Study finds big wildfires temporarily boost water supplies and flood risks

Major forest fires around the West temporarily boost both surface water supplies and the risks of flood and debris flows, researchers found in a study released this week.

In dozens of watersheds that have burned since 1984, including several on or around Arizona’s Mogollon Rim, fires that scorched at least 20% of an area increased streamflow by an average of 30%, according to the study published in the Proceedings of the National Academy of Sciences.

The effect lasts for about six years, after which encroaching vegetation and recovery of soils reduces flows again, even though the forests may take years more to fully recover.

The study, titled “Growing impact of wildfire on Western U.S. water supply,” does not indicate that the era of climate change-induced megafires could be good for the West’s long-term supplies, the authors warn. In fact, streamflow in most of the studied basins has declined since 1971. Rather, they conclude, the fires are “unhinging” streamflow from historically predictable responses to the climate.

The forests that feed the Colorado River Basin, a water source to millions of people in the Southwest, have not yet experienced such high-percentage burns on a basin-wide scale, scientist and lead author Park Williams said, so the findings are of more immediate significance to other rivers. Still, he said, climate change is creating conditions that will fuel wildfires on more of the West’s watersheds.

“Given that forest fire activity is expected to continue increasing,” Williams told The Arizona Republic in an email, “this implies that it’s likely that we’ll soon start seeing 6-year instances when (greater than) 20% of forest area burns across some large and important river basins in the West.”

Water isn’t the only thing that runs into stream and reservoirs after major fires, the scientists noted. Floods bring debris and sediment from newly denuded lands, creating both safety and water quality hazards. Heavy sedimentation — or ash deposition — can lead to costly increases in drinking water treatment. That’s one reason water providers such as Arizona’s Salt River Project are working to thin forests and reduce the risk of major fires around sources such as Cragin Reservoir, which supplies Payson.

“Yes, more water could very well have benefits,” said Williams, a climate scientist at the University of California, Los Angeles. “The West is generally a dry place where life is constrained by water. But unexpectedly high streamflow may be bad at times due to increased flood risk.”

Wildfires: How the Schultz Fire reveals long-term costs of big wildfires

The scientists examined satellite imagery for 179 minimally managed forest basins, and found 72 that had experienced at least one fire that burned a fifth of the area since 1984. These areas feed streams for which there are long-term flow measurements. By comparing flow histories with precipitation measurements, they determined how much of any year’s increased flow could be attributed to fire.

The 30% average increase held true in forests across the region, Williams said, including in Arizona.

Not surprisingly, the researchers found, the volume of water coursing into streams from burned areas increased in spring, when trees and plants would otherwise begin using some of it. More surprisingly, though, the trend held in fall, when natural water demand wanes.

This suggests that fire’s growing imprint on the Western forests will complicate the job for dam managers, who alter flows from reservoirs seasonally to alternately maximize storage and make room to absorb floodwaters.

Brandon Loomis covers environmental and climate issues for The Arizona Republic and Reach him at [email protected] or follow on Twitter @brandonloomis.

Environmental coverage on and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at and @azcenvironment on FacebookTwitter and Instagram.

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