Source: http://www.popularmechanics.com/science/...

The second problem is that, thanks to record-breaking winter temperatures over the last four years, the precipitation that has fallen across the mountain range has largely been rain, not snow. Instead of sticking as snowpack and slowly refilling reservoirs over the spring and summer, she explains, winter rains sweat off the mountainsides almost immediately, "complicating water management and, [to make full use of the water,] requiring new reservoirs where you didn't need them before," she says.

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How exactly can scientists compare today's level of snowpack against snowy seasons from centuries ago, even before the state's recorded history? Weirdly enough, by looking at tree growth rings. Trouet and her colleagues can model the mountains' snowy history by comparing the yearly size of tree rings in thousands of blue oaks, which live at the base of the Sierra Mountain range and grow at dramatically different rates depending upon changes in water—with trusted reconstructions of historical temperature.

In other words: Because they can tell how wet and how cold it was each winter, the scientists can say how much snowpack accumulated in a given year. While it's not a perfect method, this odd tree-ring reconstruction faithfully matches the last 80 years of snowpack measurements with astonishing accuracy.

Now,  Trouet is careful to point out a tricky caveat. Yes, she says, the Golden State is currently seeing a 500-year record low in snowpack. But that doesn't mean the state is also experiencing a 500-year record drought. The lack of snow reflects years of increasingly hot winters (unanimously agreed in the scientific community to be driven by climate change) in addition to California's current lack of water.