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Sediment transport in rivers is an indicator of soil eroded from various sediment sources, of which agricultural land can be a significant one, and the intensity of the phenomenon provides a measure of land degradation at a watershed level. The use of distributed models to solve problems in water resources applications including environmental impacts of land-use changes, effects of climate change on water resources, and water planning and management in a catchment, is quite complex in large Mediterranean catchments for the lack of sufficient data to characterize spatial variability, for the integration of field measurements and model parameter element, and for the imperfect representations of the real processes. In this paper the AnnAGNPS model has been used to estimate runoff, peak discharge and sediment load at the event scale in a semi-arid, medium-size watershed. The study area is the Carapelle torrent (Southern Italy), where continuous rainfall, streamflow and sediment load data are available. Nineteen flood events have been registered in the period 2007-2009 and selected for the application of the model. The aim of the paper is the evaluate the predictive accuracy of the model, given the specific conditions of the semi-arid environments. Particular attention has been focused on the definition of the local crop parameters, of the management scheduling features and to the influence of the synthetic storm distributions on peak discharge predictions. The sensitivity analysis for the most meaningful parameters has been carried out to evaluate the weight of each factor and to assign the correct parameterization avoiding a long sequence of proves to calibrate the model. The SCS Type I was found to be the suitable storm distribution as it provided the highest values of the statistical parameters used to evaluate the performance of the model. The model predictions have proven to be good for runoff and peak discharge and satisfactory for sediment yield. The relative error is lower for high events, this result being quite interesting in semi-arid environments, where most of the annual sediment yield is concentrated in few, severe events.
