Operators of the sprawling state system that supplies water to 25 million Californians from Butte County to San Diego issued their lowest-ever estimate on the amount of water they will be able to deliver.

Officials predicted Tuesday they will be able to offer only 5 percent of the total volume of water requested by California cities and farms next year. That's the smallest water allocation the agency has released since its creation in 1967.

The estimate, based on current water conditions, is only preliminary and is almost certain to rise as the rainy season wears on. Still, officials expect a multiyear drought, low reservoirs and environmental restrictions on water pumping to keep supplies well below average in 2010.

"We have to assume we're heading into a fourth year of drought and we have to respond accordingly," said Lester Snow, director of the California Department of Water Resources, the state agency that operates the network of reservoirs, pumps and pipelines known as the State Water Project.

Increase possible

If this winter were to bring relatively wet conditions, Snow said, deliveries ultimately could creep up to between 20 and 40 percent by spring. Over the last 10 years, springtime water allocations have averaged about 68 percent. Last year at this time, the department said it was likely to deliver about 15 percent. Deliveries edged up to 40 percent after late-season precipitation boosted reservoir levels and Sierra Nevada snowpack, but about 60 water agencies across the state were forced to impose some form of mandatory rationing.

For Bay Area water districts that depend on the California water project - including many communities in Napa, Solano, Alameda and Santa Clara counties - 2010 will be another year in which conservation, recycling and groundwater storage loom large.

The Santa Clara Valley Water District, which delivers about 400,000 acre feet each year to 1.8 million residents of Silicon Valley, relies on the state water project for as much as 25 percent of its supply each year (1 acre foot is equal to about 326,000 gallons).

A slim supply next year means the district will have to lean more heavily on its local reservoirs, its recycled water system and supplies from the San Francisco Public Utilities Commission's Hetch Hetchy reservoir. In addition, at its meeting Tuesday, the district is expected to extend a 15 percent water conservation target into next year.

"If we go in a fifth, or sixth or seventh year of drought, it's going to get a lot more painful," said Keith Whitman, water supply manager for the district.

Other major water agencies in the Bay Area, including San Francisco's PUC, the East Bay Municipal Utility District and the Marin Municipal Water District, operate independent water systems that aren't tied into the state's network.

As water managers attempt to squeeze every last drop out of their supplies, Gov. Arnold Schwarzenegger and others used Tuesday's announcement to underscore the importance of the state's plan to overhaul the aging water network.

"I am encouraged that we are in a position to rebuild the system to better withstand the drought situations so we are not forced to agonize over a clean, reliable water supply for future generations," Schwarzenegger said in a statement.

The plan for restoring the ailing Sacramento-San Joaquin River Delta, improving groundwater monitoring and building new dams, reservoirs and a controversial canal calls for borrowing $11 billion - a measure expected to go to voters in November.

Only a broad goal

Snow acknowledged that nothing in the water plan or bond, however, directly addresses the current crisis aside from the broader goal of reducing urban water use by 20 percent by 2020.

"We need to do a better job on conservation - it's the single most effective tool we can use now and into next spring that will have some benefit in these drought conditions," he said.