The first step in any rainwater harvesting system involves methods to increase the amount of water stored in the soil profile by trapping or holding the rain where it falls. This may involve small movements of rainwater as surface runoff in order to concentrate the water where it is wanted most. This paper presents a geographic information system (GIS) methodology based on a decision support system (DSS) that uses remote-sensing data, filed survey, and GIS to delineate potential in situ rainwater harvesting areas. The GIS-based DSS implemented as well as evaluated the existing rainwater harvesting structures in the study area. The input into the DSS included a map of rainfall surplus, slope, potential runoff coefficient (PRC), land cover/use, and soil texture. The outputs were map showing potential sites for in situ water harvesting (IWH). The spatial distribution of the suitability map showed that 1.5 and 27.8 % of the study area have excellent and good suitability for IWH, relatively, while 45 % of the area has moderate suitability. Validation of the existing IWH structures was done during a field survey using collected data and the suitability map. The validation depends on comparing rainwater harvesting/recharge dam's locations in the generated suitability map and the location of the surveyed IWH structures using the proximity analysis tool of ArcGIS 10.1. From the proximity analysis result, all the exiting IWH structures categorized as successful (99 %) were within the good suitable areas.