State and federal water officials said deliveries to farms and cities could be restricted severely next year even if the coming winter provides normal rain and snowfall.
California needs an especially wet winter if it is to fill its reservoirs and abide by court-ordered restrictions to reduce pumping by up to a third from the Sacramento-San Joaquin Delta, says John Leahigh, a top administrator with the State Water Project.
That is particularly true in the Sierra Nevada, which stores much of the state's water supply in its winter snowpack. But so far, there is no indication of wintertime salvation.
Government experts predict much of the country will have a warmer and drier winter than normal because of moderate La Nina conditions, in which air cools over the Pacific and the jet stream gets pushed farther north.
"There's not a clear signal for Northern California — whether it's going to be wetter or drier," Leahigh said during a state Department of Water Resources workshop. "It could go either way."
That's because the central part of California typically forms the southern boundary of La Nina's jet stream paths.
'Just don't know' with La Nina
La Nina conditions have contributed to dry winters at least six times since 1961. But Northern California also experienced some of its worst flooding in 1955, 1965, 1986 and 1997 because of subtropical La Ninas.
"That's the biggest message with La Nina — you just don't know," state climatologist Mike Anderson said.
The federal court order earlier this year requiring a cutback in pumping to protect the threatened delta smelt has left California's water supply more vulnerable than ever.
That decision came as a double blow, following an exceptionally dry winter.
The amount of rain and snowfall California received during the 2007 water year — measured between September 2006 and Oct. 1 of this year — was the lowest since 1988. Southern California is experiencing a record dry spell, leading officials in Los Angeles to warn about mandatory rationing for the first time since 1991.
The dry conditions have left state and federal reservoirs below normal levels. Additionally, state water managers over the summer had to draw down reservoirs to make up for the court decision that halted pumping from the delta for several weeks.
That wiped out most of the reserve water that had accumulated during the 2006 season, which was wetter than normal and produced above-average snowfall.
Largest reservoir likely to be low
For example, hydrologists say there is only a 25 percent chance that Lake Oroville, the State Water Project's largest reservoir, will be filled this coming year.
Gov. Arnold Schwarzenegger called a special legislative session to address the state's water supply, although those efforts have stalled over a disagreement about building dams. He has proposed a $10.3 billion bond to add reservoirs and underground storage, increase water recycling and promote conservation programs.
Sen. Don Perata, D-Oakland, has floated a $6.8 billion bond that would allow communities to compete for state grants to build their own dams, improve water efficiency, recycle water and store more water underground.
"People need to think about multiyear droughts and how we prepare," said Department of Water Resources Director Lester Snow, who said negotiations over a water bond are continuing.