State officials on Wednesday unveiled a new cost estimate for the hotly debated plan to change water flows through the Sacramento-San Joaquin Delta, a plan that includes twin tunnels dug beneath and around the inland estuary.

Total cost: $24.5 billion, with the lion's share going to build the underground water tunnels.

"We have to ensure against...future calamity that we know are real possibilities," said Gov. Jerry Brown in comments to reporters Wednesday afternoon. "This is not an if, it's a when."

The $24.54 billion price tag includes money for ecosystem restoration projects and land purchases, and operation and maintenance of the 35-mile long tunnel project that would take in water near Courtland and transport it to existing pumps near Tracy.

The tunnels themselves, from construction to operation, are estimated in the new documents to cost $15.9 billion.

Formal public comment on the thousands of pages of the proposal, which was released in sections this spring, begins this fall. But debate in the court of public opinion is already intense, with Delta region residents and lawmakers generally unhappy with a plan they say will still take too much water out of the system and send it to agricultural and residential users in other parts of the state.

Wednesday's release of cost estimates raised a new wrinkle in that debate -- the idea that some of the cost would be paid by a bond measure placed on the 2014 statewide ballot.

"You're looking at California taxpayers paying to correct the damage done by three water exporters," saidBarbara Barrigan-Parrilla of the group Restore The Delta.

Even so, the vast majority of the cost -- almost $17 billion over the 50 year timeline of the project -- would be expected to come from water users. About $3.7 billion would come from state funding, with more than $3.9 billion coming from the federal government.

The formal Bay Delta Conservation Plan (BDCP) spans 12 chapters, and seeks to meet the mandate set by lawmakers of an estuary proposal that balances environmental and water supply reliability. The challenge is that the project's most vocal critics think there's almost no way to meet both of those demands.

Key to the dispute are the underground water tunnels, which state officials say would transport up to 9,000 cubic feet per second of river water. Delta community groups and officials have urged, if any tunnels at all, ones that would carry only about a third of that amount.

But Wednesday's BDCP cost estimate proposal contends that would turn the project from a net benefit of $5 billion over five decades to a $1 billion net benefit loss. That estimate was immediately criticized by the plan's opponents, who maintain no real cost-benefit examination has ever been done.

A group of Sacramento and Delta region members of Congress are scheduled to hold a news conference Thursday to lash out at the plan.

The governor contends the plan is necessary to prevent the economic costs of a natural disaster or climate change in disrupting the flow of water southward through the state's water supply system -- a system that relies on Delta protection.

"It'd be catastrophic," he said Wednesday. "In the $100 billion range, maybe more. So, we just have to be careful and figure out what insurance we're prepared to buy."

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