Together with the rainfall–runoff erosivity factor, topography is a key factor on A 987306 erosion. In fact, as slope gets steeper, the higher is the velocity of overland flow, thus increasing the shear stress on the soil particles. Furthermore, as slope length increases, the overland flow and flow velocity also progressively increase, leading to greater erosion forces applied to the soil surfaces (Alexakis et al., 2013 and Ranzi et al., 2012). Both the RUSLE3D and USPED take into account the role of topography on soil erosion by means of the LS-factor, which is sliding filament model related to the slope length and the slope gradient. In particular, both models replace the slope length by upslope contributing area per unit contour width (Eq. (3)) ( Mitasova et al., 1996, Mitasova and Mitas, 1999a, Mitasova and Mitas, 1999b and Moore and Burch, 1986). The upslope contributing area was computed using a DTM of the watershed through the D8 algorithm implemented in ArcGIS Spatial Analyst extension ( Jenson and Domingue, 1988 and Tarboton et al., 1991).