More than half of Santa Clara County’s water supply comes from hundreds of miles away - first as snow or rain in the Sierra Nevada range of northern and eastern California, then as water in rivers that flow into the Sacramento-San Joaquin River Delta .
Often called “imported water,” it is brought into the county through the complex infrastructure of the State Water Project , the federal Central Valley Project , and San Francisco’s Hetch Hetchy system.
- Imported water meets about 55 percent of Santa Clara County’s water needs.
- Most of that (about 40 percent in an average year) is conveyed to the county through the Sacramento-San Joaquin Delta via the State Water Project and Central Valley Project.
- About 15 percent is diverted upstream of the Delta by the San Francisco Public Utilities Commission and delivered to cities in northern Santa Clara County
- More than 70 percent of the water delivered to Santa Clara County is sent to our drinking water treatment plants..
- Almost 15 percent of the water we import refills our local groundwater basins.
- A small percentage is used to irrigate Santa Clara County agriculture.
Vital to Managing Groundwater Levels
In the early 1900s, over pumping of groundwater caused the land surface in parts of the county to sink. Some areas of downtown San Jose sank up to 13 feet despite development of local reservoirs and an aggressive groundwater recharge program. The water district halted this trend after we began importing water from the State Water Project in the 1960s and the Central Valley Project in the 1980s.
With all the buildings, roads, gas, water, and sewer lines, and other infrastructure in place today, it would be financially and logistically damaging to Silicon Valley if this sinking were to resume. Imported water supplies are also vital to maintaining emergency groundwater reserves for dry years.
Vital Supply for Drinking Water Treatment Plants
Water that is conveyed to the county through the Sacramento-San Joaquin Delta provides about 90 percent of the water treated at the district’s three drinking water treatment plants.
Imported Water Supplies
The district has contracts for 100,000 acre-feet per year of State Water Project water and 152,500 acre-feet per year of Central Valley Project water. However, water availability and environmental conditions can impact the actual amount of water delivered. As a result, the district only receives, on average, about 170,000 acre-feet per year from these two sources combined.
Since 1996, the water district has also participated in a water banking and exchange program with the Semitropic Water Storage District located in Kern County. In wet years, the water district stores excess Delta-conveyed water in the Semitropic Groundwater Bank for later use, such as during dry years.
The water district can also store imported water supplies for shorter periods of time in San Luis Reservoir in Merced County, and locally in Anderson and Calero reservoirs.
To help manage the variability between wet years and dry years, the water district also participates in transfers and exchanges with other water agencies throughout the state, allowing water to be bought and sold as needed.
An Uncertain Future
Since 1991, increasingly stringent regulations have been imposed on the state and federal water projects to protect threatened and endangered fish species, including delta smelt, longfin smelt, Chinook salmon, steelhead, and green sturgeon.
The overall health of the Delta ecosystem has declined, and the region is susceptible to earthquakes, climate change, levee failure and other factors that threaten the district’s imported water supplies.