Relentless drought in the western United States has raised concerns about water scarcity in the coming decades. Parched landscapes are difficult to cultivate and are more susceptible to devastating wildfires. Paradoxically, such blazes could increase the flow of water to rivers and reservoirs long after containment, according to a new study.
In an analysis of watersheds throughout the western United States, researchers found that on average, streams see higher flows than expected for 6 years after a forest fire ravages the surrounding basin. Accompanying climate models show that this extra water partially offsets a decades-long deficit in streamflow.
The West Continues to Burn
The study results suggest that if more than 20% of a basin burns over a 6-year period, then streamflow should be significantly elevated for at least 1 year, said Park Williams, a hydroclimatologist at the University of California, Los Angeles, and lead author of the study. Between 1984 and 2021, just 17% of western U.S. forest area burned, according to the researchers.
“I don’t think we’re going to have a 6-year period anytime soon when 20% of all the forest area in the West burns,” Williams said. But, he added, it could happen on a regional scale. Between 2016 and 2021, for example, three of the Sierra Nevada’s biggest basins—the Sacramento, San Joaquin, and Tulare—approached that threshold.
Even a slightly drier climate could fuel enough fires to push many other important basins past that point, Williams said, and unfortunately, the West is likely to see bigger and more intense blazes in the future. According to the California Department of Forestry and Fire Protection, 12 of California’s 20 largest fires have occurred in the past 5 years. “In the next 30 years, we’re going to get more than just the tiny little nudge toward more forest fire,” Williams said.
But vegetation matters, said Sara Goeking, a forest hydrologist with the U.S. Forest Service who was not involved with the study. Dense foliage intercepts precipitation, especially snow. After a large fire incinerates a forest canopy, more of that water reaches the ground and makes its way into streams. The minimally managed watersheds considered in this study are generally found at high elevation, where snowfall is common, she said. Whether the results of the analysis apply to the lower-elevation basins that dominate the West remains unclear.
Preparing for Uncertainty
If year-round streamflow were higher than expected, water rights holders would be thrilled because more water means more crops, said Jeffrey Mount, a senior fellow at the Public Policy Institute of California who was not involved in this study. “They would cut off body parts for that.”
But regional water managers, who are responsible for maintaining and planning for future municipal water supplies, may have to contend with more and dirtier water than they expect. Water managers are bound by rules that regulate reservoir water levels, intended to mitigate flooding and optimize storage. If an unexpected deluge comes, water managers must release water, which can cause flooding downstream.
These rules are based on historic hydrology, Mount said. “But what if when the watershed burns, the [rules] are no longer correct?” As more intense wildfires torch bigger swaths of forestland in the coming years, enhanced streamflow could lead to more flooding because water managers will be forced to discharge reservoirs.
California’s Department of Water Resources—which advises regional water authorities—is working to update their models to incorporate the changes to come in the warming world, said Michael Anderson, the state climatologist, who was not involved with this study. “This is where collaboration with the research community is key because we are looking at a lot of conditions that we just haven’t seen before.”
—Jennifer Schmidt (@DrJenGEO), Science Writer