Record heat, melting snow: What does it mean for Californias reservoirs

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A record-breaking heat wave is scalding California, with major consequences for the state’s most important reservoir: its snowpack.Providing about a third of the state’s water supply, the Sierra Nevada snowpack is a vital source of spring and summer runoff that refills reservoirs when the state needs the water most.But a warm wet storm followed February’s snow, and now, March temperatures are shattering records — prompting warnings of rapid snowmelt and swift rivers.Historically, the snowpack is at its deepest in April.But climate change is shifting runoff earlier, leaving less water trickling down the mountains in warmer months for homes, farms, fish, hydropower and forests.“In an ideal world, you’d have your reservoir full right now, and this additional huge snowpack reservoir that we know will help replenish and provide more water supply,” said Levi Johnson, operations manager for the Central Valley Project, the massive federal water system that funnels Northern California river water to the Central Valley and parts of the Bay Area.This year, he said, “we’re not going to have that.”California’s reservoirs are in good shape, brimming above historic averages with many nearing capacity.

But that summertime snow bank on the slopes of the Sierra Nevada is disappearing early, and fast — dropping to 38% of average for mid-March statewide.It’s not yet the worst snowpack on record: That distinction belongs to 2015, when then-Gov.Jerry Brown stood on brown, barren slopes of the Sierra Nevada to watch scientists measure the most meager snowpack in history.But this year’s snowpack is rapidly approaching the worst five on record for April 1, state climatologist Michael Anderson said — and it’s likely to worsen still as temperatures climb.

From early to mid-March, the snowpack has been disappearing at a rate of roughly 1% per day.It’s a sharp departure fr...