In April, statewide measurements of water contents of California’s snowpack -- a major source of water for reservoirs during the summers -- showed that the water content in the snow was only 5 percent of the normal for that date, making it the lowest in a 75-year record. Further analysis of available data has now shown that the situation is much worse than previously estimated.

According to a new study, published Monday in the journal Nature Climate Change, the amount of snow in the Sierra Nevada is now at its lowest level in over 500 years.

“Our study really points to the extreme character of the 2014-15 winter. This is not just unprecedented over 80 years -- it’s unprecedented over 500 years,” author Valerie Trouet , a researcher at the University of Arizona, said in a statement. “We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures.”

California, which is reeling under a record-breaking drought that has necessitated unprecedented mandatory water restrictions throughout the state, gets most of its rainfall and snow between late fall and early spring. Though the state has typically seen wide annual fluctuations in rainfall, the wet years balanced out the dry ones during non-drought periods.

Jerry Brown California Governor Jerry Brown speaks to reporters at the site of a manual snow survey on April 1, 2015 in Phillips, California. The recorded level is zero, the lowest in recorded history for California. Gov. Brown went on to announce mandatory statewide water restrictions. Photo: Max Whittaker/Getty Images

However, recent data gathered by NASA has shown that California accumulated a rainfall deficit of almost 13 inches between 2012 and 2014, and another 7 inches during the 2014-2015 wet season -- severely depleting the amount of snow blanketing the Sierra Nevada mountains and drastically reducing the amount of water available to the state’s population during summers.

“Snow is a natural storage system,” Trouet said, in the statement. “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.”

In order to determine the snowpack levels for the past five centuries, Trouet and her colleagues used two previously published tree-ring studies. Blue oaks -- a species endemic to California -- are extremely sensitive to the amount of rainfall they receive, growing wider rings in wetter weather and thinner ones in drier weather.

After comparing this data with a historical winter temperature dataset gathered from a wide variety of trees in the region -- in order to identify both wet and cold years -- the researchers concluded that the chance that a snow drought of this magnitude would affect the entire Sierra Nevada more than once every 500 years was less than 5 percent.

“[The extremely low snowpack] has implications not only for urban water use, but also for wildfires,” lead author Soumaya Belmecheri said, in the statement.

California has seen about 6,000 wildfires this year -- about 1,500 more than the same period last year. Most recently, 400 homes were destroyed in Middletown in northern California, after a rapidly-spreading wildfire engulfed the region. As of Monday, more than 23,000 residents had been evacuated, state officials told the Associated Press.