The powerful board of Southern California’s largest urban water supplier will soon vote on whether to continue funding a large share of preliminary planning work for the state’s proposed water tunnel in the Sacramento-San Joaquin River Delta.

The 38-member board of the Metropolitan Water District of Southern California is set to consider approving $141.6 million for planning and preconstruction costs at its Dec. 10 meeting.

Gov. Gavin Newsom and his administration have requested additional financial support from suppliers that would eventually receive water from the project, and the MWD is being asked to cover its share of nearly half the initial costs.

The district, which provides drinking water for about 19 million people in Southern California, has spent $160.8 million supporting the project since 2020, and is expected to help foot the bill as requested by the state.

Newsom has said building the proposed Delta Conveyance Project is critical for California’s future. The 45-mile tunnel would transport water beneath the Delta, creating a second route to draw water from the Sacramento River into the aqueducts of the State Water Project.

The state has estimated the total cost at $20.1 billion, and Newsom has said he hopes to have the project fully permitted to move forward by the time he leaves office in early 2027.

Supporters and opponents of the project made their arguments to MWD board members at a meeting Monday. The discussion ranged widely from the vital role of the Delta’s water in California’s economy to potential alternative investments aimed at boosting the state’s supplies.

Supporters, including leaders of business and labor groups, said they believe building the tunnel would improve water-supply reliability in the face of climate change, sea-level rise and the risks of an earthquake that could put existing infrastructure out of commission.

“On the climate front, warming temperatures have put water storage capacity of the Sierra Nevada mountains in long-term decline,” said Adrian Covert, the Bay Area Council’s senior vice president of public policy.

Covert said the project would be a cost-effective way for the state to adapt, and that reliable water will also figure in future efforts to address the state’s chronic housing shortage. “Our great concern is that, without action, water scarcity will emerge as a major constraint on housing production across California,” he said.

For now, the MWD board will only be deciding on whether to agree to the state’s funding request for the next three years. The board is not expected to vote on whether to participate in the project until 2027.

“We encourage you not to pull out, stay the course and fund the study so that we can learn whether it’s good or not to buy into for the long run,” said Tracy Hernandez, chief executive of the Los Angeles County Business Federation.

She said the funding will enable the water district’s leaders to “continue shaping this project.”

Hernandez said her organization views the project as an affordable way of ensuring water reliability. Other supporters cited a recent cost-benefit analysis by the state Department of Water Resources, which concluded that building the tunnel would deliver water at lower cost than investments in seawater desalination, wastewater recycling or stormwater capture.

Opponents of the project have argued the state’s analysis is flawed and underestimates the costs while overestimating the benefits. They’ve called the tunnel a boondoggle that would harm the Delta and its deteriorating ecosystem, and have argued the project would saddle ratepayers with high costs.

“Please, stop throwing good money after bad,” said Pat Hume, a Sacramento County supervisor and chair of a coalition of Delta counties. “If these costs are this high before the project even begins, imagine what will happen to the projected costs to actually deliver the project.”

Different versions of the plan have been debated for decades — at first calling for a canal around the Delta, and later twin tunnels beneath the Delta, followed by Newsom’s current proposal for a single tunnel.

Environmental groups, Indigenous tribes, fishing organizations and local agencies have filed lawsuits seeking to block the project. They have argued the state should instead invest in other approaches in the Delta, such as strengthening aging levees and restoring natural floodplains to reduce flood risks, while changing water management and improving existing infrastructure to protect the estuary’s health.

“I believe there are a lot of alternative projects that could be explored and potentially delivered, in a more timely and more cost effective manner,” Hume said. Focusing instead on strengthening levees in the Delta and restoring tidal marshlands, he said, would ensure that water is “delivered to the doorstep of your existing pumps reliably.”

Other critics argued that California’s efforts to address its housing affordability aren’t constrained by water but rather by other issues. They noted that tribes and environmental groups are currently challenging related state water-management decisions in the Delta, and said more legal challenges are likely. Some called for continuing to increase investments in local water supplies in Southern California to reduce reliance on imported water from the Delta and the Colorado River.

“When you’re building something that creates environmental harm, environmental damage, that impacts local communities, there’s a cost to that. It impacts tribes, there’s a cost to that,” said Bruce Reznik, executive director of the group Los Angeles Waterkeeper.

Pumping to supply farms and cities has contributed to the ecological degradation of the Delta, where fish populations have suffered declines in recent years. State water managers say the tunnel would enable California to capture more water during wet periods. They also say the tunnel would lessen limitations on water deliveries linked to fish protections at the state’s existing pumping facilities.

Reznik said Southern California has a great deal of untapped potential to boost supplies locally through investments such as recycling wastewater and capturing stormwater. “There is so much we could be working on together,” he said.

The state Department of Water Resources has asked MWD to provide about 47% of the $300 million in planning and preconstruction costs, with 17 other water agencies funding the remainder.

The state’s current plans call for starting construction of the tunnel in late 2029. Construction would take about 15 years.

Deven Upadhyay, MWD’s interim general manager, called Monday’s discussion a “fantastic dialogue” that allowed board members to hear from those on different sides of the debate.

In a separate project, the district is also moving ahead with plans to build the largest wastewater recycling plant in the country. The facility in Carson, called Pure Water Southern California, is projected to cost $8 billion at full build-out and produce 150 million gallons of water daily — enough to supply about half a million homes.

The U.S. Bureau of Reclamation announced this week that the federal government will provide $26.2 million to support the project, adding to $99.2 million in federal funds committed earlier this year. The Metropolitan Water District’s managers say the plant could start operating and delivering water in 2032.

The water recycling project will benefit the entire state and the Southwest, said Adán Ortega, Jr., chair of the MWD board.

“It will help lower demands on our imported water sources from the Colorado River and on the Northern Sierra,” Ortega said. “And it will help keep the economic engine of Southern California running, regardless of the future drought conditions we may face.”