Measuring the USLE soil erodibility factor in the unit plots of Sparacia (southern Italy) experimental area

The Universal Soil Loss Equation (USLE) is still widely used to predict soil loss by water erosion and to establish soil conservation measures. In this model, the soil erodibility factor $K$ accounts for the susceptibility of the soil to be eroded due to the detachment and transport processes operated by the erosive agents. According to the USLE scheme, the $K$ factor should be measured on unit plots, i.e., bare plots of given length (22 m) and steepness (9%) tilled along the maximum slope direction, but there is little evidence that there ever existed an actual unit plot between the plots used to develop the USLE. Given the difficulty in collecting sufficient data to adequately measure $K$., the nomograph method was early developed to allow estimation of $K$ based on standard soil properties. First, in this investigation the soil erodibility factor was experimentally determined for the clay soil of the Sparacia (Sicily) experimental station, based on the available measurements collected in two unit plots. Although a limited database was available for this analysis, a very low value (0.0038 t ha h ha−1 MJ−1 mm−1) was determined, which was an order of magnitude lower than the nomograph value. Then, the values of the plot steepness factor $S$ were determined using soil loss measurements collected on plots varying in steepness from 9 to 26% and resulted higher than the estimated values by a well-known literature expression. Finally, the plot length factor $L$ resulted independent of the plot length and equal to one. The former result was explained by the different flow transport capacity in the unit plot and plot with increased steepness, while the result of a constant length factor was supported by other experimental investigations.