New reservoir rules in the West could keep more winter water to combat drought

September 17, 2020

By Trina Kleist

Lake Mendocino, California — At Lake Mendocino in northern California, researchers tested a new system for predicting winter storms, rainfall and how much water would flow into the reservoir. Their result: Water managers were able to store 20 percent more water in the reservoir, saving it for the dry summer to come. The findings are expected to lead to new rules governing wintertime water levels across the American West. Photo by Alexey Komarov/Wikimedia Commons.

It’s November 2075, and all the makings of a monster storm are brewing in the western Pacific Ocean. By February, a river of warmish rain will pummel the Sierra Nevada. In the midst of the century’s fourth 500-year drought, water agency managers know they need that precious liquid for summer. Thanks to research decades earlier, they also know within a smidge how much rain will fall and how much water will pour off the high mountains. That tells them how much to let gush out of their reservoirs to make room for what’s coming. And, with their grandparents’ memories of the disastrous winter of ’17 oft retold — about the year that heavy snows followed by a warm winter rain created huge early run-off, threatening California’s Oroville Dam with collapse — these managers also alert local emergency officials, who start preparations months ahead of potential calamity.

Hydrologist Adrian Harpold of the University of Nevada, Reno, thinks a lot about that 2075 scenario. He and teammates are working toward new tools to forecast wintertime storms and monitor run-off flowing downstream amid a warming climate. Their work includes tracking signals as seemingly small as the sap flowing up trees.

Research Ava Cooper and a teammate work on setting up a weather balloon, which is about a meter in diameter. — In addition to studying snowmelt and rain run-off, researchers also are studying weather to better understand how to manage water in western reservoirs. Researcher Ava Cooper, left, and Peter Yao prepare a weather balloon to send up for a research flight. Photo by Carolyn Ellis/Scripps Institution of Oceanography.

These studies — part of the Forecast-Informed Reservoir Operations project– could lead to changes in rules for federally controlled reservoirs, including Boca, Stampede and Prosser, which store water for Reno and northern Nevada. The rules tell water managers how much they can keep in reservoirs during winter. Usually, water levels stay pretty low. The idea is to leave room to capture a sudden surge of wintertime run-off, minimizing disastrous floods.

Changing the rules just a little would let reservoirs keep more of that water, adapting to the shifts in the West’s water cycle caused by global warming. Yet, the rules would still protect downstream communities from floods, a recent experiment shows.

And when it comes to managing water in the American West, “a marginal improvement is worth a lot of money,” Harpold said. “A mistake is going to be on the national news.”

Ava Cooper works on a large piece of equipment with a dam and reservoir in the background. — Researcher Ava Cooper, with the Scripps Institution of Oceanography, checks on a sensor to study weather, snow and water movement near New Bullards Bar Reservoir in the Feather-Yuba watershed of northern California. The work is part of the Forecast-Informed Reservoir Operations project. Photo by Carolyn Ellis/Scripps Institution of Oceanography.

Experiment: 20 percent more water saved

Those reservoir-level rules were last updated decades ago, said Anna Wilson of the Scripps Institution for Oceanography. Since then, water managers and researchers have learned a lot more about the patterns of the West’s highly variable water cycle. In addition, warming climate has shifted much of wintertime’s snow to rain and made the rules inefficient. Some winters, water district managers have to let early run-off spill over and flow away, so they can keep reservoir levels at the federally required low level. Come summer, if it’s hot and dry like this year, they may wish they had that water.

Federal officials see the value of a rule change, but it makes them nervous. “There’s no tolerance for the risk of flooding,” said Wilson. She’s an environmental engineer for Scripps’ Center for Western Weather and Water Extremes. She’s working with Harpold and others across the West to put together the puzzle pieces to create a forecasting tool leading to that 2075 scenario.

The researchers came up with a new reservoir water-level rule and tested it in 2019. Their guinea pig is Lake Mendocino, which stores water from the Russian River basin toward the central California Coast.

“We showed they were able to keep about 20 percent more water in the reservoir over what they would have had with the existing” federal rule, Wilson said. That amounts to 11,000 acre-feet of fluid, enough to supply 11,000 average households for a year. “That much water was there at the end of the water year (on Sept. 30), which is very exciting.”

She described the collaboration as “huge,” including the United States Army Corps of Engineers and local water agencies. It was led by by Scripps weather center Director Marty Ralph and Sonoma Water Agency Chief Engineer Jay Jasperse.

Environmental engineer Anna Wilson installs measuring devices near Bodega Bay, Calif., as part of the Forecast-Informed Reservoir Operations project. The work aims to develop better forecasting and monitoring tools that would help reservoir managers save more wintertime run-off. Wilson is field research manager for the Center for Western Weather and Water Extremes, part of the Scripps Institution of Oceanography, based in San Diego, Calif., which is leading the FIRO project with the Sonoma Water Agency. Photo by Douglas Alden/Scripps Institution of Oceanography.

On the path to the Sierra Nevada

The project to create a new water-level rule is called Forecast-Informed Reservoir Operations. Wilson and collaborators are expanding the FIRO tests to a second reservoir in southern California and a third in the Feather-Yuba basin, which embraces Lake Oroville.

“We’re now on the path,” Harpold said. “We’re getting the tools better; we’re getting the pieces better. It’ll probably take 10 years to take this to the Sierras.”

A map of the Feather-Yuba watershed in northern California — The Feather-Yuba watershed, in northern California’s Sierra Nevada, includes Lake Oroville and Oroville Dam, in the Feather River. In February 2017, a winter rain falling over Sierra snow caused heavy run-off, which undermined the dam’s safety. Better tools to forecast storms and their resulting streamflow would give water managers more precise information and longer lead-time to prepare for such challenges. Map by Shannon1/Wikimedia Commons.

That will be an important jump. Sierra Nevada snow provides about 60 percent of water for California, as well as significant liquid for the greater Reno area and northwestern Nevada. So, if experiments like the Russian River FIRO trial could create decision support tools for managing Sierra Nevada water more efficiently, the pay-off would be huge.

“There are a lot of stressors on our infrastructure now,” Wilson said. “Instead of building new infrastructure, can we use our existing infrastructure better and more efficiently?”

The answer, they hope, will be ‘yes.’