Utilization of ground water in the Santa Maria Valley area, California

Overdraft in the Santa Maria Valley ground-water basin since about 1946 has resulted in a significant decline in water levels throughout the basin as ground water has been removed from storage. In 1959 approximately 2,200,000 acre-feet of ground water was in storage above sea level in the ground-water reservoir. Estimates of storage depletion are not consistent with estimates of groundwater recharge and discharge. The natural perennial yield of the basin probably is about 50.000 acre-feet, on the basis of estimated recharge and natural discharge. The augmented perennial yield probably is about 70,000 acre-feet and includes 21,200 acre-feet of water per year released at Twitchell Dam. Storage depletion, not estimated in the seaward ends of the aquifers, will result as the fresh watersea water interface moves landward in response to the continuing decrease in hydraulic gradient in the aquifer system.

Evidence of sea-water intrusion into the basin has not been observed, but limited sea-water encroachment may have occurred at the offshore ends of the aquifers. Additional observation wells will be necessary to provide supplemental data to insure that hydraulic heads and gradients in the deeper aquifers are properly monitored.