Californians are taking out roughly twice as much water from the Delta as is environmentally sustainable, according to a new report demanded by lawmakers and informed by some of the state's top scientific experts.
The document from the State Water Resources Control Board bears no teeth: The state set no requirement to follow its recommendations.
But its conclusions could affect water supplies in most of California, from the entire Bay Area to San Diego, and from Sierra Nevada towns to Northern California rice fields to San Joaquin Valley orchards.
The key finding is that about 75 percent of all the snowmelt and rain that flows or falls into the Delta's watershed, which covers 40 percent of California, should flow through the Delta into the Bay.
Today, about 50 percent of the flow passes through the Delta on average as nearly all of California taps into its tributary rivers and the Delta itself.
In rough terms, it would require reducing Delta water use by half in order to meet the 75 percent target.
"It would obviously devastate water supplies," said Roger Patterson, assistant general manager for the state's largest water district, the Metropolitan Water District of Southern California.
But, Patterson said, regulators are not moving to impose the targets contained in the report, and the targets are based on the needs of an estuary severely degraded by pollution, invasive species and other issues. If other threats to the Delta are reduced, more water potentially could be taken out.
The report, released Wednesday, was required in a sweeping package of water reform laws passed last year. It is meant to determine, based solely on science, how much water needs to flow through the Delta to maintain "public trust" values, especially fish populations that are meant to be passed on to future generations.
The results could be used in Delta planning and are expected to be used as a yardstick for plans to build water tunnels under the Delta that water agencies say would improve their supplies and comply with endangered species laws.
"Flows are critical to protecting this estuary," said Gary Bobker, program director at The Bay Institute, an environmental advocacy organization. "In order to restore the species and habitats you have to dedicate significantly more water than we do today."
The report also details flow requirements for the two biggest rivers flowing into the Delta — the Sacramento and the San Joaquin.
Meeting all of those requirements would require San Joaquin farms, Southern California and portions of the East Bay and South Bay that rely on pumps in the southern Delta to cut their Delta water use by one-third in addition to recent cutbacks required to meet endangered species rules.
For other water users upstream, including utilities serving Oakland and San Francisco, the effect could be even worse — up to 70 percent, because the goal to increase river flows would make more water available in the Delta for pumps to export.
But those figures do not take into account water rights laws that say agencies with older rights — including some in the Bay Area — should not have to give up water for newer users, and that agencies closer to water sources also should not have to give up water to those relying on Delta pumps.
Big water agencies that rely on the pumps criticized the report in news releases as imbalanced and "purely theoretical."
Environmentalists cautioned that reducing the state's reliance on the Delta did not have to be painful because conservation, desalination, water recycling and other strategies could be more aggressively pursued.
In the Contra Costa Water District, for example, a water supply cut of 25 or 30 percent that might be a goal for agencies drawing out of the south Delta is not that far from the state goal set in law last year to conserve 20 percent more water by 2020.
"We're well on our way to that," said Greg Gartrell, assistant general manager at the Concord-based water district.
Gartrell said the report is consistent with numerous recent scientific reports that have all concluded "California is going to have less dependence on the Delta in order to restore fisheries."
In the 1980s and 1990s, the state board's staff came to similar conclusions about the need for more water to flow through the Delta but intense political pressure squelched those reports.
Those earlier reports would have required cutbacks.
The report released Wednesday, by contrast, is merely informational.
The report is due for consideration and possible changes at state board meeting Aug. 3.
Mike Taugher covers the environment. Contact him at 925-943-8257.