Nearly half of the water used in Santa Clara County is pumped from groundwater, one of the county's greatest natural resources. Groundwater is pumped by local water retailers, companies, and individual well owners to serve many beneficial uses, including municipal and domestic needs, agriculture, and industry. Since the 1930s, the district has worked to protect and augment groundwater supplies through the coordinated use of surface water and groundwater.
Although groundwater is replenished naturally by rainfall and other sources, the amount of groundwater pumped far exceeds natural recharge. To help offset groundwater pumping and prevent groundwater overdraft, salt water intrusion, and land subsidence, the district uses local and imported surface water to replenish groundwater through district recharge facilities, including recharge ponds and creeks. The district's managed recharge of local and imported water accounts for the majority of groundwater used in the county. Just as important as direct recharge are the district's "in-lieu" recharge programs, including treated water deliveries, water conservation, and water recycling. These activities indirectly maintain groundwater supplies by reducing demands on the groundwater subbasins.
The county's groundwater subbasins serve several important functions in that they transmit, filter, and store water. Water from the district's recharge program and from rainfall enters the subbasins through recharge areas and undergoes natural filtration as it is transmitted into deeper aquifers. The groundwater subbasins also have vast storage capacity, which allows the district to store excess water in normal and wet years. This stored water serves as the county's best protection against droughts or other outages.