Packed onto the is a precious source of water for California 鈥� a frozen reservoir that climate change is already transforming.
As the planet warms, the spring snowpack . The snow is creeping up mountainsides , melting and seeping into dry soils rather than washing into rivers and streams that feed reservoirs.
The risks are no longer futuristic or theoretical: The state鈥檚 projections for how much water to expect from the Sierra Nevada were so far from reality last spring that reforming the process has become increasingly urgent.
The calculation for the Sacramento River region , leaving the state鈥檚 reservoirs with far less water supply than expected.
鈥淚f you鈥檝e changed the climate and then you try to use statistics 鈥� which relies on what happened in the past 鈥� to predict the future, you鈥檙e already running into an issue,鈥� David Rizzardo, manager of the California Department of Water Resources鈥� hydrology section, told CalMatters.
State officials are altering their forecasts to take into account the myriad ways climate change is reshaping California, from warming temperatures to soil dryness. The stakes are huge: The Sierra Nevada snowpack provides of California鈥檚 water supply.
Some California water watchers wonder: What鈥檚 taken so long?
鈥淲e鈥檙e way past the time when we could ignore climate change,鈥� said Peter Gleick, a climate and water scientist who co-founded the Pacific Institute, a global water think-tank. 鈥淭he water agencies really need to get on the ball here. Can you tell I鈥檓 a little frustrated?鈥�
The process is complex, requiring a massive expansion of data collection from the state鈥檚 snowpack and watersheds, and an overhaul to the forecast calculations.
鈥淲e鈥檝e been forecasting since 1930. This is a complete overhaul,鈥� said Sean de Guzman, manager of the state鈥檚 Department of Water Resources鈥� snow surveys and water supply forecasting section.
Snowpack: How it鈥檚 measured and why it matters
When the weather warms and the rain stops, melting snow courses into waterways, then into reservoirs, faucets and sprinklers 鈥� supplying California鈥檚 homes, farms and wildlife right when they need it most.
To keep , engineers plunge tubes into the snow to gauge its depth and water content, blanket remote mountains with and scan the snow cover from planes flying over watersheds.
De Guzman鈥檚 team plugs the snow measurements, along with information about rain and streamflow, into their calculations to forecast how much snow is expected to melt and run off into rivers and reservoirs. The calculates its own forecasts in parallel, he said.
The results are critical for managing California鈥檚 precarious water supply year-round.
Reservoir managers use them to determine when to hold on to water and when to let it flow.
Operators of state and federal water supplies rely on them to determine how much water to send to the cities, growers and water suppliers dependent on water pumped south through the Delta to hundreds of miles of canals, tunnels and pipelines.
Weekly from February through mid-June help the powerful Westlands Water District, the largest agricultural water agency in the nation, game out the year ahead 鈥� planning how much supplemental water to buy and how much to charge growers.
鈥淭hose forecasts drive all the finances in our estimates, when we set rates at the beginning of the year,鈥� said Jose Gutierrez, Westlands鈥� chief operating officer.
Flood control, power generation and maintaining water quality for people, ecosystems and threatened and endangered species all rely on the runoff forecasts. Even outdoor enthusiasts benefit from the snowmelt predictions. 鈥淲e get a lot of calls saying, 鈥楬ey, you guys must know when the waterfalls in Yosemite are going to be going,鈥欌�� Rizzardo said.
The problem? The forecasts haven鈥檛 yet factored in how climate change has altered snowmelt.
鈥淐limate change,鈥� Rizzardo said, 鈥渉as thrown a monkey wrench at all this.鈥�
Climate change upends calculations
As climate change drives temperatures ever hotter, the snowpack is retreating up mountain sides to higher altitudes and melting earlier in the season. And the wet season is into a shorter, sharper .
The future, said state climatologist Michael Anderson, will continue to bring more rain and less snow and shift the surviving snowpack from the north鈥檚 lower peaks to the central and southern Sierra鈥檚 higher elevations. The shift will mean having to change water infrastructure to manage snowmelt storage and increased flood risks from rain mixing with snow.
鈥淚f you think of Lake Tahoe, we鈥檒l get to a future where at lake level there won鈥檛 be any snow but in perhaps the mountains, there still will be snow,鈥� Anderson said. 鈥淎nd then we watch it start to move upslope.鈥�
Scientists predict that in the next 35 to 60 years, if carbon dioxide and other greenhouse gases continue unchecked, the West鈥檚 snowpack could shrink even more substantially and even disappear .
鈥淲e鈥檒l get to a future where at lake level there won鈥檛 be any snow but in perhaps the mountains, there still will be snow. And then we watch it move upslope.鈥�MICHAEL ANDERSON, STATE CLIMATOLOGIST
California already has seen a preview of this future, said , a research scientist at Lawrence Berkeley National Laboratory and professor at the University of California, Berkeley.
In April 2015, former Gov. Jerry Brown watched as state staff measured a snowpack that didn鈥檛 exist 鈥� right when it should have been at its peak. It was the height of the last drought, which stretched from , and patched with dry grass. Behind him rose the bare slopes of what should have been snow-capped peaks.
鈥淭o know that this change is happening, and yet we鈥檙e all just living our lives and turning on the tap and using water as we always have done鈥t gives me a sense of appreciation for the fragility of the system that we have,鈥� Jones said.
Though drought grips California once again, the snowpack wasn鈥檛 as scarce last year as it was in 2015. It was calculated at about 59% of normal . But it took only one month for that snowpack to dwindle to 22% of normal in May. And, worse still, the rapidly melting snow didn鈥檛 refill rivers and reservoirs as expected.
Instead, it soaked into thirsty soils or disappeared into the air. By May, the runoff forecast for the Sacramento Valley had dropped by about 700,000 acre feet 鈥� enough water to supply 2.1 million Southern California households. All told, the forecasts overestimated runoff by 68% for the Sacramento River region and by 45% or more for major watersheds farther south, .
鈥淭hat was basically something we had never seen before. We have these various relationships that tell us if we have this much snow, you can expect this much water,鈥� de Guzman said. 鈥淎nd that basically fell apart in 2021.鈥�
Gleick said the overestimate had massive ramifications for the environment and the year ahead. For instance, when there was less water than projected, operators of the state and federal water projects to relax requirements aimed at preventing saltwater from tainting key Delta water supplies in order to preserve more water in storage.
The shortfall was no surprise to Gleick, in the 1980s warned that climate change would shrink the snowpack.
鈥淚 would have suggested fixing the algorithms by the year 1990. But that didn鈥檛 happen,鈥� he said. 鈥淪o the best time to do it is right now.鈥�
Turning up the forecasts
Revamping runoff forecasts will require collecting better data about the dwindling snowpack and creating more comprehensive models that better capture the changing conditions.
鈥淚t鈥檚 an understandable concern, (but) it isn鈥檛 easy science,鈥� Rizzardo said.
鈥淲hat last year did was say, 鈥極kay, we just need to kick all this into high gear, and figure out a way to get it done.鈥欌��
Better data is already in the works. Ten years ago, the Department of Water Resources teamed up with to conduct detailed surveys of snow cover from airplanes equipped with a remote sensing device called lidar and other instruments.
So far, the surveys have been limited to five of the state鈥檚 watersheds. Though 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 for feeding new, data-hungry models informed by climate factors and incorporating more information about the watersheds themselves, such as vegetation, temperature and soil moisture.
New technology, including sensors that quickly assess the snowpack鈥檚 temperature and how much water it contains, are now being test-driven by the and state officials.
The question is whether the scientists will have to start from scratch and build a new model 鈥渙r are there ways that we can tweak the existing models to really make them more accurate again?鈥� said , the snow lab鈥檚 lead scientist and station manager.
鈥淭he old models that have been developed for this runoff no longer really apply to today鈥檚 climate, because the climate has changed already,鈥� he said.
鈥淲e have these various relationships that tell us if we have this much snow, you can expect this much water. And that basically fell apart in 2021.鈥�SEAN DE GUZMAN, DEPARTMENT OF WATER RESOURCES
As part of a pilot project this year, will include data from the airborne snow surveys of the Feather River and San Joaquin watersheds, and spit out forecasts that the scientists will compare to their current approach.
They already tried using machine-learning techniques to weigh factors like atmospheric dryness, soil moisture and temperature, but the multi-year effort yielded only slight improvements, de Guzman said.
This year, the team is working on what he calls a major tuneup, incorporating more recent rain, snow and runoff data that better captures the relationships under climate change.
鈥淏y changing a lot of the datasets that we鈥檙e feeding into the models, that will hopefully help give us a better picture of what we鈥檙e now seeing,鈥� he said.
Despite the challenge of forecasting the future, some state officials don鈥檛 expect as significant a gap between expectation and reality this year. Although storms late last year built up the snowpack and soaked the earth, priming conditions for more snowmelt to reach reservoirs.
Rizzardo, though, is less optimistic, particularly after the Berkeley snow lab reported a . 鈥淭his is also part of the question mark, because we鈥檙e seeing things we鈥檝e never seen before. And so we can鈥檛 say with certainty, 鈥楾his is what it鈥檚 going to be.鈥欌��
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