Hydrologic, digital-model, and economic analyses were made to determine the most efficient balance of conjunctive use of local ground water and surface water--specifically, whether additional ground-water supplies can be developed in the Oak Glen study area, San Bernardino County, Calif., for local use and also for export to the adjacent Yucaipa area, and what will be the effects of imported water available in 1980.

The hydrologic analysis showed that transmissivity values of the aquifer in the Oak Glen study area ranged from 1,000 to 6,750 gallons per day per foot or their equivalent 134 to 902 feet squared per day (12 to 84 metres squared per day) and that net annual recharge in the area was about 1,940 acre-feet (2.39 cubic hectometres) per year. The volume of ground water in storage in 1970 was about 86,000 acre-feet (106 cubic hectometres).

The digital-model analysis included building and verifying a steady-state and a non-steady-state model. The steady-state model was considered to be verified when the model-generated water levels approximated the measured 1949 water levels, which were assumed to represent steady-state water-level conditions. The non-steady-state model was verified for 1949-70, and the model satisfactorily reproduced the measured water-level changes for the study area during that period. Water-level changes in the ground-water basin were predicted from 1971 to 1980 using as representative average annual pumpage that from 1966 to 1970 and 1971. The model also predicted water-level changes from 1971 to 1980 using the maximum pumpage capacities of the wells under recharge conditions for average, wet, and dry periods.

The predicted water-level changes in the Oak Glen study area were used to calculate the average costs of pumping water for 1980. The comparable average costs of importing water from an alternative supply and the capital costs of well construction were also determined. The economic evaluation suggests the following: (1) Increase ground-water pumping for local use, (2) reduce well-water import to the lower parts of the Oak Glen study area from adjacent areas to the west, (3) short term prior to 1980 overdevelopment appears to be feasible with export to Yucaipa, and (4) tunnel development or new shallow wells at the higher altitudes could help alleviate the water problem.