Diverging wildfire regimes and doubling of burn area in western United States water supply watersheds

Recent increases in western United States wildfire activity are raising concerns for regional water supplies. Analyzing the wildfire histories (1940-2024) of 3,100 watersheds, we identify a significant regime shift in the late 20th century. Watersheds with higher runoff ratios, critical for regional water provisioning, have significantly increasing burn area trends. The burned areas double following the late 20th century changepoints. Analysis of 17,000 watersheds reveals a diverging fire regime: a transition from moderate burns (1-20% area) towards minimal (<1%) and extensive (> 20%) events. Vulnerability is greatest in forested and mountainous regions, specifically those providing water to 100,000 or more downstream consumers. An integrated watershed management approach, one that considers land management and downstream water operations, may help secure water supplies and mitigate the cascading hazards from wildfire.