Water from the Sierra Nevada, local reservoirs and streams and groundwater are the primary sources of supply in Santa Clara County. Nearly half is from local groundwater aquifers, but more than half is imported.
The district imports water in the Bay Area because there is not enough local supply to serve the growing population. The Bay Area is a semi-arid region with a limited annual rainfall of 10 to 20 inches per year, often less. In comparison, the average precipitation (including snow) in the Midwest is 100 to 200 inches per year.
California has the largest water transport system in the world. Most of the state, except the northernmost areas, imports water. Santa Clara County's imported water comes from the Sierra Nevada mountains via the Sacramento-San Joaquin River Delta. This water is delivered by both the State Water Project (SWP) and the federal Central Valley Project (CVP).
Imported water is conveyed to the county through three main pipelines: the South Bay Aqueduct, which carries water from the SWP, and the Santa Clara Conduit and Pacheco Conduit, which bring water from the CVP.
The San Francisco Water Department conveys water from the Hetch-Hetchy reservoir into Santa Clara County through its own facilities.