Simulated Effects of Ground-Water Management Alternatives for the Salinas Valley, California

A two-dimensional digital ground-water flow model was developed to analyze the geohydrology of the ground-water basin in the Salinas Valley. The ground-water model was calibrated for steady-state and transient simulations by comparing simulated with measured or estimated inflows, outflows, and water levels for 1970-81. Preliminary estimates of hydraulic properties and some inflows and outflows were adjusted during model calibration. The simulated mean annual water budget for the basin was 559,500 acre-feet per year each of outflow and inflow. Inflow components consisted of Salinas River recharge (38.3 percent), percolation of irrigation water (34.0 percent), small stream and Arroyo Seco recharge (20.9 percent), seawater intrusion (3.4 percent), and other sources (3.4 percent). Outflow components consisted of agricultural pumpage (91.5 percent), municipal pumpage (4.0 percent), and riparian phreatophyte evapotranspiration (4.5 percent).

For the steady-state calibration, 70 percent of the simulated water levels were within 9 feet of measured water levels for 1970-81. A sensitivity analysis determined the overall stability of the model results. The model input variable that probably contributes most to the uncertainty of the results is the quantity of ground-water recharge contributed by irrigation return flow to the unconfined aquifer. A 15-percent change in the estimate of this variable causes an 11-percent change in the simulated river-seepage rate and a 6-percent change in the simulated seawater-intrusion rate.

The calibrated model was used to investigate several water-resources management alternatives. Projected pumpage increase at a rate of 1 percent per year for 20 years caused declines in mean annual water levels of 10 to 20 feet in some areas and an increase in seawater intrusion from 18,900 to 23,600 acre-feet per year. Pumpage decreases in the coastal area decreased seawater intrusion more effectively than pumpage decreases farther inland. When pumpage was decreased near the coast, seawater intrusion decreased oneseventh as much. When pumpage was decreased uniformly throughout the valley, the decrease in seawater intrusion was only one-fourteenth the decrease in pumpage. Simulations indicated that replacement of ground-water pumpage with imported surface water in a 9,000-acre service area near the coast would result in a decrease in seawater intrusion equaling nearly onehalf the quantity of imported water. This further confirmed that the rate of seawater intrusion is most sensitive to pumpage near the coast.