In the West, Signs in the Snow Warn That a 20-Year #Drought Will Persist and Intensify — Inside #Climate News #snowpack #runoff #aridification

PHOTO CREDIT: McKenzie Skiles
McKenzie Skiles (right) measures snow density, which is used to estimate the amount of water in the snowpack.

From Inside Climate News (Bob Berwyn and Judy Fahys):

Critical April 1 snowpack readings once again spell trouble, and new studies show the warming climate is lengthening dry spells and shrinking the snowpack, even in winter.

Lack of monsoon rainfall last summer and spotty snowfall this winter combined to worsen the Western drought dramatically in the past year, and spring snowmelt won’t bring much relief. Critical April 1 measurements of snow accumulations from mountain ranges across the region show that most streams and rivers will once again flow well below average levels this year, stressing ecosystems and farms and depleting key reservoirs that are already at dangerously low levels.

As the climate warms, it’s likely that drought conditions will worsen and persist across much of the West. Dry spells between downpours and blizzards are getting longer, and snowpack in the mountains is starting to melt during winter, new research shows. The warming atmosphere may also be suppressing critical summer rains from the western monsoon.

West Drought Monitor April 6, 2021.

A year ago, when California and Colorado experienced their worst fire seasons on record, drought conditions spanned about half the West, and no areas experienced “extreme” or “exceptional” conditions. But going into this year’s dry season, about 90 percent of the region is now in drought, with 40 percent in those two most severe categories.

At the end of last month, the U.S. Drought Monitor showed exceptional drought spreading across roughly half of Nevada, Arizona, Utah and New Mexico, and extending up to northern Colorado. Utah was persistently dry from January 2020 through January of this year, with some welcome snow piling up in just the last few weeks, but too little and too late to stop the state’s steady drying. California’s snowpack is about half of average, according to that state’s April 1 snow survey…

April 1, 2021 streamflow forecast Colorado Basin River Forecast Center.

Forecasters expect this year’s annual flow into Lake Powell, a key Colorado River reservoir that helps distribute water to 40 million people and vast croplands, will only be around 45 percent of normal.

Other forecasters see little chance for drought-busting storms in April, the last full month of the western wet season. Off the West Coast, a bulge of warm and dry air known as the “Ridiculously Resilient Ridge” that shifts storms northward is forming in the same general area where it contributed to the extreme drought California experienced from 2012 to 2016. As a result, the outlook for the rest of the month in the state is trending toward “extremely dry.”

This year’s parched conditions aren’t surprising to climate scientists who study the region. Nearly all recent research suggests that the West is experiencing what climatologists call aridification—decades of drying with rare relief from occasional wet years.

Research published this week by the American Geophysical Union analyzed daily temperature and precipitation readings collected between 1976 and 2019 from 337 weather stations spread from North Dakota and Texas to the West Coast.

The analysis showed that, across the West, the longest dry spells between measurable precipitation are increasing by 2.4 days per decade, while precipitation is declining about 0.09 inches and temperatures are increasing at 0.36 degrees Fahrenheit every 10 years.

Hardest hit is the Desert Southwest, where average dry spells increased by 50 percent—from 31 to 48 days—during the 45-year study period, and total annual precipitation declined by 0.75 inches per decade.

Bill K. Smith, an author of the study from the University of Arizona who studies how drought in the Southwest affects plants, said those trends have accelerated since 2000. The new study and other research shows that changes in the timing and intensity of rainfall are likely to disrupt plant communities, he said. Species that grow quickly after an intense downpour and can survive dry spells in dormancy are likely to replace those needing a steadier supply of moisture…

More Winter Snowmelt Could Disrupt Ecosystems and Water Management

One reason that less moisture is trickling down from mountains to arid landscapes below is that more snow is melting during winter, when it should be piling up, as shown in another recent study published in Nature Climate Change. That research, led by Keith Musselman, a climate scientist at the University of Colorado Boulder, is the first to compile a long-term data record from 1,065 automated weather stations in the Western United States and Canada.

One-third of those stations showed that snowmelt is increasing throughout the winter, but especially in November and March. This year, the winter snowpack in Colorado was already in steep decline on April 6, when it’s typically at its peak.

The findings show that the Western snowpack is more sensitive to warming than suggested by the commonly used measurement of the snow’s water content, Musselman said, and more winter snowmelt will complicate water resource planning. Water managers around the West have historically relied on the April 1 snowpack measurements to decide how much water to store or release from reservoirs, and how much will be available for seasonal irrigation.

The findings bolster projections that global warming will disrupt the water cycle that’s been relatively stable for at least the past few hundred years, said snow scientist Jeff Deems, part of the research team at the Western Water Assessment.

“At a basic level, this study helps quantify something you suspected, anecdotally,” he said. “We’ve grown up in a world in which snowpack has been a reliable reservoir, but it’s not that way anymore.”

The research shows that the widely used measurements from snow-sensing stations and manual surveys don’t tell the whole story of how global warming is changing the snowpack, he added.

Combined photos of the Senator Beck basin in the Colorado San Juan mountains during increasingly warmer months. (Credit: Jeffrey Deems/CIRES and Matthew Kennedy/CU Boulder Extreme Ice Survey)

“We can’t rely on history anymore to tell us what is up there,” said Deems, cofounder of a company that does aerial snow measurements. Keeping up with the rapid changes warming is making to the water cycle requires more emphasis on broad-scale measurements of snow from planes and satellites, he said.

The April 1 snowpack readings worked well in the past, Musselman said, but when you add in new and rapidly changing factors like winter snowmelt, “you’re eroding your predictive capacity.”

Big Impacts Trickle Down From Winter Melting

Early snowmelt can raise winter soil moisture, which could “prime the pump for an extreme response, something like a rain-on-snow flood event,” Musselman said. Wetter cold season soils may also emit more carbon dioxide, and increasing winter runoff can wash more pollution into rivers, worsening water quality, he added.

In Yosemite National Park’s Tuolumne Meadows, Jessica Lundquist, with the Mountain Hydrology Research Group at the University of Washington, found that 2017 storms that delivered record-setting snowfall to California’s Sierra Nevada mountains also brought rain and extreme cold temperatures, which formed ice layers within the snowpack that left a “bathtub ring of damage” to trees encased by the ice. Similar frozen layers in the snowpack can also make it harder for animals like bighorn sheep to find food.

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