A definition of the hydrologic system of the upper San Pedro basin was obtained by developing a numerical ground-water model to evaluate a conceptual model of the system. Information on hydraulic properties of the basin fill, recharge from bordering mountain ranges, discharge by evapotranspiration, and exchange of water between aquifer and stream was available from previous measurements or estimates. The steady-state calibration procedure and subsequent transient simulations demonstrated that the original conceptualization can be reasonably simulated. An analysis of model sensitivity to increases and decreases in certain hydraulic properties indicated a low sensitivity to aquifer anisotropy and a low to moderate sensitivity to stream leakance and evapotranspiration rate. An analysis to investigate the effects of generalizing aquifer conductivity and recharge showed that flow components and water-level response to stress could be simulated adequately but that steady-state water-level conditions could not. During equilibrium conditions, recharge to and discharge from the basin was about 16,500 acre-feet per year. Modeling results indicated that by 1978 the storage depletion rate had reached 5,600 acre-feet per year resulting from a ground-water withdrawal rate of 10,500 acre-feet per year. (USGS)
