Arnold Schwarzenegger, the Governor of California, raised the spectre of emergency water rationing throughout the state for the first time in its 160-year history amid a severe drought that some are blaming on global warming.

The drought is expected to push up food prices further as the farmers in Central Valley - the US's primary source for tomatoes and grapes, among other food products - write off their crops because of a lack of water to irrigate them. The US Bureau of Reclamation has already said that it will cut water supplies to some Central Valley areas to less than half the usual levels.

Scientists have long claimed that a big fall in the snowpack in the Sierra Nevada mountains caused by rising temperatures could leave parts of the state - including Los Angeles - uninhabitable by the end of this century. The run-off from this melting snow keeps many of California's rivers flowing and supplies the state with water during summer.

Measurements taken last month found that the Sierra Nevadas were holding 69 per cent of average winter's snowfall. Meanwhile, run-off into the state's rivers was at 55 per cent of a normal year, while the big reservoirs were at 50 to 60 per cent capacity at a time when they should be full.

At a news conference this week Mr Schwarzenegger - known for his progressive environmental policies - declared the drought to be official and told Californians that they “must recognise the severity of the crisis we face” by cutting their water use by a fifth. It is the first time a statewide drought has been declared in 17 years. “There is no more time to waste because nothing is more vital to our economy, our environment and our quality of life,” the Governor said.

Mr Schwarzenegger is using the drought to promote a $12 billion (£6 billion) bond issue that would fund new dam projects. The scheme is opposed by Democrats, who argue that conservation is more important. Although California has suffered drought-like conditions for several years, this winter was one of the driest on record. Rainfall in the Golden State during the winter months was 1.2 inches, or 22 per cent of the average for the 114 years since records began.

Conditions could become even more serious if there is another dry winter. “We need at least above normal in terms of our snowpack, and then we're still going to be tight,” Lester Now, director of the state's water resources, said. “The idea is to put programmes in place now to soften the impact in 2008 and to prepare for a potential third year of drought in 2009.”

Mr Schwarzenegger is demanding that residents and water managers cut water use or face rationing next year. Aside from destroying crops, he said, the unusually dry conditions were harming water quality and causing extreme fire hazards - as was seen this week when part of Universal Studios in Los Angeles burnt down.

While the state's water supplies are dwindling its population is rising, largely thanks to immigrants from Mexico. Until recently, southern California could simply ship in water from the north to help to ease a drought. But last year a federal judge ordered more of northern California's water to be left in the Sacramento-San Joaquin delta to aid declining fish populations.

“We're suffering the perfect storm, if you will,” Timothy Quinn, head of the Association of California Water Agencies, said, adding that the state had never resorted to such rationing. “The purpose of the Governor's declaration is to send a wake-up call.”

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