The snow that blanketed the Sierra Nevada in California last winter, and that was supposed to serve as an essential source of fresh water for the drought-stricken state, was at its lowest levels in the last 500 years, according to a new study.

The paper, published on Monday in the journal Nature Climate Change, used tree-ring data from centuries-old blue oaks to provide historical context for the mountain range’s diminished snowfall. As of April 1, the snowpack levels were just 5 percent of their 50-year historical average.

The paper is the first to create a model that describes temperature and precipitation levels on the Sierra Nevada that extend centuries before researchers started measuring snow levels each year.

“The 2015 snowpack in the Sierra Nevada is unprecedented,” said Valerie Trouet, one of the authors of the study and a paleoclimatologist at the University of Arizona. “We expected it to be bad, but we certainly didn’t expect it to be the worst in the past 500 years.”