California lacks the capacity to store water that atmospheric rivers dump on the state

President Donald Trump is obsessed with how California manages its water supply, demanding changes as one price of giving the state billions of dollars in aid to cope with Southern California’s deadly and destructive wildfires.

However, Trump’s specific complaints are not grounded in hydrologic or managerial reality — such as his insistence that a lack of water from Northern California was a factor in either the fires’ eruption or the firefighting efforts. Hydrants dried up largely because systems were designed to deal with individual structure fires, not widespread wildfires involving thousands of buildings.

That said, there’s much to criticize in how California, once a global leader in large-scale water management, has faltered. Population growth and evolving agricultural practices have increased demand, while federal and state environmental laws, judicial decisions, political foot-dragging and climate change have restricted supply.

One major failing has been a slow response to an obvious need for more water storage — either in reservoirs or underground aquifers — to capture winter rains and spring snowmelts as a buffer for dry years.

Scientists believe that even if California’s overall water supply from rain and snow storms doesn’t decline, wet and dry cycles have become more intense, and more precipitation is coming as rain instead of snow. Thus the natural reservoirs of snowpacks in the Sierra and other mountain ranges are becoming less dependable, increasing the need for supplemental storage.

California’s most recent experiences — two wet winters that defied some forecasts — underscore the need.

A new report from the Public Policy Institute of California points out that the atmospheric rivers that dropped immense quantities of rain and snow on the state this month, following a very dry January, did not result in substantial new storage in the state’s major reservoirs.

“Rather than storing all the water they can, during the winter reservoir operators are required to maintain enough space in their reservoirs to capture high inflows and reduce the risk of flooding downstream,” PPIC researchers Jeffrey Mount and Greg Gartrell wrote.

“When the February storms arrived, the surge of water into the state’s two largest reservoirs — Shasta and Oroville — quickly filled the flood reserve space. Because the winter flood season is far from over, dam operators had no choice but to let the water go to make space for possible future floods.

“And they let go a lot of water. Between February 1 and 18, those two reservoirs alone released more than 2 million acre-feet of water into the Sacramento and Feather Rivers to maintain space for future stormwater.”

Read More: How can California boost its water supply?

An acre-foot is 325,851 gallons and 2 million-acre feet equates to more than half of Oroville’s capacity, or about 20% of what Californians consume each year for non-agricultural purposes.

Overall, Mount and Gartrell calculated, 5.1 million acre-feet of water flowed into the Sacramento-San Joaquin Delta from storm runoff and reservoir releases during that period in February. Just 4% of it could be diverted into storage because of insufficient capacity and operational mandates.

Even a relatively tiny increase in storage capacity could pay huge dividends when wet winters such as this evolve into periods of drought. Had the long-proposed Sites Reservoir on the west side of the Sacramento Valley existed, it could have banked as much as 1.5 million acre-feet of that excess flow.

California’s water managers acknowledge the need for more storage to take advantage of high-precipitation winters such as this one, but clearing all of the legal and financial hurdles and actually building it take decades. Sites, first proposed seven decades ago, is just now beginning to appear feasible.

The hydrological reality of California’s water supply is changing faster than our willingness to deal with it. The outcome of that disparity is perilous.