Number of days without summer rain increasing Western fires, UM study says

A drop in summer precipitation and longer dry spells play a significant role in the size and number of large fires across the American West, a new study conducted in part by the University of Montana has found.

The research, completed in partnership with the U.S. Forest Service, was published this week in the Proceedings of the National Academy of Sciences.

“Summer dry periods are tightly coupled to how warm and dry the air is during the fire season,” said Zack Holden, a Forest Service scientist and lead author of the study. “Longer windows without rain lead to more surface heating, which dries out woody fuels.”

After a record 46 days without measurable precipitation, rain finally fell on Missoula on Tuesday. It also soaked portions of a Glacier National Park, which has seen fire activity intensify over the past few weeks.

Longer, hotter summers and dipping precipitation have become a common story across portions of the West.

“This new information can help us better monitor changing conditions before the fire season to ensure that areas are prepared for increased wildfire potential,” said Matt Jolly, a Forest Service research ecologist and one of the study's co-authors. “It may improve our ability to predict fire season severity.”

The study found that a decline in summer precipitation from 1979 to 2016 across nearly 45 percent of the forested areas in the West were strongly associated with fires.

If trends identified in the study persist, a decrease in summer precipitation would lead to more fires with “far-reaching ecological and socioeconomic impacts.” It could also carry impacts for one of Montana's largest economies in agriculture.

“Decreases in precipitation and the increasing length of dry spells during the summer – when crop water demand in the arid West are highest – is not only exacerbating wildfires but could also have serious implications for Western agriculture, especially in states highly reliant on rainfed crops,” said Marco Maneta, a UM professor of hydrology and co-author on the paper.

The study was funded by NASA and the U.S. Department of Agriculture.