California, in the fourth year of an ongoing drought, is now drier than it's been in 500 years, researchers say.

Temperatures are so high that what little rain has fallen is evaporating almost as soon as it hits the Earth, they say in a study set to appear in the journal Nature Climate Change, with the result that the Sierra Nevada snowpack the state relies on for water hasn't been this low since the 1500s.

"Snow is a natural storage system," says senior study author Valerie Trouet of the University of Arizona. "In a summer-dry climate such as California, it's important that you can store water and access it in the summer when there's no precipitation."

Trouet and her colleagues began analyzing tree rings in the state's Central Valley to gauge the amount of mountain snow available, prompted by an announcement by California officials in April that they found "no snow whatsoever" in the Sierra Nevada Mountains for the first time in 75 years of making such measurements.

Thirty percent of the state's water supply comes from snowpack melt along the 400-mile mountain range that feeds streams, rivers, lakes and reservoirs — all of which are approaching historic lows, the researchers point out.

"Our study really points to the extreme character of the 2014-15 winter," Trouet says. This is not just unprecedented over 80 years — it's unprecedented over 500 years."

The cause was a double whammy of drought and high temperatures that left the 2015 Sierra snow water equivalent — a measure of water content — at just five percent of the average over the past five centuries, the researchers explain.

The team says it chose California oak trees for study because they yield reliable records of annual rain amounts in the winter season, producing wider rings after wetter winters.

There were some study findings that suggest the Sierra snowpack could in fact be at its lowest level in more than 3,000 years, the researchers note, and even though that analysis comes with a higher margin of error, it is more evidence that current levels are setting unfortunate records.

Additionally, with climate warming and increasing human impacts on it, such conditions are even more likely in the future, Trouet says.

"We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures," she says. "Anthropogenic warming is making the drought more severe."