The GIS-based distributed hydrological model, WetSpa, whose flow routing method is described in this paper is suitable for flood prediction and watershed management on catchment scale. The model predicts outflow hydrographs at the basin outlet or at any converging point in the watershed, and it does so at a user-specified time step. The model is physically based, spatially distributed and timecontinuous. This paper focuses on the GISbased diffusive transport approach for the determination of rainfall runoff response and flood routing through a catchment. The watershed is represented as a grid cell mesh, and routing of runoff from each cell to the basin outlet is accomplished using the first passage time response function based on the mean and variance of the flow time distribution, which is derived from the advection-dispersion transport equation.The flow velocity is location dependent and calculated in each cell by the Manning equation based on the local slope, roughness coefficient and hydraulic radius. The hydraulic radius is determined according to the geophysical properties of the catchment and the flood frequency. The total direct runoff at the basin outlet is obtained by superimposing all contributions from every grid cell. The model is tested on the Ziarat _Gorgan watershed with 4 years of observed hourly rainfall and discharge data, and the results are in excellent agreement with the measured hydrograph at the basin outlet