San Francisco  - After three exceptionally dry years, California Governor Arnold Schwarzenegger on Friday declared a state of emergency amid a prolonged drought.

"This is a crisis, just as severe as an earthquake or raging wildfire, and we must treat it with the same urgency by upgrading California's water infrastructure to ensure a clean and reliable water supply for our growing state," Schwarzenegger said, calling on residents to save water.

In March, water supplies for agriculture will be curtailed during certain periods. The measure will likely hit hard farmers in the state's Central Valley that is among the country's chief agricultural areas and where massive fields must be irrigated. Schwarzenegger estimated that the drought could cost the state's agriculture sector more than 2 billion dollars.

If residents do not take voluntary measure to curb their water usage, the state could institute water rationing - restricting lawn sprinkler systems, car washes and the filling of swimming pools, he said.

After an extremely dry start to the winter, the last two weeks saw hardly any rain or snow and meteorologists predict only a 15 per cent possibility of a return to normal conditions this year.

March, April and May of last year were among the driest since measurements began, and supplies of drinking water have reached an historic low.

The emergency declaration frees up money for affected areas.

Much of the state's water comes from snow melt in the Sierra Nevada mountains. The snow cover last year was just 69 per cent of normal levels. (dpa)