Lack of sufficient good quality water to fulfill the rural population's needs in many countries, such as Mexico, has become one of the main issues governments have to cope with to provide adequate living conditions. The situation has worsened due to rainfall pattern variability influenced by climate change. Water harvesting is an ancient practice that provides an additional water supply for different uses. The lack of sufficient readily available decision tools within these environments has hindered adequate decision-making. The computer aid presented in this paper serves the purpose of a decision tool for use in marginal rural areas where a water harvesting system is to be implemented. Input variables are easily obtained and the system has preloaded information for use in the water balance. The interface is user-friendly, enabling users to plug local information in to obtain outputs on which to base a decision. Input variables can be changed as needed to achieve an acceptable output. The system generates rainfall amounts stochastically enabling posterior analysis to be used in terms of designing roof water harvesting under different runoff occurrence probabilities.