Phillips Station in April of 2021. April 1 is typically when California’s snowpack is the deepest and has the highest SWE. Photo by DWR.

NOTEBOOK FEATURE: In 2021 people used all the runoff in the Delta watershed—how it happened and lessons learned

by Robin Meadows 

This year is a classic example of the way California’s water system was designed to work. Winter storms dropped towering snowpacks on the mountains, and spring was so cool that the snowmelt will likely last through the summer.

But just two years ago, California was in a severe drought: 2021 was the state’s second driest year on record and was also exceptionally hot. The snowpack vanished before summer even began.

“It went from 70 percent in the northern Sierra Nevada in April to zero percent by May,” says California Department of Water Resources director Karla Nemeth. DWR operates the State Water Project, which extends more than 700 miles from northern to southern California, and delivers water to 27 million people and 750,000 acres of farmland.

The Sierra Nevada snowpack was 70% of average in late March 2021 …
… versus 234% in early April 2023. Images by NASA.

Lessons from 2021 are helping the state prepare for future droughts. In hindsight, it’s easy to see what went wrong. “It was a combination of bad luck and steep runoff,” Nemeth says. Runoff is water that sheets across land and flows into streams, rivers and lakes.

The bad luck included the weather. Besides hastening snowmelt, the warm spring prompted water requests from farmers eager to take advantage of the early growing season. There was also far less runoff than anticipated based on water supply models.

Low water in Lake Oroville during the summer of 2021. Photo by DWR.

“It wasn’t until later that we understood the magnitude of the problem,” Nemeth says. By then it was too late: the state had already allocated a lot of water to farmers.

The loss of runoff in 2021 took the agency by surprise because water supply models hadn’t accounted for a key factor: it was the second dry year in a row. The previous year had left the ground so parched that, in the spring of 2021, the thirsty soil drank much more snowmelt than anticipated, resulting in far less runoff than had been predicted.

“There was about 700,000 acre-feet less runoff than expected,” says Jay Lund, an environmental engineer who is vice director of the UC Davis Center for Watershed Sciences and former chair of the Delta Independent Science Board. This missing water would have been enough to supply up to 1.4 million households or nearly 200,000 acres of farmland.


That runoff would also have helped keep enough water flowing through the Sacramento-San Joaquin Delta. Forty percent of California’s runoff comes from the Delta watershed, which supplies water to about 30 million people and six million acres of farmland.

Comparing Delta flows in wet and dry years. Figure by PPIC.

Flows are vital to keeping the Delta’s water fit for human use and aquatic life, including salmon, smelt and other imperiled species. The Delta connects to the San Francisco Bay which then connects to the ocean, and tides can push salty water deep inland. “You need enough outflow to keep the salinity at bay,” Lund says.

“In the drought of the 1930s, there were no regulations for Delta flows,” Lund continues. “The salinity was too high as far as Stockton.” This Central Valley city is more than 80 miles east of San Francisco.

But in 2021, as documented in a Public Policy Institute of California analysis, people used all the available water in the Delta watershed. Lund suspects this also happened in previous dry years, ticking off the droughts of 1976-77, 1991, 2014 and 2015.

Jeffrey Mount, a river scientist and PPIC water policy fellow, calls this complete use of runoff astonishing. “One hundred percent of the runoff in 2021 was used upstream and in the Delta—there was nothing for Delta outflow but the water stored in reservoirs.”


While California naturally cycles between drought and deluge, most of the last decade has been exceptionally dry and warm. This trend will likely intensify with climate change. “This year is wet, and we’re lucky, but it’s always funny how quickly we can forget the dry years,” says Sarah Null, an environmental water management expert at Utah State University who did her graduate work at UC Davis. “Our memories are so short.”

That’s beginning to change. Thanks to a lesson drawn from the previous drought of 2012-2016, for example, there was enough stored water to supply Delta flows in 2021. This is because instead of allocating all the water in Lake Oroville in 2020, also a drought year, the Department of Water Resources saved some for the next year.

Lake Oroville is the State Water Project’s largest reservoir and the water saved at the end of 2020 was considerable, amounting to 20 percent or 300,000 acre-feet. This carryover storage from one year to the next is a hedge against future drought years.


Moreover, new strategies for tightening water supply forecasts and for managing supplies prudently are underway. The forecasting side includes physical models of runoff, which will incorporate data from a planned network of soil moisture sensors in the Sierra Nevada. Work on these models entails collaborations with federal agencies, which stalled under the previous federal administration but are now “massively accelerated,” Nemeth says.

The Tuolumne River Basin seen by the Airborne Snow Observatory in 2013. Photo by NASA/JPL-Caltech.

“We can’t just rely on past hydrology,” Nemeth says. “One of the lessons of 2021 is that we need to account for upstream hydrology—we need to know how much water is going to overall dry conditions.”

“We need to modernize and be nimble,” Nemeth continues. “We need data to make real-time decisions like curtailments, and to inspire confidence in the water rights system—people need predictability.”

The physical models also require accurate inputs on how much snow there is and how fast it’s melting. A recently launched program, called the Airborne Snow Observatory, measures the snowpack via airplanes outfitted with lidar instruments that shoot lasers toward the snow. The deeper the snowpack, the faster the lasers bounce back compared to baseline rates measured when there was no snow on the ground. This provides a high-resolution, three-dimensional map of the snowpack.

How lidar measures the depth of the snowpack. Figure by Thomas Painter.

“It goes all the way up and over the crest of the Sierra Nevada,” Nemeth says. Traditional snowpack estimates are based on instruments that can only be placed up to a certain altitude. In 2022, the Airborne Snow Observatory expanded to mapping the snowpack in the Feather watershed, which drains into Lake Oroville.

The management side includes routinely saving water from year to year. “We need to get in the business of carrying over more to avoid drastic cutbacks,” Nemeth says, citing the 2022 curtailments. Water deliveries to Sacramento Valley growers dropped to 18 percent, fallowing hundreds of thousands of acres and resulting in devastating economic and job losses.


The environment needs a reliable water supply too, Utah State University’s Null says. She hopes to see more water allocated to stewarding ecosystems through dry times. “The environment is outside the water rights process,” she says. “Most freshwater ecosystems suffer during drought—it’s even more of a problem in back-to-back dry years.”

As Null notes in a 2021 California WaterBlog post, one option would be to dedicate a share of reservoir and aquifer storage to waterways and aquatic life. “When we allocate all the water to upstream users, there’s no room for error,” she says. “We need to get ahead of the problem.”

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