Rain and runoff have a decisive influence on the transport and deposition of sediment detached or under the impact of raindrops or by runoff exerts shear stress on the ground. In this study, we try to characterize the risk of erosion associated with rainfall data from the rainfall station Mostaganem (West Algeria). This will explain the importance and variability of water erosion of soils in the study area. We calculate its index by various methods namely Fournier, Arnoldus and Deffontaines with series of data collected during the period from 2000 to 2010.