In this paper, the modeling is based on division of the catchment into a grid mesh. Each cell has a unique response function independent of the functioning of other cells Summation of the flow responses from the cells, result the flow hydrograph from this area basin.A method is presented to simulation the flow hydrograph within a river basin using the hydrological model WetSpa. WetSpa is a GIS-based distributed hydrological model, which operates on catchment scale and developed for flood prediction and watershed management. The model is physically based and simulates hydrological processes of precipitation, snowmelt, interception, depression, surface runoff, infiltration, evapotranspiration, percolation, interflow, groundwater flow, etc. continuously both in time and space, for which the water and energy balance are maintained on each raster cell. The model combines topography, landuse and soil maps, and observed daily meteorological time series to predict discharge hydrographs and the spatial distribution of hydrological parameters in the catchment. The digital maps of topography, land use and soil type are 3 base maps used in the model in GIS form using 100×100m cell size. The model was applied on the Dinvar Watershed in the Karkheh river basin of Kermanshah, with 72 months of meteo-hydrological measurements. Results of the simulations show a good agreement between calculated and measured hydrographs at the outlet of the basin. The model predicts the daily hydrographs with a good accuracy, between 66% during the calibration according to the Nash-Sutcliff criteria and 72% during validation period by Kling-Gupta Efficiency. تفاصيل المقالة