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California Geography And Its Water Needs

Picture of some children at a Drinking Fountain
From Northern California's watersheds, the SWP provides about
20 million Californians with at least part of their drinking water supply.

California is blessed with many water resources. Northern California receives the most abundant rainfall and runoff from mountain snowpack. However most of California’s population lives in Southern California and most irrigated farmland lies in Central California. These regions are mostly arid and heavily depend on water imported from other areas. Many communities—both north and south—have developed their own water projects, but they must often seek additional supplies, especially to meet shortages during dry years or the demands of increasing populations.

Picture of irrigation of crops
The project also provides water to about 600,000 acres of California farmlands.

A look at the State’s geography will explain California's need for a system to convey water supplies from one region to another.

Water and Geography

California is a state of different climates, from dry, hot deserts to snow-peaked mountains to foggy coastlines. Its water supply varies widely from year to year, season to season, and area to area, depending mainly on runoff from the Sierra Nevada Mountain Range. Sometimes floods and droughts occur in the same year.

Precipitation amounts can vary from less than an inch in California’s Death Valley desert to about 56 inches along the North Coast. Precipitation is not only captured and stored in reservoirs but can also recharge groundwater basins. These underground aquifers are capable of holding six to 10 times the amount of surface water reservoirs. However many are overdrafted (over-pumped) and some are contaminated by toxins such as MTBE, an ingredient in gasoline or arsine, a naturally occurring substance. Other aquifers are too deep to reach economically.

Competing Water Needs

Picture of the bridge over Lake Oroville
Lake Oroville

There is a finite amount of water for Californians to use. Experts say the same amount of water that exists today is exactly the amount that was present during prehistoric times.

Nature provides about 200 million acre-feet of precipitation to California in average year. Of this total, 65 percent is lost through evaporation and transpiration by trees and other plants. The remaining 35 percent stays in the State's system as runoff. More than 30 percent of this runoff is depleted as outflow to the Pacific Ocean or other salt sinks. The rest is used by agricultural, urban, and environmental purposes. Runoff is usually stored in reservoirs until needed.

For more information on California's water supply sources and uses, visit our Water Supply page.

During dry years, and even normal years, water supply shortages can occur because of competing demands from farmers, cities, and the environment. California's population is expected to increase 15 percent, exceeding 47.5 million, by 2020, With a growing population comes the need for improved infrastructure such as conveyance facilities, recycling and desalting plants, and environmental restoration projects.

Picture of men rescuing fish in the Delta
The Skinner Fish Protective Facility, and wildlife studies.

Environmental laws and water quality regulations have reserved more water for wildlife refuges and listed species from the delta smelt to the fairy shrimp. Fish hatcheries and screens help nurture and safeguard fish, while fresh water releases from the SWP’s Lake Oroville help control salinity levels in the Sacramento-San Joaquin Delta.

Picture of farmers harvesting crops
Agriculture is a major user of water.

California’s agriculture is a multi-billion dollar industry as the number one producer in the nation. Some crops use great quantities of water. For example, a hamburger patty takes more than 1,300 gallons to produce. However, the amount of water that goes to agriculture has dropped in the past decade, as more stringent environmental laws have been passed, and agriculture has increased efficiency of water use.

Some say California has an ample water supply compared to other states such as Nevada. California's challenge is how best to conserve, protect, and deliver water to meet needs where and when they occur.