Western wildfires increasingly intersect critical water supply watersheds
Study of more than eight decades of wildfire activity finds growing burned -area exposure in watersheds important to drinking water supplies across the western U.S.
New research examining more than eight decades of wildfire activity found that fires are burning more land in Western watersheds that supply water to millions of people.
The study published in analyzed wildfire trends from 1940 through 2024 across approximately 17,000 watersheds in 11 Western states. Led by Dalston J. Karto, a 麻豆传媒映画 computational science doctoral student, and Alicia M. Kinoshita, the study was conducted in collaboration with researchers from NASA鈥檚 Jet Propulsion Laboratory and the U.S. Geological Survey. The researchers found that some of the watersheds most important for generating runoff and supplying downstream drinking water are experiencing significant increases in burned area.
In California, the study captured fires affecting watersheds connected to major water supplies, including the 2018 Carr Fire near Whiskeytown Lake and the 2013 Rim Fire near Don Pedro Reservoir.
The findings highlight a growing challenge for water security: Wildfire can affect water resources long after a fire is extinguished by increasing sedimentation, degrading water quality and altering runoff patterns. Those effects can reduce reservoir capacity, increase water treatment demands and create additional risks for downstream communities.
Among the study鈥檚 most significant findings, the average area burned in watersheds supplying drinking water to at least 100,000 people nearly doubled after a shift in wildfire patterns in the late 20th century, rising from 4.7% to 9.1%.
鈥淢any water systems were designed under the assumption of stable and predictable bounds, but are no longer valid under converging wildfire, aging infrastructure, water demand, and climate pressures,鈥� said Alicia M. Kinoshita professor in 麻豆传媒映画鈥檚 . 鈥淭his research helps identify the watersheds most at risk so limited resources can be directed toward better monitoring, infrastructure, and proactive management to protect communities before the next fire rather than responding afterward.鈥�
Watersheds serving 10,000 to 99,999 people experienced a similar increase in the average area burned, from 4% to 8%. In watersheds serving 1,000 to 9,999 people, the average increased from 4% to 11%. Over the past 30 years, the average number of people relying on fire-affected watersheds
鈥淩ather than seeing wildfire activity increase everywhere across the western United States, we found that certain watersheds are being affected disproportionately, particularly water-rich, forested and mountainous watersheds that serve large downstream populations,鈥� said Karto. We also found that wildfire patterns are becoming more extreme, with more repeated and extensive burning and fewer moderate burns.鈥�
A changing wildfire pattern
The researchers identified another notable shift across the West: Wildfires are increasingly burning either very little or a large portion of a watershed.
Across approximately 17,000 watersheds, the proportion experiencing either minimal area burned annually, less than 1% of watershed area, or burning of more than 20% of the watershed increased significantly. Meanwhile, watersheds in intermediate categories of 1% to 19% area burned declined.
鈥淭he pattern of wildfire is moving toward the extremes. We are seeing fewer moderate burns and more repeated or extensive burns, while some places experience very little burning at all,鈥� said Karto.
Rather than identifying a single abrupt transition across the West, the study describes a diverging wildfire regime in which many watersheds remain largely unburned while a growing subset experiences extensive fire. The amount of a watershed that burns can influence what happens downstream. Previous research cited in the study has associated a 20% burned area threshold with measurable increases in watershed yield, while extensive wildfire can exacerbate erosion and sediment movement through river networks, which can impact water quality.
The study also found that the area burned annually increased across a majority of the western U.S. ecoregions examined. Nineteen of 35 ecoregions showed statistically significant increases, with some of the strongest trends occurring in forested and mountainous landscapes, including the Wasatch and Uinta mountains, Southern Rockies and Klamath Mountains.
Where fire and water meet
As part of the study, researchers also examined the relationship between wildfire activity and runoff ratios in approximately 3,100 watersheds,a measure used in the study as an indicator of a watershed鈥檚 long-term runoff generation potential. Watersheds in the higher runoff ratio groups, among the region鈥檚 more productive water sourcing areas, experienced significant increases in burned area between 1940 and 2024.
The findings highlight an important overlap: Landscapes capable of efficiently generating runoff can also contain dense vegetation that contributes to fuel availability when conditions become conducive to wildfire.
The study does not attribute the observed changes to a single cause but rather is a statistical analysis that identifies long-term trends and approximate shifts in wildfire activity. The researchers note that climate conditions, fuel accumulation and historical fire management practices can interact to influence wildfire extent and behavior.
鈥淚t was surprising to see watersheds serving more than 100,000 downstream consumers experience a doubling of burned area, exposing a vulnerability in our regional water supplies,鈥� said Kinoshita.
The research also underscores the connection between managing landscapes upstream and protecting water systems downstream. Post-fire sedimentation can affect reservoir capacity, while nutrients, metals and other contaminants can degrade source water quality. Changes to vegetation and soil properties after wildfire can also alter runoff, creating challenges that may continue for years after a fire.
An integrated approach that considers watershed and land management alongside downstream water operations is needed. Potential strategies discussed in the study include targeted prescribed burning, riparian restoration, post-fire runoff monitoring, resilient treatment technologies and watershed recovery programs.
As wildfire increasingly intersects with watersheds serving large populations, the findings indicate that planning for wildfire resilience may also require planning for the resilience of the water systems communities depend on.
The research team included Dalston J. Karto, Brittany Barreto Martinez, and Alicia M. Kinoshita of 麻豆传媒映画; Christine M. Lee of NASA鈥檚 Jet Propulsion Laboratory at the California Institute of Technology; and Brian A. Ebel of the .
