The immediate impacts of large and severe wildfires on water runoff have long been known to researchers, land managers, and, increasingly, the communities in their path. Devastating mudslides and millions of dollars in flood damage occur each year following fires that compromise vegetation and soils that would otherwise absorb and regulate the flow of post-fire rainfall.
Though forests and rangelands provide more than half of U.S. water supplies, the long-term impacts of fires, including wildfire and prescribed fire, on annual water yield have been less understood. Three case studies, recently published in the journal Ecohydrology, provide some answers.
Researchers from the U.S. Forest Service Eastern Forest Environmental Threat Assessment Center and partners from the Southern Research Station and U.S. Department of Energy’s Oak Ridge Institute for Science and Education developed an analytical framework to study three watersheds with different characteristics and climates that experienced different types of fire. Their study combined pre- and post-fire streamflow data, fire records, remote sensing images, and computer models in an effort to tease out climate effects and reveal the true impacts of fire on water yield.
In Arizona’s Wet Bottom Creek watershed, which has wide-ranging slopes, a temperate climate with hot and dry summers, and evergreen forest over half its land, researchers found a 266 percent increase in annual water yield in the five years after the 2004 Willow Fire, which affected more than 80 percent of the watershed and burned at mostly low to moderate severity. Only 24 percent of this increase was attributed to precipitation trends.
In California’s Del Puerto Creek watershed, which receives relatively little rainfall and has more shrub and grass cover than forest, streamflow increased 38 percent in the five years after the 2003 Deer Park Fire, which burned about 14 percent of the watershed with varying severity. But annual water yield decreased 64 percent due to low precipitation.
South Carolina’s humid subtropical Black Creek watershed, a relatively flat, low altitude area with about half of its land covered in forests and grass, received high precipitation following periods of low severity prescribed burning across 40 percent of its land, but researchers could find no apparent change in annual water yield that could be attributed to fire.
“The variable magnitude of increased water yield after a wildfire surprised us,” says Dennis Hallema, a hydrologist with the Eastern Threat Center and the study’s lead author. “Wildfire has a much greater effect on water supply than prescribed burning, because it consumes more vegetation and its heat alters the ground in a way that reduces the amount of water that can infiltrate during a rainstorm.”
Researchers found the task of disentangling true wildfire effects from climate factors to be a complex, but critical task. “High-severity wildfires across a large area do affect watershed hydrology greatly and the effects can last many years, but post-fire precipitation patterns can mask the hydrologic effects of fire alone,” says Eastern Threat Center research hydrologist and co-author Ge Sun. “For example, recent drought events in California may hide the big potential of wildfires to enhance water yield.”
With such great variability in climate, watershed characteristics, and hydrologic dynamics across the United States, researchers say that management and restoration activities must be tailored to unique local conditions with the future in mind.
“We now know that post-wildfire management requires an integrated, long-term approach. Prescribed burning at an interval that matches the natural fire frequency before humans arrived is an efficient method to partially prevent and reduce damage from wildfire and associated flood risk,” according to Hallema, who says that land managers and local stakeholders can use the framework outlined in the published study to determine by how much an actual or hypothetical wildland fire can affect annual streamflow in their area.
Sun explains that this research, which is part of a Joint Fire Science Program-supported project, is ongoing. Researchers are applying the framework to study other U.S. watersheds to generate additional insight into the interactions between wildfire and water. “Forest management practices such as thinning dense forests and prescribed burning have been viewed as a means to reduce wildfire risks while providing more water from forestlands (due to reduced water use by understory vegetation)—a so called ‘win-win’ scenario. Our study will offer more empirical evidence to test this hypothesis in different geographic regions.”
For more information, email Dennis Hallema at email@example.com.