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California's Water: Facts on "Beyond Drought:Factors Affecting California's Water Supply "

Background

Water Supply Overview

  • Precipitation varies widely from year to year. In average years, close to 200 million acre-feet (MAF) of water falls in the form of rain or snow in California. One acre-foot is about 326,000 gallons, or enough water to supply two typical families for a year.
  • Over half of that water soaks into the ground, evaporates or is used by native vegetation. That leaves somewhere around 82 million acre-feet of usable surface water in average years. Of that water:
  • 48% goes to environmental uses such as instream flows, wild and scenic river flows, required Delta outflow and managed wetlands.
  • 41% is used by agriculture.
  • 9% is used by cities and industry.
  • About 75% of California’s available water occurs north of Sacramento, while about 80% of the demand occurs in the southern two-thirds of the state.

Water Delivery System

Two of the most important projects are the federal Central Valley Project (CVP) and the State Water Project (SWP). The CVP and SWP bring water from Northern California through the Sacramento-San Joaquin River Delta, the hub of the state’s water supply, for delivery to users in the San Joaquin Valley, parts of the San Francisco Bay Area and Southern California.

Factors Affecting the State’s Water Supply

  • California is prone to both droughts and floods. The most recent prolonged dry spell was a six-year drought from 1987 to 1992. The most severe drought on record occurred in two consecutive years, 1976 and 1977, in which California received very little precipitation and surface water reservoirs were extremely low.
  • However, drought is one of many factors that affect the state’s water supply.
  • The Sacramento-San Joaquin River Delta has been in the headlines lately because of a crisis involving the threatened Delta smelt, a minnow-like fish that has been listed as threatened since 1993, and the pumps that supply water to more than 25 million Californians and more than 7 million acres of farmland.
  • On August 31, a U.S. District Court judge ordered the State Water Project and federal Central Valley Project to reduce pumping in the Delta to protect the smelt. The ruling translates into an average loss of as much as one-third of previously available water supplies, depending on year type. In wet years, the loss of water for people, farms, and the environment will be greater.
  • Earlier this spring, because of fears of harming the smelt, the Department of Water Resources voluntarily shut down the pumps that send water from the Delta to parts of the Bay Area, Central Valley and Southern California for 10 days in June.

The effects of this temporary shutdown were felt across the state:

  • During the 10-day shutdown, areas that rely on water from the Delta were compelled to use groundwater, emergency reserves or other alternative supplies. Some agencies, such as Alameda County Zone 7 Water Agency, asked their customers to reduce water use and have extended that call through the end of 2007
  • Kern County Water Agency, which receives water from the Delta via the State Water Project, declared a local drought emergency, citing reduced water supplies and crop losses.
  • San Luis Reservoir, near Los Banos was drawn down to only 20% capacity in order to continue to supply water to the Bay Area, Central Valley and Southern California.
  • The water level in the San Luis Reservoir level dropped two feet a day, raising concerns about the structural integrity of the exposed earthen banks of the reservoir.
  • Uncertainty over Delta pumping is creating challenges for many local water agencies, with potentially significant water supply cutbacks ahead if curtailments occur