The model HEC-HMS 3.5 has been applied to calibrate (from 2004 to 2008) and validate (from 2009 to 2012) with a daily time step, to offer the chance for describe hydrology in a Mediterranean watershed "Oued Laou" located in northern of Morocco. We based this study on two complementary approaches; observation and trend detection in the site and hydrological simulation using the HEC HMS model, these two approaches are reinforced by remote sensing, also the geographic Information System (GIS) techniques were used to determine the geometric and hydrological parameters required to estimate the parameters of the model. The model performance measured in the output of simulation give satisfactory results with an average Nash greater than 70%. We note that the model tends to underestimate volumes as well as flood peaks. These results lead to a realistic modeling of the hydrological processes in the Laou watershed and could fill a lack of information concerning the hydrological functioning of this basin. This study shows that the hydrological model HEC-HMS 3.5 can be used to model the Laou river basin, also in other similar contexts.