Major water districts in California are quietly considering using property taxes — and possibly raising them without a vote of the public — to help fund Gov. Jerry Brown’s $25 billion plan to build two massive tunnels through the Sacramento-San Joaquin River Delta.

Most property tax hikes require a two-thirds vote, as required under California’s landmark Proposition 13, which voters passed in 1978. But the water agencies contend they are not bound by that requirement.

They say they were given the authority to raise property taxes to pay for the State Water Project, a vast system of dams and canals, in both a 1959 law and a year later in a statewide ballot measure. And those predate Proposition 13.

The Santa Clara Valley Water District, which serves 1.8 million customers in Silicon Valley, this month discussed raising property taxes from $36 a year to $60 a year on the average assessed residential property over the next decade to help pay some of the $228 million it expects to contribute to the Delta tunnels project between now and 2024.

“Because this is a tax that was already voted on, it was a pre-Prop. 13 measure,” said Jim Fiedler, the district’s chief operating officer. “Because it was adopted by voters prior to that date, it doesn’t qualify for the two-thirds requirement.”

Asked if there was any limit to how much higher property taxes could be raised if the Delta tunnels project has cost overruns, similar to the Bay Bridge or high-speed rail projects, Fiedler said he didn’t know.

“That’s a good question. That’s a decision our board would make,” he said. “But it would be in public hearings, not behind closed doors.”

Until now, it had been assumed that water agencies supporting the tunnels project would be raising water rates to pay for the tunnels.

As word has trickled out about the property tax strategy, anti-tax activists, environmental groups and even other water agencies are raising concerns.

The Metropolitan Water District of Southern California, which provides water to 19 million people in and around Los Angeles, is also discussing using that agency’s property tax authority to help pay its share of the cost.

But its property tax rate, now about $10 a year on the average home in Southern California, wouldn’t be increased, said general manager Jeff Kightlinger.

“If there are cost overruns, water rates would go up,” he said. “It has been a policy of our board to not raise property taxes.”

Both the Metropolitan and Santa Clara Valley districts are expected to decide the issue this fall.

Anti-tax groups say they fear that residents of Silicon Valley, Los Angeles and other urban areas could be left on the hook for major cost overruns and that their property taxes could be increased significantly without a public vote.

“We’re worried they could tax people to the hilt. We will be watching it very carefully,” said Jon Coupal, president of the Howard Jarvis Taxpayers Association in Sacramento.

Coupal said that his group questions whether water districts can even use property taxes to pay for Brown’s tunnels on the grounds that the taxing authority only extended to the original State Water Project, championed by the governor’s father, Gov. Pat Brown.

“There is a strong parallel here between this tunnels project and high-speed rail,” Coupal said. “Both don’t have an adequate financial plan. If we conclude using property taxes is illegal, we will use whatever legal means we have to stop it.”

The Contra Costa Water District, East Bay Municipal Utility District and San Francisco Public Utilities Commission are not participating in the tunnels’ planning or funding process in a major way.

Brown’s plan is to build two tunnels, each 40 feet in diameter and 35 miles long, under the Delta to make it easier to move water from Northern California to cities and farms further south. The idea is to reduce reliance on the massive pumps at Tracy, which are sometimes shut down to protect endangered salmon, smelt and other fish.

But environmentalists, Delta farmers and some Northern California lawmakers are trying to kill the plan, saying it could eventually be used to draw even more of Northern California’s water south. The Brown administration has released an environmental study. It expects water agencies to pay roughly $17 billion, with $8 billion coming from state water bonds and the federal government.

“With water rates, if you use less you pay less. But with property taxes, you pay at a fixed amount regardless of how much water you use,” said Doug Obegi, an attorney with the Natural Resources Defense Council in San Francisco.

It’s unclear how many water agencies are considering using property taxes to pay their share of the tunnels project, although more than 20 large water agencies have the pre-Proposition 13 property taxing authority. So far, water agencies have spent $240 million on studies related to the project, but have at least $1.2 billion more to study the plan. They aren’t expected to decide until the fall how to divide those costs or the costs of building the project. Some agencies, particularly in farm areas like Kern County and Fresno, may drop out if they are not convinced the investment will deliver them more, or more reliable, water.

Some agencies are demanding more details.

“We think for one of the largest water projects in state history, perhaps human history, that we ought to have more solid answers to these questions around property taxing authority before we embark on spending the next billion dollars in planning,” said Dennis Cushman, assistant general manager of the San Diego County Water Authority.

It’s clear, however, that Wall Street financiers are looking at property taxes as a major way to guarantee that investors who buy the bonds would get their money back.

A March 19 proposal by Goldman Sachs to the State Water Contractors Authority, seeking to win the contract to underwrite the construction bonds, cited property taxes as a “primary” element “that would certainly need to be preserved.”

Brown administration officials say it will be up to each water agency to determine how to fund its portion of the tunnels project.

“We are agnostic on the financing method,” said Nancy Vogel, a spokeswoman for the state Department of Water Resources. “We would not dictate to state project customers how they pay for their share.”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN.