Dr. Swain said he and his colleagues were struck that their computer models showed such a consistent increase in risk across the West, even though the region’s climate is so varied. California has dry summers and wet winters, while in Colorado, both flooding and wildfire peak during the warm season.
It doesn’t take much rain to trigger a debris flow on a recently burned slope, said Jason W. Kean, a hydrologist with the United States Geological Survey in Golden, Colo., who was not involved in the study. In some areas, as little as a fraction of an inch falling in 15 minutes might be enough, he said.
But as more wildfires occur in places where they hadn’t been a big problem before, scientists are working to understand how the thresholds might differ in those wetter climates, Dr. Kean said. “It’s a scramble for us to stay ahead of the game,” he said.
Dr. Touma conducted most of the analysis for the new study when she was a postdoctoral researcher at the University of California, Santa Barbara, not far from Montecito, which was devastated by post-fire mudslides in 2018. The authorities there had urged residents of certain areas to evacuate, but many chose not to.
“There was a lot of evacuation fatigue from the fire just one month before,” Dr. Touma said.
Residents of the West are generally very conscious of the risks of flooding and mud flows in burn zones, said Samantha Stevenson, an environmental scientist at the University of California, Santa Barbara, who also worked on the study. But “the degree to which they are increasing as a result of climate change, and the rapidity of that increase, is something that we should maybe try to be more aware of,” she said.