Why some creeks run stronger after wildfire--and why it doesn't last

New UBC Okanagan research shows wildfire can change how much water remains in streams during the driest months of the year.

Wildfires don't just burn forests. They can also change how much water is left in creeks and rivers in summer, when water is scarce and demand is high, according to new research led by UBC Okanagan.

Published in Forest Ecosystems, the study looks at stream flows between July and September--after spring snowmelt and before fall rains.

These flows matter because they determine how much water is available for drinking, irrigation, fish habitat and emergency response during heat waves and drought.

"This is the kind of study that helps move us from 'wildfire changes flow' to 'here's why, when and through which pathways,’" says Shixuan Lyu, lead author and a doctoral student with UBC Okanagan's Department of Earth and Environmental Sciences.

"In water management, mechanisms matter because they affect what you can plan for and how long changes might last."

They used a combination of long-term stream measurements and chemical "fingerprints" that revealed where the water originated. Researchers found that burned watersheds in the Okanagan Valley had more water flowing later into the summer than unburned watersheds, but with considerable variations.

At first glance, that might sound like a benefit. But the researchers say the apparent boost comes with important caveats.

"Low flows are the pinch point for communities, agriculture and fish habitat," says Dr. Adam Wei, senior author and a hydrologist with UBC Okanagan. "Understanding how wildfire reshapes the seasonal balance between snowmelt, groundwater and water loss to the atmosphere is key to building realistic watershed strategies in a warming climate.

"It’s also important to understand that every watershed is unique, with different water responses to wildfires, so management strategies must be tailored to local watersheds."

After wildfire, fewer trees mean less water is pulled back into the atmosphere, and more snowmelt reaches streams and underground storage earlier in the year. That can temporarily increase summer low flows; however, as forests recover, water losses are expected to rise again. In some cases, they may exceed pre-fire levels.

The findings suggest wildfire can briefly reshape water availability during the dry summer, but they also underscore the need for long-term monitoring and careful planning as extreme weather, wildfire and water demand increasingly intersect.

"This isn't a new source of water," says Lyu. "It's a shift in timing and pathways, and those shifts don't last forever."

The research was conducted in partnership with the Okanagan Indian Band and the Okanagan Nation Alliance, whose leadership in watershed stewardship supported this work. Funding was provided by Mitacs Accelerate and the Okanagan Basin Water Board's Water Conservation and Quality Improvement Grant Program.

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