California's "frozen reservoir" is melting fast.
Unusually high temperatures this spring have acted like a blow-drier on accumulated winter snows, despite a healthy boost during the stormy month of March.
One thing is undeniable: the Sierra snowpack is a lot more robust than the previous winter's, which was off-the-charts ugly. Around the first of May, water content (which is what really matters) of the snowpack stood at 60 percent of the long-term average. That's not great news unless you compare it to May 1 of last year, when it was 1 percent. That's not a typo.
On April 1, when hydrologists generally reckon the snowpack to be at its peak for the season, it stood at 85 percent of the average for that date.
This spring the most intense period of melting is running about two to three weeks earlier than normal, according to Nina Oakley, assistant research climatologist at the Western Regional Climate Center in Reno.
"Typically the steepest declines are in late April to early May," she says. But this year, Oakley says monthly average temperatures in the early spring were running 2 to 5 degrees above normal at the higher Sierra elevations.
Plus there's a "snowball" effect, or what Oakley calls a "positive feedback," in which some snow melt leads to more.
"As soon as you start melting enough snow that you expose trees, rocks, soil underneath," she explains, "these features are not reflective like the snow and so they'll start to absorb solar energy and facilitate further snow melt."
The good news is that a lot of this early melt is ending up in reservoirs. The bad news with an early melt is that as consumers start drawing down those reservoirs, there's less runoff to replace it during the peak growing season, when it's most needed.
"We try to capture every drop we can," says Doug Carlson of the state's Department of Water Resources.
The big Northern California reservoirs are filling fast. The two biggest, Shasta Lake and Lake Oroville, are both at more than 90 percent of capacity.
But Carlson concedes that significant volumes of water are let go for flood control and to push back encroaching salt water and maintain river ecosystems.
"It's a big juggling act, frankly, to manage the water resource here," says Carlson. "We think we're capturing a good deal of the snow melt right now."
An early melt can also have environmental impacts, especially on fish. If the cold water pulses come too early, lower flows and higher water temperatures later in the season can make it difficult or impossible for some species to survive.