Water erosion is a serious threat to the environment, sustainability of agriculture and socio-economic development. It is widespread in the Mediterranean regions, particularly in Tunisia. The upstream watershed of the Nebhana dam (855 km2), located in central Tunisia, is among the regions most affected by this global issue. The objective of this study is to estimate and spatially assess water erosion in the basin, produce an erosion risk map and identify priority areas to facilitate the intervention of decision makers and managers in developing appropriate policies for conserving soils and protecting the upstream Nebhana-dam against sedimentation. The adopted methodology uses the Revised Universal Soil Losses Equation (RUSLE) and the MEDALUS approach in a GIS environment. The obtained results showed that RUSLE model was efficient in quantifying the average soil loss in the basin, given that estimated value (10.94 t/ha/year) was very close to the value obtained from bathymetric measurements in the dam reservoir (11.25 t/ha/year). Furthermore, RUSLE estimated that 34% of the total area is suffering from an erosion risk exceeding the soil loss tolerance which is in agreement with results found by MEDALUS approach revealing that 30% of the total area were vulnerable to erosion and were considered as areas with highest priority for land management intervention.