A mathematical model for personal computers was prepared as a planning tool for development of micro catchment water harvesting systems. It computes runoff from natural or treated catchments, using estimated or actual parameters. The model also computes the water balance of the soil zone in the cultivated area and the water balance of the reservoir system which serves it. The model was calibrated with hydrolologic data and site characteristics for a location near Tucson, Arizona. Its prediction of cotton and grain sorghum yields was comparable to that of Morin (1977). An attempt was made to use weekly or monthly rainfall data for areas where daily data are unavailable. Lack of direct rainfall and runoff durations and infiltration characteristics made this attempt unsuccessful. This option cannot be used with the model in its current form.