LOS ANGELES — A Southern California water agency has approved key funding needed to support plans for a giant underground tunnel that would reroute a big part of the state’s water supply.

The board of the Metropolitan Water District of Southern California, which along with local agencies provides water to 19 million people, voted Tuesday to allocate about $142 million for pre-construction and planning costs for the Delta Conveyance Project. State officials say the project is critical to help shore up water supplies in the nation’s most populous state due to aging infrastructure and climate-change induced shifts in precipitation.

“After careful consideration, our board took this step because it allows us to gather critical information about the project’s benefits and costs that will allow us to evaluate whether we will participate in the full construction of the project,” said Metropolitan board Chair Adán Ortega, Jr. said in a statement.

Metropolitan is the largest of a dozen water agencies that have voted to continue funding for preconstruction and planning of the 45-mile (72 kilometer) long tunnel. A key vote is expected in 2027 on whether it should move forward.

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Gov. Newsom Advocating Water Agencies to Continue Funding

Gov. Gavin Newsom has been advocating for water agencies to continue providing funding for what he has called “the most important climate adaptation project” in the country.

“Nothing else comes close because nothing is more important,” he said earlier this week. “Because this impacts 27 million Californians.”

California officials want to build the tunnel over concerns about the reliability of existing water infrastructure and say it will help capture more water during massive storms and move it from the northern part of the state to the south, where most of the state’s people live. But critics say the project’s $20 billion price tag is too high and there are other ways to strengthen the state’s water system with less impact to the Sacramento-San Joaquin River Delta’s fragile ecosystem.

Barbara Barrigan-Parrilla, executive director of Restore the Delta, said the tunnel aims to transport too much water to farm operations that should be scaling back on water use as the state faces a potentially drier future. California grows much of the country’s fresh produce.

“There are certain elements that just want to build things for the sake of building things rather than doing the analysis of building the right things,” she said.

The tunnel has been proposed — and disputed — for many years, earning widespread opposition from Central Valley communities that say it would harm their economies and the delta. Last year, the state completed a key environmental review for the project, but it still must pass additional review and obtain various state and federal permits.