(AP) — Water districts and households across California could be compelled to help pay for Gov. Jerry Brown’s plans to build two giant tunnels to ferry water to cities and farms mainly in central and Southern California, under newly revealed plans to shore up funding for the struggling $16 billion project.

The tougher state funding demands pivot from longstanding state and federal assurances that only local water districts that seek to take part in the mega-project would have to pay for the twin tunnels, the most ambitious re-engineering of California’s complex north-to-south water system in more than a half-century.

The Associated Press obtained new documents from the state’s largest agricultural water agency and confirmed the expanded funding demands in phone and email interviews with state and local water officials.

With no major water district yet signing on voluntarily to help pay for the project amid uncertainty about its costs and benefits, state and local promoters of the tunnels now contend that dozens of local water agencies representing millions of Californians are obligated to help foot the bill under their existing contracts.

While speculation of that arrangement has swirled privately, “this is the first acknowledgement that we’ve heard” from the state that those water agencies would be on the hook, said Paul Gosselin, director of Northern California’s Butte County water district.

His agency would get no water from the tunnels and has been seeking written state and federal guarantees that its customers would not have to pay for them. He’s gotten no such assurances.

“Any of these funding mechanisms has been in a black box — none of it’s been described to us, the contractors, or the public,” Gosselin said.

Brown’s administration intends to exclude from the funding obligation a half-dozen Northern California water districts, like Gosselin’s, that would get no water from the tunnels, although just how hasn’t been worked out, said Lisa Lien-Mager, spokeswoman for the state Natural Resources Agency.

The two tunnels — each 35 miles long and the width of a three-lane highway — would tap into Northern California’s Sacramento River to provide more reliable supplies for points south.

Brown says the tunnels would modernize the existing water delivery system built under his father, then-Gov. Pat Brown. The younger Brown is pushing to see his water project launched before he leaves office next year.

But it’s been beset by controversy. Opponents say the tunnels could further threaten struggling native species and drain Northern California dry. Federal auditors also said last week that authorities improperly used $50 million in taxpayer money for the project.

The state’s newly revealed funding plan hinges on its contention that the tunnels would be an update, not a new project. As such, the 29 water districts that get water from the existing state system of aqueducts, reservoirs and pumps would be obligated to help bear the costs of the new tunnels, state and local water officials said Friday.

Rather than deciding whether to opt in to the project, any state water clients that “make an affirmative decision not to participate, it would be up to them to reach an agreement” with some other water contractor to take on their share of the project’s cost, Lien-Mager said.

Asked if California intended to cut off state water deliveries to districts that refuse to help pay for the tunnels, Lien-Mager said only that “opting out would not affect their existing contracts, but their actual water supplies from the SWP could become less reliable in the future,” referring to the current State Water Project.

That message has begun trickling out as water agencies decide whether to raise rates to pay for the tunnels.

“That’s what we’re being informed — our contract ends if we don’t participate,” said Richard Santos, a board member of the Santa Clara Valley water district, which supplies water to Silicon Valley.

If it plays out that way, Santos said, he will fight.

“If they say they’ll cut off our allocations if we don’t participate, then let the courts take it on,” he said.

The funding arrangements also are described in documents released Thursday by Westlands Water District, which covers 1,000 square miles of the farm-rich Central Valley. With lobbyists and strong ties to Washington, the rural water district is one the most politically influential in a state where water is the most fought-over resource.

Westland says in a new report to its board that the tunnels’ costs “will be allocated to all state water contractors in proportion … to their contract amount,” meaning how much water they currently get from the state.

A staff recommendation that the Westlands board could vote on Tuesday calls for Westlands to sign on to the mega-project — on two conditions.

One, which is spelled out in the meeting documents, says water agencies that get their supply from a separate, federally run water project in California, the Central Valley Project, should also be compelled to help foot the bill for the tunnels.

The second is that California not impose “unreasonable” restrictions on the amount of water Westlands can take from the Sacramento River.

Without stronger assurances on more water for the Central Valley, the richest U.S. agricultural region, the higher water costs from the project may not be worth it, Westlands board members said.

“It really upsets me or makes me sad,” said Sarah Woolf, a farmer and Westlands board member. “I could very well vote against something that infrastructure-wise I know we need.”