The water supply for the Tucson basin, Arizona, is derived entirely from ground water. The average annual pumpage for 1962-64 was about 165,000 acre-feet and was greater than the natural rate of ground-water recharge. Water-level declines of as much as 70 feet occurred from spring 1940 to spring 1965 as a result of the overdraft. An electrical-analog model of the hydrologic system was constructed to provide a tool for determining the possible future effects of ground-water management schemes. Basic data required for the simulation of the hydrologic system in the model included periodic water-level measurements, determinations of transmissibility, and pumpage and recharge values. The model was analyzed using steady-state and storage-depletion techniques. The steady state analysis served to determine the average annual recharge to the hydrologic system and to verify the pattern of transmissibility. The steady-state analysis indicated that 97,000 acre-feet of water was entering and leaving the ground-water reservoir annually prior to extensive development. The storage-depletion analysis for 1940-64 was made to verify that the model was a valid analog of the hydrologic system and, therefore, could be used for the prediction of future water-level conditions. The storage-depletion analysis indicated areas where some of the basic-data values and (or) the conceptual design of the hydrologic system used in the model were in error. After all the hydrologic variables simulated in the model had been adjusted, the analog model reasonably simulated the historical field data. Based on the assumption that pumpage and recharge would continue at existing rates and locations, the model was then used to predict water-level conditions in spring 1985. The results of the projection indicate a maximum water-level decline of 140 feet for 1940-84. The predicted overall shapes of the cones of depression will remain about the same as in the historical period, except that a large amount of lateral development will take place in all the cones.