By Published: July 11, 2022

When Western wildfires break out, water may first come to mind as a critical resource for helping extinguish it. But what about after the flames finish?

A 2022 CU study on the growing impact of wildfire on the Western U.S. water supply found that large forest fires can significantly increase the amount of water in surrounding streams and rivers up to six years after a fire, impacting regional water supplies and increasing risks for floods and landslides. The results suggest that water and natural hazard management will need to be more prepared for wildfire impacts. U.S. wildfires — which have quadrupled in size and tripled in frequency since 2000 — are only projected to escalate.

“It is something organizations need to educate fire-prone communities about, so we can be prepared for short- and long-term impacts.”

“We’re likely going to see a lot more fires,” said Ben Livneh, co-author of the study and assistant professor of civil, environmental and architectural engineering. “Like we saw with [Boulder County’s] NCAR and Marshall fires, this is going to be a clear and present danger.”

Historically, forest-based streams and rivers increased in predictable amounts in response to rain or snowfall. However, from 1970 to 2021, those amounts declined due to warming and evaporation.

Wildfire adds another layer to the equation.

“When you bring so much fire into the mix, it fundamentally alters that relationship,” said Livneh, who also serves as director of the Western Water Assessment and is a fellow in the Cooperative Institute for Research in Environmental Sciences (CIRES).

The study examined 35 years of data from 179 forest basins in the Western U.S. between 1984 and 2019, including 72 sites where at least one large wildfire occurred. In areas where 20% or more of the forest burned, area streamflow was 30% greater than expected, for an average of six years post-fire.

It’s the first paper to show this increase persists in all four seasons after a fire, in all manner of vegetation, topography and elevation.

This water surplus could in part be a good thing, given the overall decline in the past 40 years. But it also comes with elevated landslide risks and a need for Western communities to invest in a greater diversity of water sources, as ash-laden water is low quality and expensive to treat, according to Livneh.

Due to the uncertainty of where or when future forests will burn, wildfire is not currently factored into assessments of the effects of climate change on Western U.S. streamflow.

“It is something organizations need to educate fire-prone communities about, so we can be prepared for short- and long-term impacts,” said Livneh.

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