The development and the application of rainfall-runoff models have been a corner-stone of hydrological research for many decades. In general, the purpose of the development of these models is two-fold. The first is to advance our understanding and state of knowledge about the hydrological processes involved in the rainfall-runoff transformation. The second is to provide practical solutions to many of the related environmental and water resources management problems. Sinai falls in the arid and semi-arid region, which is characterized by unpredictable rainfall patterns, sometimes of high intensity, short duration and uneven distribution. In Egypt, rainfall distribution varies considerably from one year to another with changeable average annual rainfall volume. Sinai is under going a rising development, and it needs an accurate evaluation of its water resources. Therefore, this study investigates the watershed modeling of different wadis in Sinai with respect to their meteorology, geomorphology, and geology and hydrology. These wadis are wadi Sudr in the Southern West part of Sinai and Wadi AlArbain at South of Sinai. The Watershed Modeling System (WMS, WMS – Hydro, Version 2) has been applied to analyze and simulate the surface runoff storms using HEC-1 model. This model was used to simulate runoff volumes and hydrographs at the two main wadis for different real world measured storms. The digital terrain modeling functions of WMS were used to create terrain models using Geographic Information Systems (GIS) data, and girded Digital Elevation 2 Models (DEMs). These data were used to delineate watersheds, streams and sub-basins. In this study, different unit hydrograph methods, different loss estimation methods and different methods of lag-time computation have been analyzed. The range of the Curve Number (CN) was obtained from standard tables according to the soil type and cover of each basin. Since there are no definite calibration procedures in this software, the method of lag-time (TL) computation was selected according to the lag-time of each storm at each basin; and the CN in order to match the derived hydrograph with the observed one with respect to the volume, the peak, and the time to peak.