Snow cover in the Sierra Nevada mountain range, a water lifeline for California's cities and agriculture, has hit its lowest level in 500 years, a study said Monday.

Measured on April 1, the natural, frozen reservoir was barely five percent of the 1950-2000 average, threatening tens of millions of Californians and the state's $50-billion (44-billion-euro) agriculture sector with chronic water shortages, its authors warned.

And things were set to get worse, they wrote in the journal Nature Climate Change.

"We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures," lead author Valerie Trouet, a professor at the University of Arizona, said in a statement.

"Anthropogenic" –- or manmade –- global warming "is making the drought more severe," she added.

The "snowpack" of Sierra Nevada, central California's 650-kilometre (400-mile) spine, provides more than 60 percent of the state's distributed water supply, including all or part of the drinking water for 23 million people.

Greater Los Angeles and the San Francisco Bay Area are among the zones affected.

On April 1 -– when the snowpack is generally at its maximum -– California Governor Jerry Brown declared the state's first-ever mandatory water restrictions. He made the announcement while standing on dry ground that a few years ago would have been covered by a man-high snow blanket.

Extremely low winter rainfall combined with record high temperatures during the first three months of 2015 are to blame, said the study.

Scientists had already established that this year's snowpack was the smallest since annual measurements began in the 1930s, but the new research goes even further back in time.

- Tree rings -

The team measured the rings of some 1,500 ancient blue oaks in the Central Valley -- which runs parallel with the Sierra Nevada –- for a record of annual rainfall in winter, when California receives 80 percent of its precipitation.

Given that the same storms which water the oaks also dump snow on the mountains, tree-ring width is a good proxy for what the snowpack would have been in any given year.

The team then compared their tree-ring data with a reconstruction of winter temperatures for the period 1500-1980, to create a year-by-year snowpack profile.

"This is not just unprecedented over 80 years –- it's unprecedented over 500 years," Trouet said of the findings.

California is in the grips of a four-year drought, and ravaged by wildfires that have destroyed more than 100,000 acres (40,470 hectares), hundreds of homes and businesses, authorities say.

According to the US Geological Survey, more than 120 of the state's reservoirs are less than a fifth full, and 190 under half.

This also threatens California's hydroelectric power sector -- a key source of renewable energy.

Scientists warn that climate change will likely boost the frequency and intensity of droughts.