San Francisco  - California is facing its "worst drought ever" after an extremely dry January left the crucial Sierra Nevada snowpack well below normal levels for the third year running, state officials said Friday.

"We may be at the start of the worst California drought in modern history," Lester Snow, director of the state Department of Water Resources, said in a statement. "It's imperative for Californians to conserve water immediately at home and in their businesses."

Measurements of the snowpack - which is the backbone of the water system in the most populous state in the country - came in at only 61 per cent of normal levels, according to the Department of Water. Adding to the severity of the problem is a growing population, an aging infrastructure and court-ordered reductions in water pumped through the Sacramento-San Joaquin River Delta to preserve natural ecosystems.

With the state suffering drought conditions in the last two years most of its major reservoirs are now less than a third full, and farmers in the fertile Central Valley are bracing for drastic water reductions that will force them to leave fields, orchards and vineyards fallow.

Lake Shasta, the largest reservoir in California, is at 31 per cent of its capacity, down from 74 per cent in 2007.

The state is usually battered by a series of soaking storms in January, February and March. But a high-pressure system parked over Northern California has kept skies clear and warm through most of the winter. There is now little chance of making up the shortfall through the remainder of the winter months, meteorologists say, making water rationing likely throughout the state. (dpa)