Runoff ratio as a factor in the empirical modelling of soil erosion by individual rainstorms

A number of factors that influence erosion have separate and differing effects on flow discharge and sediment concentration, depending on local conditions. Empirical erosion models that do not have mechanisms to help account for these separate and differing effects often lack the capacity to predict event erosion adequately in many locations. In this paper, the product of the EI(30) index, the erosivity index used in the Universal Soil Loss Equation (USLE) and the revised version (RUSLE), and the runoff ratio (Q(R)) is discussed in relation to its capacity to act as an event erosivity index where sheet and rill erosion occur either separately or together in a rainstorm. An analysis of runoff and soil loss data shows the index to be superior to the EI(30) index as an event erosivity index for storms on bare fallow plots at Holly Springs, Mississippi. The inclusion of runoff as an independent term in the USLE/RUSLE results in a need to determine new values for the soil erodibility factor, K. Existing USLE/RUSLE equations for determining L and S (topographic factors), C (a crop and crop management factor), and P (an erosion control practice factor) may be used as first approximations provided that the values of the new index are determined for the unit plot condition. Since many of the factors that determine L. S, C; and P influence runoff, new methods to determine these parameters need to be developed in the future if the new index is to De used most effectively.