iordano G

Estimating the USLE Soil Erodibility Factor in Sicily, South Italy

The Universal Soil Loss Equation (USLE) is used by professionals and technicians to predict soil loss by water erosion and to establish soil conservation measures. One of the key elements of the USLE is the K factor, which is a measure of the soil erodibility. Given the difficulty in collecting sufficient data to adequately measure K, early in the USLEs history the soil erodibility nomograph method was developed to allow estimation of K based on standard soil properties. Since the nomograph approach was developed based on a small number of soils in the United States, it is necessary for other contexts to check the nomographs ability to predict the soils true erodibility. Considering that soil organic matter data are difficult to obtain, an estimation procedure of the soil erodibility factor, K, based only on soil textural data is desirable. In this investigation, the soil erodibility factor was first experimentally determined for the clay soil at the Sparacia (Sicily) experimental station. A relatively low value (0.039 t ha h ha-1MJ-1mm-1) was determined, and summer erodibility was found to be more than twice the value of winter erodibility. This measured K value was 1.85 times the nomograph K, which for many practical applications is not a large difference. Finally, using 1813 data points, a procedure for estimating K using only soil textural data was developed for Sicily. The errors of the predictions did not exceed a factor of two and three for 94.4% and 99.2% of the data points, respectively, suggesting a satisfactory ability of the developed procedure to yield an estimate of K with a reduced input dataset.

MONITORING AND PREDICTING SEDIMENT YIELD IN A SMALL SICILIAN BASIN

Identifying areas of a basin that are most sensitive to erosion have stimulated the study of within–basin variability nof the sediment–delivery processes and the use of spatially distributed models. To verify the reliability of a sediment–delivery ndistributed model applicable at the morphological unit scale (i.e., the area of clearly defined aspect, length, and steepness), nexperiments were carried out at mean annual and event scales in a small Sicilian basin. A Geographical Information System nis briefly presented into which the measurements carried out at the basin outlet (runoff, sediment yield, etc.) and other point nand areal information (soil erodibility, digital terrain model, etc.) were entered. nFor validating the model at mean annual temporal scale, the sediment yield spatial distribution calculated by the model nwas compared with the net soil erosion spatial distribution obtained by cesium–137 activity measurements. At morphological nunit scale, the agreement between measured and calculated sediment yield values showed a good predictive ability of the nmodel at mean annual temporal scale. nFinally, the model was calibrated and tested using five rainfall–runoff events measured at the outlet of the experimental nbasin. The analysis showed that the coefficient x01, appearing in the expression of the sediment delivery ratio of each nmorphological unit, is independent of subdivision criterion and can be estimated by the hydrological characteristics of the nrainfall–runoff event. The comparison between measured and calculated sediment yield values showed that the sediment ndelivery distributed approach has a good predictive ability at event scale, too.