This method was developed primarily as a practical solution to the problem of reservoir yield determination in arid areas where there is a paucity of hydrological and meteorological data. By examination of the physical characteristics of the catchment, surface runoff is related to rainfall and catchment wetness. The latter is determined by the quantity of water held in the 'soil cover reservoir', the area of which extends over the entire catchment and the depth of which is defined by the average depth to which evapotranspiration effects the depletion of this reservoir. Results show that short-term runoff records can be extended with confidence. The extended record is used in a simple and rapid method to determine the optimum design of a reservoir.