‘We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures …’
LINZ — A new tree ring study suggests this past winter’s snowpack in the Sierra Nevada may have been the lowest in 500 years — and there may be more of the same ahead, warned to researchers who analyzed the data.
“We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures,” said Valerie Trouet, an associate professor of dendrochronology at the University of Arizona Laboratory of Tree-Ring Research. “Anthropogenic warming is making the drought more severe.
“Our study really points to the extreme character of the 2014-15 winter. This is not just unprecedented over 80 years — it’s unprecedented over 500 years,” Trouet, said.
On April 1 of this year, California Gov. Jerry Brown declared the first-ever mandatory water restrictions throughout the state while standing on dry ground at 6,800-foot elevation in the Sierra Nevada. The historical average snowpack on the site for that date is more than five feet, according to the California Department of Water Resources.
The lack of snow in 2015 stems from extremely low winter precipitation combined with record high temperatures in California in January, February and March, Trouet said. About 80 percent of California’s precipitation occurs in the winter months, she said. Snowpack level is generally measured on April 1 each year, a time when the snowpack is at its peak.
“Snow is a natural storage system,” she said. “In a summer-dry climate such as California, it’s important that you can store water and access it in the summer when there’s no precipitation.”
In past years the snows of the Sierra Nevada slowly melted during the warmer months of the year, and the meltwater replenished streams, lakes, groundwater and reservoirs. In a winter with less snow or with winter precipitation coming as rain rather than snow, there is less water to use during California’s dry summers.
“This has implications not only for urban water use, but also for wildfires,” said Soumaya Belmecheri a postdoctoral research associate at the UA tree-ring lab.
The study involved analyzing existing tree-ring data that shows annual winter precipitation in central California from 1405 to 2005 and annual snowpack measurements since the 1930s. The team also used a previously published reconstruction of winter temperatures in southern and central California that spanned the years 1500 to 1980.
Other researchers had already measured the width of tree rings for 1,505 blue oaks in California’s Central Valley from 33 different sites. Belmecheri and her colleagues put those measurements together as one long chronology, meaning the scientists had a blue oak tree-ring record that reached back reliably to the year 1405.
For those particular oaks (Quercus douglasii), the width of their annual rings reflects the winter precipitation they receive. Because the same storms that water the oaks also dump snow in the Sierra Nevada just to the west, the width of the blue oaks’ rings is a good proxy for snowpack in the Sierras, Trouet said.
Wahl had already published a reconstruction of central and southern California February-March temperatures from 1500 to 1980 that is independent of the blue oak tree-ring records.
Snowpack in the Sierras has been measured approximately since the 1930s, so the researchers checked their snowpack estimates from tree rings and the temperature reconstruction against actual snowpack measurements for 1930 to 1980.
The different measurements all lined up – when winter precipitation was lower and temperature was higher, snowpack was lower.
Peak snowpack is the measurement that hydrologists use to predict the amount of runoff that will occur in the summer, Trouet said.
The team’s next step, she said, is investigating and reconstructing the atmospheric circulation patterns that contribute to the California drought and the Sierra Nevada snowpack.
Trouet, Belmecheri and their colleagues’ report, “Multi-century evaluation of Sierra Nevada snowpack,” is scheduled for online publication in Nature Climate Change on Sept. 14, 2015. The National Science Foundation, the U.S. Geological Survey and the Swiss National Science Foundation funded the research.