Searching for new water supply alternatives for rural households and isolated communities, a research about the availability of rainwater over a region of the Colombian Andes is performed, in which a new methodology is proposed. Based mainly on the availability of information, a geographic context is chosen and a description of the rain field and annual cycle is made. The methodology to evaluate the feasibility (relative to the amount available) of the water supply systems, is based on the Normal Operation Rule to regulate a percent of the daily average precipitation over 1 m 2 of horizontal catchment area. It allows to find the volume of reservoir required to guarantee a daily amount of water with confidence a. This procedure obeys to an iterative completing process of calculus. Non­dimensional regulation curves are constructed and analysis of the system operation is presented. Finally an application of this methodology is fulfilled to find the volume of reservoir required for an especial condition with historical rain information.