This paper presents an assessment of the potential of Tambakboyo basin as one of the potential new water sources due to the water shortage in some areas of the city of Yogyakarta. It is a retention basin located in the Sleman Regency, Special Province of Yogyakarta, Indonesia. The basin has overflow elevation of +147,00 m and retrieval pipe elevation of +141,00 m. This assessment includes an analysis of the inflow, rain fall on the surface of the reservoir, outflow for water requirements, water loss from evaporation and seepage. By using the scales of water balance and simulation of water retrieval, we are able to determine water potential of the basin for clean water.