The California State Water Resources Control Board (SWRCB) conducted an extensive investigation on water supply in Santa Cruz and Monterey Counties in 1953. It was concluded that the Pajaro Valley groundwater basin was in a state of overdraft causing saltwater intrusion. By the 1970's,groundwater levels in Watsonville were below sea level the majority of the year. In 1980, the SWRCB identified the Pajaro Valley basin as one of eleven California basins with critical conditions of overdraft. By 2000, 54 square miles of the basin were below sea level.
Overdraft is caused by pumping more groundwater than is naturally put in. Incoming sources of water that recharge the basin include rainfall, snowmelt, rivers, and lakes. One result of the imbalance of input v. output of water is the reversal of the natural flow of the basin. Saltwater begins to replenish the basin instead of freshwater.
The PVWMA estimates the current saltwater intrusion rate to be 100-250 ft/yr and with its effects already extending several miles inland. The SWRCB estimated a loss of 300,000 af (acre-feet) of freshwater storage in the basin from 1964-1997, with approximately 200,000 af of loss due to saltwater intrusion and 100,000 af due to chronic overdraft.
PVWMA groundwater modeling has indicated that a sustainable yield for the Pajaro Valley groundwater basin to be 24,000 af/yr. However, the sustainable yield can be increased to 48,000 af/yr if coastal pumping is eliminated and the groundwater basin is replenished with a different source. In 2008, 62,000 af were pumped and if recent trends continue, the volume of pumped water will continue to increase in the future (see Pajaro Valley Water Usage and Precipitation Table). The PVWMA estimates that by 2025, the water demand for its service area will be 76,900 AFY (acre-feet per year).
Pajaro Valley Hydrologic Model Base-Case Scenario
See PVWMA Hydrologist Brian Lockwood's presentation to the Board of Directors regarding a new hydrologic model for the Pajaro Valley as presented March 23, 2011.