The Sierra Nevada has not provided as much water as predicted. Now the state is scrambling to revise its snow runoff forecasts.
by Rachel Becker, CalMatters Network
A valuable water source for California is stored in the foothills of the Sierra Nevada , a frozen reservoir that climate change is already transforming. As the planet warms, the snow cover decreases . Snow is melting down mountainsides toward higher elevations and seeping into dry soils instead of melting into rivers and streams that feed reservoirs.
The stakes are no longer futuristic or theoretical: The state’s projections for the amount of water expected from the Sierra Nevada were so far off the mark last spring that reforming the process has become increasingly urgent. The water estimate for the Sacramento River region decreased by 68% , leaving the state’s reservoirs with far less water supply than expected.
“If the weather has changed. And according to the statistics, which are based on what happened in the past to predict the future, a problem has already been found,” David Rizzardo, manager of the California Department of Water, told CalMatters.
State officials are adjusting their forecasts to account for the myriad ways climate change is changing the state of California, from rising temperatures to drying soil. The stakes are high: The Sierra Nevada snowpack provides about a third of California’s water supply.
Some California water experts ask: Why has it taken so long?
“The time to ignore climate change is over,” said Peter Gleick, a climate and water scientist who co-founded the Pacific Institute, a water think tank. “Water agencies need to start working on fixing this problem as soon as possible.”
The process is complex and requires a massive expansion of the state’s snow pack and watershed data collection, and a revision of forecast calculations.
“We’ve been forecasting since 1930. This is a complete overhaul,” said Sean de Guzman, manager of the snow studies and water supply forecasting section at the state Department of Water Resources.
Snowpack: how it is measured and why it is important
When temperatures start to rise and it stops raining, melting snow turns to water, and when it’s needed most, it’s the primary source of water for California homes, farms and wildlife.
To closely monitor this precious resource , engineers like Sean de Guzman plunge tubes into snow to measure its depth and water content, blanket remote mountains with sensors and weather stations, and scan snowpack from planes flying over watersheds.
De Guzmán’s team incorporates the snow measurements, along with rainfall and streamflow information, into their calculations to forecast the snow that is expected to melt and run off into rivers and reservoirs. California ‘s Nevada River Forecast Center calculates its own forecasts, he said.
The results are critical to managing California’s precarious year-round water supply. Reservoir managers use them to determine when to hold water and when to let it flow. State and federal water supply operators rely on these experts to determine how much water they can send to cities. Weekly forecasts from February through mid-June help the Westlands Water District, the nation’s largest agricultural water agency, plan for the year ahead: they plan what additional water to buy and how much to charge producers.
“Those forecasts are very helpful when we set the rates at the beginning of the year,” said José Gutiérrez, director of operations for Westlands.
Flood control, power generation, and maintaining water quality for people, ecosystems, and threatened and endangered species depend on forecasts. Even outdoor enthusiasts benefit from melt forecasts.
“We get a lot of calls saying, ‘Hey, you guys need to know when the waterfalls in Yosemite are going to be open,’” Rizzardo said. The problem? Forecasts have not yet taken into account how climate change has altered melting. “Climate change,” Rizzardo added, “has thrown a monkey wrench into all of this.”
Climate change upsets calculations
As climate change drives temperatures higher and higher, snowpack is melting earlier in the season and moving down mountainsides to higher altitudes. With all this, the rainy seasons are getting shorter and shorter .
The future, state climatologist Michael Anderson said, will continue to bring more rain and less snow and shift the surviving snowpack from the lower northern peaks to the higher elevations of the central and southern Sierra. The change will mean having to change water infrastructure to manage melted snow storage and increase flood risks from rain mixing with snow.
“If we think about Lake Tahoe, we’re going to get to a future where there’s no snow at lake level, but maybe in the mountains there is,” Anderson said. “And then it will start moving uphill.”
Scientists predict that in the next 35 to 60 years, if carbon dioxide and other greenhouse gases are left unchecked, Western snow cover could shrink even more substantially and even disappear for a decade or more .
“We will get to a future where at lake level there will be no snow, but maybe in the mountains, there will still be snow. And then it will start moving up.” michael anderson, state climatologist
California has already seen a foretaste of this future, said Andrew Jones , a research scientist at Lawrence Berkeley National Laboratory and a professor at the University of California, Berkeley. In April 2015, former Governor Jerry Brown demonstrated how the state measured snow cover that didn’t exist, just when it should have been at its peak. This was when the last drought started ( 2012 and continued until 2016 ).
“Knowing that this change is happening and we are not aware of the seriousness of the problem and we continue to waste water, it makes me feel sad because we are facing a very serious problem and nothing is being done to solve it,” said Jones.
Although drought grips California once again, snow cover was not as light last year as it was in 2015. It was estimated to be about 59% of normal in April 2021 . But it only took a month for that snow cover to drop to 22% of normal in May. And, worse yet, the rapidly melting snow did not refill the rivers and reservoirs as expected. Instead, it soaked into sedimentary soils or disappeared into thin air. By May, the runoff forecast for the Sacramento Valley had been reduced by approximately 700,000 acres.
Still, it’s enough to supply water to 2.1 million homes in Southern California. In all, forecasts estimated runoff at 68% for the Sacramento River region and 45% or more for major watersheds south of the state, according to a state report .
“That’s something we’ve never seen before. We have these various relationships that tell us that if we have that much snow, we will also have a lot of water,” de Guzmán said. “And that fell apart in 2021.”
Gleick said that all the changes that are being seen have been caused by climate change and are affecting the amount of water available. For example, when there was less water than projected, state and federal water project operators asked regulators to relax requirements needed to prevent saltwater from contaminating Delta water supplies so they could conserve more water in storage.
The problem did not surprise Gleick, who in one of his investigations in the 1980s warned that climate change would reduce snow cover.
“I would have proposed to fix the algorithms for the year 1990. But that didn’t happen,” he added. “So the best time to do it is now.”
Changing the forecasts
The priority for water experts is to collect better data on declining snow cover and create more comprehensive models that better capture climate conditions that are currently changing.
“It’s an understandable concern, (but) it’s not easy science,” Rizzardo explained. “What was done last year was to say, ‘Okay, we just have to get this all up and running and find a way to do it.'”
Work is already underway to get better data. Ten years ago, the Department of Water Resources partnered with NASA’s Jet Propulsion Laboratory to conduct detailed studies of snow cover using aircraft equipped with a remote sensing device called lidar and other instruments.
So far, investigations have focused on just five watersheds in the state. Although the partnership with NASA has ended, the list will almost double this year with the addition of the Feather, Yuba, Truckee and Carson rivers. These measurements will be critical in helping to create new data models that help with all climate factors and will incorporate more information about watersheds, such as vegetation, temperature and soil moisture.
The new technology, including sensors that rapidly assess the temperature of the snowpack and the amount of water it contains, is now being tested by the University of California, Berkeley’s Central Sierra Snow Laboratory and state officials.
The question is whether scientists will have to start from scratch and build a new model “or are there ways to modify existing models to actually make them more accurate again?” said Andrew Schwartz , chief scientist and manager of the snow lab station. “The old models that have been developed for this runoff no longer really apply to the current climate, because the climate has already changed,” he added.
“We have various data that tells us that if we have a lot of snow, we will also have a lot of water. But that fell apart in 2021.” Sean de Guzman, Department of Water Resources
As part of a pilot project this year, a new data model from airborne snow surveys of the Feather River and San Joaquin watersheds will return forecasts that scientists will compare with their current approach. They have already tried using machine learning techniques to measure factors such as atmospheric drought, soil moisture and temperature, but the multi-year effort found slight improvements, de Guzmán said.
This year, the team is working on what they call a major change, incorporating more recent rain, snow, and runoff data that capture better data with climate change.
“We have been introducing new equipment to get better data and hopefully that will help us get a better picture of what we are seeing now,” Guzman said.
Despite the challenge of forecasting the future, some state officials don’t expect such a significant gap between expectations and reality this year. Although dry conditions now persist, storms late last year built up snowpack and soaked the land, setting the stage for more snowmelt to reach reservoirs. Rizzardo, however, is less optimistic, particularly after the Berkeley Snow Labreported a new drought record .
“This is also part of the question mark, because we are seeing things that we have never seen before. And so we can’t say with certainty, ‘This is what’s going to happen.’”
This article was originally published by CalMatters .
