In summary

As climate change threatens to affect California’s water supply, outgoing Gov. Jerry Brown tries to do a big picture water deal in the final days of his governorship.

As the Civil War raged, William Brewer, a young botanist from upstate New York, spent five years cataloging California’s natural attributes for its Legislature.

As he and his crew traversed the state by mule in their annual sojourns, living off the land, Brewer found much to commend. But in letters to his brother, decades later assembled into a must-read book (Up and Down California), Brewer also wondered whether its climate would impede its development.

He was particularly negative about what we now call the San Joaquin Valley, seeing it as an inhospitable desert and unsuitable for agriculture.

Of course, California’s Central Valley is now one of the planet’s most productive agricultural regions, thanks to dams, reservoirs and canals that capture runoff from winter snows in the Sierra to support irrigation during the summer growing season.

It’s a stark example of one of California’s abiding truths: A reliable supply of water is the single most important factor in the state’s evolution from the sparsely populated, semi-arid wilderness that Brewer surveyed into a global powerhouse.

Protecting that supply stands atop California’s civic agenda, but doing so is increasingly difficult, as a new report from the Public Policy Institute of California underscores.

Myriad factors are placing new strains on California water, PPIC says, with the effects of climate change potentially the most intractable.

“California’s climate is highly variable, with frequent droughts and floods,” PPIC points out. “Climate models predict significant changes: warmer temperatures; shorter, more intense wet seasons; and more volatile precipitation – with wetter wet years and drier dry years.”

It continues: “Warming has complex and interrelated effects: it reduces the share of precipitation falling as snow, causes earlier snowpack melting and increased winter runoff, raises water temperatures, and amplifies the severity of droughts and floods.”

PPIC’s report was issued just as California undergoes a change of political leadership, and as outgoing Gov. Jerry Brown tries to make a big water deal.

The State Water Resources Control Board, appointed by the governor, threatens to shift huge amounts of water from farmers into stronger flows through the Sacramento-San Joaquin Delta to protect wildlife habitat. Farmers, of course, don’t like it, saying it will drive them out of business and/or force them to rely more on already overdrafted underground pools.

Just as the board was poised to vote this month, it delayed action at the request of Brown and his successor, Lt. Gov. Gavin Newsom. Brown told reporters that he hoped to work out a compromise within the next month.

Brown’s lame-duck status may work to his advantage. The pending decree gives him overt leverage with farm interests, as does an implicit threat that a more liberal Newsom might be less willing to deal.

Lurking in the background, meanwhile, is Brown’s long-sought project to bore twin tunnels beneath the Delta that would divert water into the California Aqueduct near Tracy.

Environmental groups are leery about the tunnels because they would – or at least could – deprive the Delta of flows. Implicitly, shifting more water from farms could offset the tunnel diversions and might dampen opposition to the project from environmentalists.

Together, in effect, the water board plan and the tunnels would enhance both the Delta and water transfers to Southern California cities at the expense of farmers.

Newsom, obviously, wants Brown’s drive for a big picture water deal to succeed, because it might allow him to set aside the ever-present water issue, at least for a while, and sidestep the tunnel controversy altogether.

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Dan Walters has been a journalist for more than 60 years, spending all but a few of those years working for California newspapers. He began his professional career in 1960, at age 16, at the Humboldt Times...