Applying the QREI30 index within the USLE modelling environment

The USLE/RUSLE model was designed to predict long-term (~20years) average annual soil loss by accounting for the effects of climate, soil, topography and crops. The USLE/RUSLE model operates mathematically in two steps. The first step involves the prediction of soil loss from the 'unit' plot, a bare fallow area 22.1m long on a 9% slope gradient with cultivation up and down the slope. Appropriate values of the factors accounting for slope length, gradient, crops and crop management and soil conservation practice are then used to adjust that soil loss to predict soil loss from areas that have conditions that are different from the unit plot. Replacing EI ₃₀, the USLE/RUSLE event erosivity index, by the product of the runoff ratio (Q _R) and EI ₃₀, can enhance the capacity of the model to predict short-term soil loss from the unit plot if appropriate data on runoff is available. Replacing the EI ₃₀ index by another index has consequences on other factors in the model. The USLE/RUSLE soil erodibility factor cannot be used when the erosivity factor is based on Q _REI ₃₀. Also, the USLE/RUSLE factors for slope length, slope gradient crops and crop management, and soil conservation practice cannot be used when runoff from other than the unit plot is used to calculate Q _R. Here, equations are provided to convert the USLE/RUSLE factors to values suitable for use when the erosivity factor is based on the Q _REI ₃₀ index under these circumstances. At some geographic locations, non linear relationships exist between soil loss from bare fallow areas and the Q _REI ₃₀ index. The effect of this on the slope length factor associated with the Q _REI ₃₀ index is demonstrated using data from runoff and soil loss plots located at the Sparacia site, Sicily.