They say warmer temperatures will lead to less snowpack in the Sierra, requiring more water to be stored for use in increasingly drier years.

While water experts agree there will be less snow, they say building new reservoirs is expensive and less important than being able to move water more efficiently in response to changing weather patterns.

"People will use climate change to advocate for whatever they advocated for without climate change," said Jay Lund, a professor at UC Davis' department of civil and environmental engineering, who specializes in reservoir operations theory.

"There's fairly little value for expanding surface storage relative to expanding conveyance capacity," said Lund, the lead author of an 86-page study of the effect of climate change and population growth on California's water supply over the next 93 years.

Regardless of whether more reservoirs are needed because of climate change, Schwarzenegger's call to put a water bond on the November 2008 ballot has sparked new debate over California's water policy.

"One of the silver linings of this bond proposal is that water is an important issue and, as a whole, it has been underdiscussed or hasn't been taken as seriously as it should have been for years," said Tom Graff, director of Environmental Defense in Oakland. "I'm glad the governor put a proposal on the table -- it shouldn't be the final proposal -- but putting a proposal out there is good."

The bond, which calls for $2 billion for the reservoirs and $1 billion for yet-to-be-determined projects in the Sacramento-San Joaquin River Delta, is the only part of the Republican governor's ambitious agenda that faces withering opposition from Democrats who control the Assembly and Senate.

Democratic lawmakers have complimented elements of Schwarzenegger's proposed changes to the state's health care system, and the governor's continued efforts to reduce greenhouse-gas emissions have earned him praise from environmentalists.

Despite disliking the governor's dam proposal, Graff and other environmentalists say they are heartened that Republicans -- opponents of last year's landmark bill to curb greenhouse-gas emissions from refineries and power plants -- are citing climate change as justification for new reservoirs.

"The growth expected in the state and the changes in our hydrology mean within a relatively short time we will be 6 million acre-feet in deficit to meet our water needs," said state Sen. Dave Cogdill, R-Modesto, who voted against AB32, the greenhouse gas bill.

Cogdill is carrying the legislation that would put the bond on the ballot. Eleven co-authors of his bill, SB 59, also voted last year against the measure to combat global warming.

Environmentalists say they welcome the Republican lawmakers' acknowledgement of climate change.

"I'm looking forward to working with them on other global-warming issues. We can certainly use the help," Graff said.

Recent debate over water has largely focused on flood-control issues, such as the Central Valley's deteriorating levees.

Schwarzenegger has now raised the issue of supply with his proposal to help build the 1.8 million acre-foot Sites Reservoir in Colusa County 55 miles northeast of Sacramento and a dam at Temperance Flat above Lake Millerton on the Upper San Joaquin River near Fresno.

One acre-foot is 325,851 gallons -- roughly the amount of water used in a year by a family of four.

Schwarzenegger believes the state's growth requires more reservoirs.

"It's not something I dreamed up. The state needs it,'' he said in a recent interview with The Chronicle.

Predictions of how much water California will need by 2030 vary substantially. Demand for water for farming is expected to decline, while water for urban needs will increase. The Department of Water Resources has developed models that show if current trends continue, overall demand will decrease slightly over the next two decades, while a scenario that envisions more intensive water consumption would increase demand by 4 million acre-feet.

Schwarzenegger notes that his new bond proposal includes money for other ways of storing water, such as conservation and refilling depleted aquifers.

"Conservation is very important -- we totally agree with the environmentalists,'' he said in the interview. "But I think at the same time we need to build above-the-ground storage. That's something we need to work out with them and make them more comfortable with it.''

Environmentalists -- and Democrats -- show no signs of getting comfortable with Schwarzenegger's bond.

"Economically, Temperance Flat is the dumbest dam in America," said Barry Nelson, senior policy analyst for the Natural Resources Defense Council. "The Sites suffers from a more complicated problem because nobody knows what it will be used for."

Assembly Speaker Fabian Núñez, D-Los Angeles, flatly said the lower house won't consider the bond. Senate Democrats, led by Senate President Pro Tem Don Perata of Oakland, introduced a package of four bills they will back instead of the governor's bond.

"We don't believe new dams at this point are needed," said Perata at the unveiling of the Senate Democrats' plan. "They cost billions and take years to build. We want more water supply and better flood protection as quickly and as cheaply as possible."

Temperance Flat -- which would largely benefit Central Valley growers -- is misguided, Nelson and Perata said, because in 7 out of every 10 years, the existing dam diverts the entire flow of the San Joaquin, leaving the river downstream bone dry. In those years, a new dam would store no additional water.

Sites would be filled by water channeled off the Sacramento River. Environmentalists fear the project would further damage the delta's ailing ecosystem.

Senate Democrats and environmentalists argue that cheaper, less environmentally harmful ways of increasing water supply should be used first -- groundwater storage, recycling, desalinization -- to meet the state's supply needs.

A key objection by Democratic lawmakers to Schwarzenegger's bond is that he wants to place it before voters in 2008 even though environmental and feasibility studies of the two projects won't be completed until after the November 2008 election, according to the state Department of Water Resources' Web site.

"The timing of these proposals is not consistent with the environmental process," said Sen. Darrell Steinberg, D-Sacramento and chairman of the upper house's Natural Resources and Water Committee. "We should not be voting for or against major water projects before the environmental review is completed."

Both projects have already been studied for more than six years. Lester Snow, director of water resources, said neither is risky.

"We have pretty good cost numbers. We have all the environmental information we need," he said. "By the time the Legislature starts holding hearings on it, we will make that information available."