Assessment of the flash flood hazard in arid regions is a serious challenge, especially as the climate changes. The Kingdom of Saudi Arabia lies in one of the most arid regions of the world and is known for recurring flash floods in many basins, particularly in the southwest. Many dams have been constructed in some basins; however, flash floods sometimes cause catastrophic damage for mankind and infrastructure when the floods' intensity and capacity are higher than those the dams and reservoirs were intended to contain. This study aims to assess runoff and reservoir capacity of the Jazan Basin in different return periods of rainfall events based on integrations between two models deals with morphometric parameters and hydrologic characteristics. These models are, Watershed Modeling System (WMS) and the Hydrological Modeling System (HEC HMS) in addition to Geographic Information Systems (GIS). Our results show that the dam peak storage in the condition of zero and full reservoir storage ranges from 55 x 10(6) to 98 x 10(6) m(3) and from 84 x 10(6) to 124 x 10(6) m(3), respectively. The corresponding area of the dam reservoir (lake) in all scenarios ranges from 9 to 11.3 km(2), respectively, for 5 and 100 year return periods. Sub-basins 2 and 3 contribute about 56% of the total runoff volume of Jazan Basin, while Sub-basins 1, 4, and 5 contribute about 44%. These results highlight the value of using an integrated modeling approach to assess and mitigate flash flood hazards in arid environments worldwide.