Sierra Nevada Snowpack Worst in Five Centuries

Tree rings suggest that California's dwindling snowpack is truly epic.

California's snowpack isn't just historically low, it's pre-historically low, according to a new study that uses tree rings to reconstruct 500 years of seasonal snowpack stored up in the Sierra Nevada. Out of those 500 years, the end-of-wet-season snowpack on April 1, 2015, could be the lowest.

"When we saw the April 1 snowpack numbers coming out and how they were at 5 percent of average since the 1930s, we discussed how someone should put that into a longer term context," said Valerie Trouet, a researcher at the Laboratory of Tree-Ring Research at the University of Arizona in Tucson.

That someone became Trouet and her colleagues, who looked to the blue oak trees of California's Central Valley to get help. Blue oaks are very sensitive to the amount of rainfall they receive, growing wider or thinner rings in direct response to the wetter or drier weather. And since these trees grow between the Pacific Ocean -- the source of California's winter storms -- and the Sierra Nevada, they are probably a very good indicator of the amount of moisture that is heading into those mountains.

But because rainfall could come down as just rain if it's not cold enough, the researchers added to the blue oaks data a good, but less precise, winter temperature dataset gathered from a variety of trees throughout the region. The idea was that if they could match wet years with cold years, they could identifying the big snowpack years.

"By adding temperature we were able to develop an even better model of snowpack back to 1500," said Trouet. What's more, they could calibrate their model by looking at the actual snowpack levels at 108 Sierra Nevada snow stations since the 1930s and comparing them directly to growth rings on those same years on blue oaks.

As for the possibility that summer rains could throw off this model by nourishing the blue oaks at a time of year when snow doesn't accumulate on the mountains, that's not a problem, Trouet explained. Significant rainfall in California between April and November is very rare. So essentially all the rain the oaks are recording in their rings is a good proxy for winter snow up in the Sierra -- so long as the winter weather is cold enough.

"When temperatures are high more precipitation will fall as rain than as snow," Trouet said. Which was certainly the case in 2015, with record low snowpack corresponding with record high temperatures for January through March. Global warming is expected to make that scenario more likely, which is bad news for the snowpack which feeds California though the dry season.

"This study falls right in line with with a whole slew of other studies that have come out recently," said Park Williams, a climate researcher at Lamont Doherty Earth Observatory at Columbia University. The estimates of past snowpack will get even better when researchers have more direct tree ring data from trees in the Sierra Nevada itself, as well as more refined past temperature information. "This really highlights the need for more tree ring data for the Sierra Nevada."

California's water managers determine their annual snowpack based on measurements taken on April 1, because by that date the state has received all of the rain it's likely to get until late fall or winter. So the rivers and streams are thereafter fed by melting snow – which ultimately is the state's primary water supply.

These two natural-color satellite images of the snow cover in the Sierra Nevada in California and Nevada show the last year with average winter snowfall, 2010, compared with 2015.

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