Forest fires can have a significant impact on the amount of water in nearby rivers and streams, and the effects can persist for years even after the smoke has cleared.
Now, as the number of wildfires in the western U.S. increases, they are increasingly affecting the region’s water supply and increasing the risk of flooding and landslides, according to A study published today inside Proceedings of the National Academy of Sciences.
The researchers examined river flow (the amount of water in rivers and streams that changes over time) and climate data for 179 watersheds. (A basin is an area of land where precipitation is collected and discharged into a public outlet.) All areas are located in the western United States and were affected by forest fires between 1984 and 2020.
Using mathematical models they developed, the scientists found that water flow in the years following a fire tends to be higher than scientists expected based on climatic conditions alone, and that larger fires tend to be accompanied by larger increases in water flow.
In watersheds where more than 20 percent of forests were burned, river flows averaged 30 percent higher than expected based on climate conditions alone. The effects lasted an average of six years.
UCLA lead author Parker Williams said the wildfires enhanced streams because they burned vegetation that would otherwise draw water from the soil and prevented precipitation before it reached the soil. Intense bushfires can also “cook” soils, making them temporarily waterproof, he said.Williams is also an adjunct fellow at Columbia University Lamont-Doherty Earth Observatory.
From 1984 to 2020, the area of forest burned annually in the West increased 11-fold. This trend is expected to continue or even accelerate due to climate change.
“So we’re starting to see a sequence of years in which most of the forest is burned in some very important hydrological basins like the Sierra Nevada in California,” Williams said.
The study shows that wildfires will soon become another important consideration for those in charge of water supplies and distribution. Every year, water managers in the region must carefully calculate the amount of water available and determine how to conserve and allocate water.
In a sense, the increase in streams from forest fires could be beneficial, Williams said. “This could be good news for arid cities like Los Angeles because it can actually improve water availability,” he said.
But other results can be troubling. For example, excess water could overwhelm reservoirs and other infrastructure in the coming decades, and could increase the risk of catastrophic flooding and landslides in and around burned areas.
To accommodate the increased risk of flooding, California water managers may have to lower reservoir levels in the fall and winter to make room for excess water from heavy rains and snowstorms, Williams said. That strategy could avoid catastrophic flooding in some cases, but it could also put communities at risk of having too little water during the state’s increasingly hot and dry summers.
Water after forest fires also tends to be heavily polluted, carrying mud, debris and large amounts of sediment. So even if water availability increases after the fires, water quality could deteriorate, he said.
Williams said he hopes the findings will help water managers and climate scientists better predict water availability and flood risk.
“Water is a very heavy and destructive thing,” he said. “It’s great when it comes to the volume we expect. When it comes up unexpectedly, it’s catastrophic.”
The study was co-authored by Jason Smerdon, Benjamin Cook, Arianna Varuolo-Clarke and Caroline Juang of Lamont-Doherty; and researchers from the University of Colorado Boulder; and the Cary Ecosystem Institute.
Adapted from a UCLA press release.
