Sierra Nevada snowpack at 500-year low — report

Source: Scott Streater, E&E reporter • Posted: Tuesday, September 15, 2015

Snowpack levels in California’s Sierra Nevada are the lowest in at least five centuries, according to a sobering new academic report that warns that the ongoing drought could continue for the foreseeable future.

At issue is a¬†report, led by researchers at the University of Arizona, that used tree-ring data to estimate that low 2015 snowpack levels in the Sierra Nevada are “unprecedented in the context of the past 500 years,” according to the two-page report, published online today in the journal¬†Nature Climate Change.

“This is not just unprecedented over 80 years, it’s unprecedented over 500 years,” said Valerie Trouet, an associate professor of dendrochronology at the university’s Laboratory of Tree-Ring Research and one of the report’s authors.

The researchers were able to “reconstruct” snowpack levels in the Sierra Nevada back to roughly the year 1500 by collecting and analyzing a set of blue oak tree-ring samples that the report says “reflects large-scale California winter precipitation anomalies” over many years.

The historically low snowpack level is the result of little snow falling between February and March, when 80 percent of California’s winter precipitation occurs, and higher temperatures that quickly melt the snow that is there, the report says.

“Our study really points to the extreme character of the 2014-15 winter,” Trouet said.

Snowpack levels in the Sierra Nevada aren’t likely to rebound anytime soon, according to the report, which is billed as the first to estimate how 2015 snowpack levels that have deepened California’s ongoing record-setting drought compare historically with other drought periods.

“Anthropogenic warming is projected to further increase the probability of severe drought events, advance the timing of spring snowmelt and increase rain-to-snow ratios,” the report says. “The ongoing and projected role of temperature in the amount and duration of California’s primary natural water storage system thus foreshadows major future impacts on the state’s water supplies.”

The University of Arizona report is more bad news for California, which depends on Sierra Nevada snowmelt filling water reservoirs that supply nearly one-third of the state’s water.

In normal years, the snows of the Sierra Nevada slowly melt during the summer months, allowing the snowpack to act as a “natural storage system” for water, replenishing streams, lakes, groundwater and reservoirs, Trouet said.

But by late May this year, the 98 electronic sensors in the Sierra Nevada measured zero snowpack, prompting one state water official to call 2015 one of the earliest snowmelts ever (Greenwire, June 1).

Gov. Jerry Brown (D) was forced this spring to implement mandatory statewide water consumption cutbacks, as water reservoirs in California were already low because the drought has been ongoing since 2012.

Scientists and water managers have been monitoring the dwindling snowpack levels for years in many parts of the Sierra Nevada and Cascades in California, Oregon and Washington.

The Agriculture Department’s Natural Resources Conservation Service earlier this year reported that low snowpack levels are due in large part to higher temperatures in February that caused more precipitation to fall as rain rather than snow (E&ENews PM, March 11).

California cannot expect relief anytime soon from the Sierra Nevada snowpack, Trouet said.

“We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures,” Trouet said. “Anthropogenic warming is making the drought more severe.”