Flood management needs to identify and prioritize of watershed flood source area. The most studies in the field of flood management are based on the application of the integrated-conceptual models. These models have not appropriate precision to identify watershed-wide flood source area and moreover they have restriction to investigate the spatial distribution of effective components in runoff generation. The main aim of this study is to determine the severity of flooding and priority determination of flood generation potential area based on unit response approach using Mod-Clark distributed (spatial) hydrological model for Qaranqu watershed. In this study, digital elevation model (DEM) with a cell size of 30 m was prepared in the GIS using the topographic maps. Furthermore the watershed gridded file, based on the Standard Hydrologic Grid (SHG), was created using HEC-GeoHMS as input data for the HECHMS model. In order to choose the suitable grid scale, two different resolutions of 1*1 and 2*2 kilometers were utilized. Although the results revealed that the 1*1 km grid scale could more accurately specifies the boundaries of flood area source, there is no significant difference between results in comparison to 2*2 km grid. Outcomes of the current study suggest 1*1 km resolution for priority determination of the flood source area. By iterative execution of Mod-Clark model for different scenarios, flooding severity index of the basin was spatially determined. Based on the flooding severity index the Almalochai sub-watershed was recognized as the most flood source areas from point of flood peak flow generation view.