California teeters on the edge of the worst drought in the state's history, officials said Thursday after reporting that the Sierra Nevada snowpack - the backbone of the state's water supply - is only 61 percent of normal.

January usually douses California with about 20 percent of the state's annual precipitation, but instead it delivered a string of dry, sunny days this year, almost certainly pushing the state into a third year of drought.

The arid weather is occurring as the state's water system is under pressure from a growing population, an aging infrastructure and court-ordered reductions in water pumped through the Sacramento-San Joaquin River Delta - problems that didn't exist or were less severe during similar dry spells in the late 1970s and late 1980s.

"We're definitely in really bad shape," said Elissa Lynn, chief meteorologist with the state Department of Water Resources. "People can expect to pay higher prices for produce ... and more agencies may be rationing ... some raising fees. We just don't have enough water."

In Sonoma County, water managers are expected to take a bold step Monday - telling residents to prepare for severe rationing within weeks.

"We have entered uncharted territory," said Pam Jeane, deputy chief engineer of operations at the Sonoma County Water Agency. "A 30 percent mandatory rationing order is just the beginning. Further decline in reservoir levels could necessitate 50 percent cutbacks."

After two consecutive dry years and with a third on the way, Lake Mendocino, one of two main reservoirs that supply 750,000 residents in Marin and Sonoma counties, contains only 32,000 acre-feet of water - about one-third of its capacity of 90,000 acre-feet. (One acre-foot is the amount of water needed to cover one acre at a depth of one foot, enough to meet the needs of one to two families each year).

The picture is similar around the state. Lake Shasta, the largest reservoir in California, is at 31 percent of its capacity, down from 74 percent in 2007.

By now, water planners had hoped a series of strong storms would fill up reservoirs and make further rationing unnecessary. But a high-pressure system parked over Northern California has kept skies clear and warm through most of the winter.

Thursday's snow survey by the Department of Water Resources found that the snow's water content - the snowpack - across the Sierra Nevada was 61 percent of normal for this time of year. Last year at this time, the snowpack was 111 percent of normal, but the driest spring on record led to a drought.

The saturation level of snow, or the snow's water content, is the most important factor affecting the crucial spring runoff levels, which help water planners determine water supplies for their districts each season.

If Sonoma County institutes 50 percent rationing, it would be the Bay Area's most drastic measure so far to address the drought. Last spring, the East Bay Municipal Utility District announced 15 percent rationing and added extra drought fees.

Many other districts are relying on voluntary cutbacks. San Francisco has asked users to trim water use by 10 percent. But the city and others could move to rationing this spring, potentially barring customers from filling pools, washing cars or watering lawns.

No customers would suffer more than agricultural districts in Central Valley, where farmers expect they will receive no water from the U.S. Bureau of Reclamation for their alfalfa, corn, pears and almonds. Critics charge farmers with wasting water by flooding fields and growing low-value crops. But growers insist they are increasing efficiency at the same time they supply the nation with critical food supplies.

Wine is not a critical food supply, but the economies of Napa and Sonoma counties stand to lose tens of millions of dollars if the drought squeezes this year's grape harvest. Already, there are signs that vines are sprouting early - exposing grapes to frost for a longer period of time.

Like commercial farmers in the Central Valley, grape growers might be forced to water some plots and not others.

"If you have limited water, and you don't have the irrigation to keep it going ... it's not going to be a bumper crop," said Nick Frey, president of the Sonoma County Winegrape Commission.

Gov. Arnold Schwarzenegger used Thursday's snow survey to pitch his answer to California's water crisis - a $9 billion-plus water bond that would funnel money toward new dams, reservoirs, water recycling programs and conservation efforts. The governor also supports building a giant pipeline, called a peripheral canal, around the Sacramento-San Joaquin River Delta to reduce pressure on the ailing water system.

In 2007, a federal judge said operators of the giant export pumps near Tracy must curtail pumping in order to save the endangered delta smelt, a tiny fish.
Tips for saving water

With the state heading for drought, here's what you can do:

Indoors

Toilets: Replace older toilets with high-efficiency ones.

Showers: Install shower heads that use no more than 2.5 gallons of water per minute.

Washing: When showering, wet body, turn off water, apply soap, and turn on water to rinse.

Food: Wash fruits and vegetables in a bowl of water.

Leaks: Fix leaky faucets and toilets that run.

Outdoors

Watering: Do it less often and more deeply.

Irrigation: Use drip irrigation.

Plant choice: Select plants adapted to Mediterranean climates.

Mulch: Apply mulch to gardens to maintain moisture in soil.

E-mail Kelly Zito at kzito@sfchronicle.com.