirically related to rainfall and temperature in the United States but the method is untested elsewhere. The modeling approaches adopted USLE/RUSLE and RUSLE2 are extremely useful in determining soil loss for land management purposes despite any real or perceived lack of ability to predict event soil loss accurately.