In the light of the current situation of water resources in Egypt, where it has a deficit in those resources, the rainwater harvesting and assessment of its potentialities became necessary. In the present study, an attempt is made to define a decision based on scientific approach for identifying the most appropriate sites for rainwater harvesting. This approach is based on GIS data layers which include DEM, landforms, geologic setting, watershed area, rainfall amounts, drainage lines, morphometric parameters, flow accumulation, flow direction, slope, and surface runoff. It also includes a verification of the site suitability through field investigations. El Daba'a area has been chosen because it has a lack of water for agriculture and drinking purposes. In addition, this area has special interest because of a planned nuclear power plant there. The obtained results reveal that the study area contains eight basins which have slightly intensive drainage network. The morphometric analyses of the studied basins reveal their opportunities for surface runoff. Three of them are showing high hazard degrees. These basins have actual runoff (Q) ranges between 16.5 and 25 mm annually resulted from average rainfall of 164 mm. Therefore, management of rainwater harvesting and protections from flash floods were recommended. Many rainwater harvesting structures, for example concert dams, alternative earth dikes, surface water reservoirs, and cisterns, should be established.