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Imported Water: Vital to Santa Clara County

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 or directly to water conveyance systems.

Often called “imported water,” it is brought into the county through the complex infrastructure of the State Water Project (SWP), the Central Valley Project (CVP), and San Francisco’s Hetch Hetchy system. Imported water supplies support many beneficial uses in Santa Clara County.

On average:

  • Imported water provides for 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 (SWP) and Central Valley Project (CVP).
  • More than 70 percent of the water delivered to Santa Clara County through the Delta is sent to drinking water treatment plants providing over 90 percent of the water treated at the District’s three drinking water treatment plants.
  • Almost 15 percent is used for local groundwater recharge, providing about one-third of the water used in the district’s groundwater recharge program.
  • A small percentage is used to supply county agricultural irrigation

Vital to Managing Groundwater Levels

Imported water supplies are critical to prevent the return of historic groundwater overdraft and land subsidence (a sinking or settling of the land surface) in San Jose and adjacent cities.

As Santa Clara County developed as an agricultural community in the early 1900s, aggressive groundwater pumping caused the land surface to sink. In fact, some areas of downtown San Jose sank up to 13 feet despite development of local reservoirs and an aggressive groundwater recharge program. This trend was only stopped after the water district began importing water from the SWP in the 1960s and the CVP in the 1980s.

With all the buildings, roads, gas, water, and sewer lines, and other infrastructure in place today, the impacts of even a small amount of land surface subsidence would be both financially and logistically detrimental to Silicon Valley. Imported water supplies now provide, on average, half the water delivered to the district’s groundwater recharge system and are vital to maintaining emergency groundwater reserves for successive dry years.

historical groundwater chart

Vital Supply for Drinking Water Treatment Plants

The water district’s SWP and CVP supplies conveyed through the Delta are also the primary sources of supply for its three drinking water treatment plants. Approximately 90 percent of the water treated at the district’s three drinking water treatment plants is supplied from the SWP and CVP. In critically dry years these imported water supplies can make up over 95% of the treated water the District supplies to nine retail water agencies within the County.

Imported Water Supplies

The district has a contract for 100,000 acre-feet/year of SWP water and 152,500 acre-feet/year of CVP water. However, annual hydrology and environmental conditions in the Delta can profoundly impact the actual amount of water delivered. As a result, the district receives, on average, a combined allocation of about 170,000 acre-feet/year.

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 to help manage variability in imported water supplies. In wet years, the water district stores excess Delta-conveyed water in the Semitropic Groundwater Bank for use in years when it is needed, such as during dry years.

To store water in the Semitropic Groundwater Bank, the district’s imported water supplies are delivered through the California Aqueduct to Kern County. Semitropic then either uses that water in lieu of groundwater pumping, or recharges it into their groundwater basin. Because Semitropic is located south, or “downstream,” of the district in the state’s water delivery system, the district must retrieve banked water by exchange with other SWP water through the Delta. Thus in dry years, Delta-conveyed water contributes to a larger portion of the district’s water supply.

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 reserve supplies in case the following year is dry.

To help manage the variability between wet years and dry years in California, the water district also participates in both one-year, or “spot market,” transfers and exchanges as well as multi-year transfers and exchanges with other water agencies throughout the state.

When there is insufficient water to meet the county’s water demands through the water district’s regular local and imported supplies, the water district obtains water from other places in the state where there are excess supplies either by purchasing the water or promising to repay the water when the water district has excess supplies in a future year. When the water district has more water than it can use or store for later use, it looks to sell or exchange this water to areas outside the County.

An Uncertain Future

Since 1991, increasingly stringent regulations have been imposed on the SWP and CVP to protect threatened and endangered fish species, including delta smelt, longfin smelt, Chinook salmon, steelhead, and green sturgeon.

The overall health of the Bay-Delta ecosystem has declined, as food supply and water quality have both been degraded in a system that has lost 98% of its freshwater emergent marsh.

Earthquakes, climate change, levee failure, and other factors pose serious threats to water conveyance of imported water through the Delta. According to the Public Policy Institute of California (PPIC) , there is a 66% chance of major levee failure in the Delta within the next 50 years. California Department of Water Resources (DWR) estimates that a major levee failure could impair operation of the SWP and CVP pumps from six months to two years.

To address these risks, the district is actively participating in Delta planning efforts, including the California WaterFix and California EcoRestore.

 The Delta
Learn more about the Delta ecosystem
and its importance to local water supplies


WaterFix - Eco Restore-sm
What are the California WaterFix
and EcoRestore proposals?