Declining Snowpack Over Western U.S. - Xubin Zeng and team map 35 years of change at a finer scale

Photo Xubin Zeng
Thursday, December 13, 2018

A team of researchers led by UA HAS Professor Xubin Zeng has mapped exactly where in the Western U.S. snow mass has declined from 1982 to 2016. The team mapped the changes in snow mass onto a grid of squares 2.5-miles on a side over the entire contiguous U.S. "A person could practically find the trend for their neighborhood," said first author Zeng. "Grid size for previous studies was about 40 miles on a side. This is the first time anyone has assessed the trend at the 2.5-mile by 2.5-mile pixel level over the 35-year period (1982-2016)."

Researchers found that in some areas of the Western U.S., annual snow mass has decreased by about 41 percent, while the Eastern U.S. has experienced very little decrease in snow mass."If we add those areas together, the representative size is (that of) South Carolina," said Zeng. UA co-author, HAS alum, and UA SNRE research scientist Patrick Broxton said, "The big decreases are more often in the mountainous areas that are important for water supplies in the West."

Snow mass is how much water it contains, which is important in regions where winter snows and subsequent snow melt contribute substantially to water resources. Snow melt contributes to groundwater and to surface water sources, such as the Colorado River.

Zeng's team also found over the period 1982-2016, the snow season shrank by 34 days on average for squares that, if combined, would equal the size of Virginia. "The shortening of the snow season can be a late start or an early ending or both," Zeng said. "Over the Western U.S. an early ending is the primary reason. In contrast, in the Eastern U.S. the primary driver is a late beginning."

Temperature and precipitation during the snow season also have different effects in the West compared with the East. In the West, the multidecadal changes in snow mass are driven by the average temperature and accumulated precipitation for the season. The changes in the Eastern U.S. are driven primarily by temperature.

The paper, Snowpack Change from 1982 to 2016 Over Conterminous United States, by Zeng, Broxton, and their co-author Nick Dawson of the Idaho Power Company in Boise, Idaho, was published in Geophysical Research Letters on December 12.

Developing the new data set has allowed the UA-led team to examine changes in temperature, precipitation, and snow mass from 1982 to 2016 for every 2.5-mile by 2.5-mile square in the contiguous U.S., as well as to study how snow can affect weather and climate. "Snow is so reflective that it reflects a lot of the sunlight away from the ground. That affects air temperature and heat and moisture exchanges between the ground and the atmosphere," said Broxton.

Professor Zeng, who holds the Agnese N. Haury Chair in Environment at the UA, is now working with NASA to figure out a way to use satellite measurements to estimate snow mass and snow depth. NASA and the UA's Agnese Nelms Haury Program in Environment and Social Justice funded the research.

For more details about their methodology and the data collected from on-the-ground measurements used to develop their new dataset--including COOP, SNOTEL, and PRISM--see Mari Jensen's UA News.org article here.

Congratulations, Xubin!

Other Media Presentations

Press conference at the American Geophysical Union's Annual Meeting, 9 a.m. Eastern Time in the Shaw/LeDroit Park Room, Level M3, Marriott Marquis, Washington, DC. Scientific presentation on December 13, 11:20 am, Salon G, Walter E. Washington Convention Center

Phoenix KJZZ on Wednesday, December 12

Arizona Public Media FM on Thursday, December 13

UA NOW on Thursday, December 13

Arizona Daily Star on Saturday, December 15

UA Science E-News on Wednesday, December 19