Forest fires increasingly affecting rivers and streams, for better and worse

Forest fires can have a significant effect on the amount of water flowing in nearby rivers and streams, and the impact can continue even years after the smoke clears.

According to a UCLA-led study published today, the number of forest fires in the western U.S. has increased the risk for flooding and landslides.

Researchers looked at streamflow and climate data for 179 river basins. Basins are areas of land where precipitation collects and empties into a common outlet. Between 1984 and 2020 all of the areas were affected by forest fires.

The scientists used a mathematical model to show that the streamflow after a fire tended to be higher than expected, and that larger fires tend to follow larger increases in streamflow.

For an average of six years, streamflow was 30% greater than expected in basins where over 20% of the forest had burned.

Park Williams, a UCLA associate professor of geography and the study's lead author, said forest fires enhance streamflow because they burn away vegetation that would otherwise draw water from soil and block precipitation before it ever reached the soil. Fires can cook soils and make them temporarily water resistant.

Climate change is expected to cause the amount of forested area burned in the West to increase from 1984 to 2020.

We are starting to see a sequence of years when large portions of forest are burned across some important basins, such as those in California's Sierra Nevada.

According to the study, wildfires will soon become another important consideration for those in charge of the supply and distribution of water resources. Water managers in the region must carefully calculate how much water will be available and how to use it.

Williams said that the increase in streamflow from forest fires may be beneficial.

It could be good news to dry cities like Los Angeles, because it could enhance water availability.

Other outcomes could be troubling. In the coming decades, too much water could overwhelm the infrastructure and increase the risk of flooding in and around burn areas.

Water managers in California may have to lower the water levels in the fall and winter to make room for excess water from major rains and snowstorms. It is possible to avoid disastrous flooding in some cases, but it is also possible to put communities at risk for having too little water during the state's increasingly hot, dry summers.

The water after a forest fire tends to be polluted. It is likely that water quality will get worse even if the quantity of available water increases.

Williams wants the findings to help water managers and climate scientists make better predictions about water availability and flood risk.

Williams said that water is a heavy and destructive thing. When it shows up unexpectedly, it is catastrophic.

More information: Growing impact of wildfire on western US water supply, Proceedings of the National Academy of Sciences (2022). doi.org/10.1073/pnas.2114069119 Journal information: Proceedings of the National Academy of Sciences Citation: Forest fires increasingly affecting rivers and streams, for better and worse (2022, February 22) retrieved 22 February 2022 from https://phys.org/news/2022-02-forest-increasingly-affecting-rivers-streams.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.