ESTIMATION OF RUNOFF, PEAK DISCHARGE AND SEDIMENT LOAD AT THE EVENT SCALE IN A MEDIUM-SIZE MEDITERRANEAN WATERSHED USING THE ANNAGNPS MODEL

In this paper, the Annualized Agricultural Non-Point Source (AnnAGNPS) model has been used to estimate runoff, peak discharge and sediment load at the event scale in a Mediterranean watershed. The study area is the Carapelle torrent, Southern Italy (area=506km(2)), where continuous rainfall, streamflow and sediment load data are available. Nineteen flood events have been registered in the period 2007-2009 and were used for the application of the model. The aim of the paper is to evaluate the predictive accuracy of the model at the event scale, in a medium-size watershed, given the specific conditions of the semi-arid environments. A sensitivity analysis has been carried out to assign the correct parameterization: the mean normalized output variation of the most meaningful input parameters pointed out the influence of the curve number on runoff, peak discharge and sediment load predictions (values greater than 1); the MN Manning's roughness coefficient and K, C and P factors of the universal soil loss equation showed a moderate influence on sediment load simulations (values between 05 and 1). The selection of the Soil Conservation Service synthetic storm types has been based on the observed storm events analysis to improve the peak discharge simulations. The model prediction has proved to be good for runoff (R-2=074, NSE=075, W=092) and peak discharge (R-2=085, NSE=070, W=094), and satisfactory for sediment yield (R-2=070, NSE=063, W=091). The relative error is lower for high events; this result is quite interesting in semi-arid environments, where most of the annual sediment yield is concentrated in a few, severe events. Copyright (c) 2013 John Wiley & Sons, Ltd.