Predicting event soil loss from bare plots at two Italian sites

Including runoff in USLE-type empirical models is expected to improve plot soil loss prediction at the event temporal scale and literature yields encouraging signs of the possibility to simply estimate runoff at these spatial and temporal scales. The objective of this paper was to develop an estimating procedure of event soil loss from bare plots (length = 11–44 m, slope steepness = 14.9–16.0%) at two Italian sites, i.e. Masse, in Umbria, and Sparacia, in Sicily, having a similar sand content (5–7%) but different silt (33% at Sparacia, 59% at Masse) and clay (62% and 34%, respectively) contents. A test of alternative erosivity indices for the Masse station showed that the best performances were obtained by the USLE-MM, originally developed at the Sparacia station. This model includes an erosivity index equal to a power (exponent > 1) of the runoff ratio, QR, times the single-storm erosion index, EI30. The fitted models at the two stations were found to coincide for highly erosive events (i.e., QREI30 > 6 MJ mm ha−1 h−1). A parallelism of the two models was detected for low erosivity events. In conclusion, runoff was an important predictor of soil erosion, and local soil characteristics had a more noticeable effect on soil loss for events with a relatively low soil detachment and transport ability. A wider applicability of the USLE-MM scheme can be suggested than for the Sparacia station alone. Additional testing of the model in other environments and development of accurate procedures to estimate plot runoff coefficient at the event temporal scale are required.