Scale effect in USLE and WEPP application for soil erosion computation from three Sicilian basins

Accurate estimation of soil erosion due to water is very important in several environmental contexts, such as the assessment of potential soil loss from cultivated lands and the evaluation of the loss of water storage capacity in reservoirs due to sediment deposition. Several studies have been carried out to build models suitable to quantify the results of erosion processes. These models, calibrated from experimental studies on plots or fields, have been applied at quite different scales. The aim of this paper was to present the results of the application of two soil erosion models, both spatially distributed, to three large Sicilian basins upstream of reservoirs. Each basin was subdivided into hillslopes, using three different classes of average area, in order to estimate the scale effect on the sediment yield evaluation. The first model was the empirical Universal Soil Loss Equation (USLE), and the other one was the physically based model of the Water Erosion Prediction Project (WEPP). A Geographical Information System was used as a tool to handle and manage data for application of the models. Computed sediment yields were compared with each other and with measurements of deposited sediment in the reservoir, and for these cases the WEPP estimates better approximated the measured volumes than did the USLE. Neither model appeared to be particularly sensitive to the size area of the hillslopes, at least within the range of values considered. This suggests that a finer subdivision, although it may better define the experimental conditions (plot or field areas) for calibration of models, may not result in a better estimate of erosion.