Record-breaking heat melted California’s snowpack early

from Rachel BeckerCalMatters

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A record-breaking heat wave is scorching California with major implications for the state’s most important reservoir: the snowpack.

Insurance about a third of the state’s water supply, the Sierra Nevada snowpack is a vital source of spring and summer runoff that refills reservoirs when the state needs water the most.

But a warm wet storm followed the February snow, and now, March temperatures are breaking records – prompting warnings of rapid snowmelt and rushing rivers.

Historically, the snow cover is deepest in April. But climate change is displacement of outflow earlierdeparture less water flows down the mountains in the warmer months for homes, farms, fish, hydropower and forests.

“In an ideal world, your reservoir would be full right now and this extra huge reservoir with snowpack that we know will help fill and provide more water supply,” said Levi Johnson, operations manager for Central Valley Projectthe massive federal water supply system that channels Northern California’s river waters to the Central Valley and parts of the Bay Area.

This year, he said, “we’re not going to have that.”

California reservoirs are in good condition, overflowing above historical averages with very approaching capacity. But that summer snowpack on the slopes of the Sierra Nevada is disappearing early and fast… down to 38% of the mid-March average throughout the country.

It’s still not the worst snowpack on record: That distinction belonged to 2015, when then-Governor Jerry Brown stood on the brown, barren slopes of the Sierra Nevada to watch scientists measure the sparsest snowpack on record.

But this year’s snowpack is quickly approaching the top five worst on record for April 1, state climatologist Michael Anderson said — and it’s likely to get worse as temperatures rise. From the beginning to the middle of March, the snow cover was are disappearing at a rate of about 1% per day.

This is a sharp departure from close to last year’s average conditionsand presents both a challenge and a glimpse into the future for reservoir operators in the state.

Conflicting roles for reservoirs

Many of California’s reservoirs serve a dual role: stemming flood flows and storing water for drier times ahead.

Those roles sometimes conflict — as happened with Lake Mendocino, which dried up to a muddy puddle during the 2012-16 drought. Strict federal operating rules forced the U.S. Army Corps of Engineers to release vital water supplies from the dam to make way for winter floods that didn’t come.

The huge water shortage that followed stimulated experimental partnership called Forecast Informed Reservoir Operations, between the Scripps Institution of Oceanography at UC San Diego’s Center for Western Weather and Water Extremes and state, federal and local agencies.

The program incorporates advanced forecasting and meteorological observations into Lake Mendocino reservoir release decisions. That kept the reservoir from drying out during the last drought, according to Don Seymour, deputy director of engineering at Sonoma Water, which operates the reservoir.

Now, 165 miles into the Sierra foothills, the Yuba Water Agency intends to adopt the same program for New Bullards Bar, a reservoir roughly eight times the size of Lake Mendocino fed by Sierra snowmelt on the North Yuba River.

The reservoir supplies water to more than 60,000 acres of farmland in Yuba County as well as to users south of the Delta. But early snowmelt complicates efforts to store that water.

“We’re seeing snowmelt conditions in mid-March that we don’t normally see until at least mid-May,” said general manager Willie Whitley. “It’s pretty obvious that it’s the runoff — it’s the snowmelt — and it’s just happening about two months earlier.”

The tank is almost full at 114% of the average for that date and 84% of the total capacity.

But when snowmelt arrives early, the agency can’t capture it after the reservoir reaches a certain level — even when there are no storms in the immediate forecast. Federal rules require Yuba Water to maintain a certain amount of empty space until June to absorb potential flooding, according to Whittlesey.

Yuba Water has been working with the U.S. Army Corps of Engineers to update that regulation for decades, Whitley said, but until then it must request a special permit to store the extra water.

Although the agency has received permission in the past, this year it also struggled with a major pipe rupture to one of its hydropower facilities, forcing the agency to hold more water behind the dam.

Whittlesey said he suspects the combination of flood control and damage control requirements after the pipe failure likely cost them tens of thousands of acre-feet of snowmelt.

The California Department of Water Resources, which manages Lake Oroville — the state’s second-largest reservoir — told CalMatters it is storing water beyond normal flood control limits, with a permit from the U.S. Army Corps of Engineers.

In the Bay Area, the East Bay Municipal Utility District, California’s second largest municipal water provider, owns and operates Kamanche and Pardee tanks in the foothills of the Central Sierra.

“We’re working to conserve every drop in light of the high temperatures we’re experiencing now and in light of all the zeros we’re seeing in the rain or snow forecast,” spokeswoman Andrea Puck said. “The last time we escaped this early was in 2015.”

Pook said the district is releasing less water from its reservoirs now to preserve more for the fall, when salmon migrate upriver to spawn.

“We’re tracking to not necessarily be in a drought situation. But I’m not confident we’ll fill our reservoirs by July 1, which is our usual goal,” Pook said.

Improved predictions after a big miss

Even as California suffers record heat and early snowmelt, the state is better prepared than in the past.

five years ago state forecasters missed their runoff predictions badly — overestimation of snowmelt expected to refill reservoirs by up to 68%. Dry soils and a parched atmosphere drank the runoff before it could flow into the reservoir. Farms and cities faced a drought as supplies fell far short of expectations.

This year is different. The main tanks are now above historical averagesand early-season storms wetted the soil beneath the snowpack, making it less likely to absorb runoff.

The state is also working on better forecasts.

“Things have improved significantly,” said Andrew Schwartz, director of UC Berkeley’s Central Sierra Snow Lab, in an email to CalMatters.

Johnson, of the federal Central Valley Project, said state and federal water systems are in better shape than they were five years ago, and that forecasts haven’t gotten much wrong since then.

But an early season melt could still leave a gap.

“He’s going to get us through this year well,” Johnson said. “But it’s not as ideal as having that extra snow reservoir ready to run out in the summer and replenish what we’re going to release.”

Improved modeling of snow cover and soil moisture assessmentaccording to Water Resources Department..

Still, between the state budget shortfall and federal cuts, challenges remain, Anderson said.

Efforts to install more soil moisture sensors on national forests have run into permitting delays at the U.S. Forest Service, which has laid off thousands of employees under President Donald Trump.

“You’re waiting in line a lot longer,” Anderson said. “That’s the biggest restriction lately. There’s just nobody.”

This article was originally published on CalMatters and is republished under Creative Commons Attribution-NonCommercial-No Derivatives license.