The number of wildfires burning across the western United States over the past 6 decades has been steadily increasing, and those fires are growing larger and more severe, especially in mountain areas where more than 65% of clean water resources for the West’s 75 million people originate. What happens when fires intersect water resources is the subject of two new papers in Hydrological Processes.
The watersheds of the Sierra Nevada deliver water to more than 25 million people, primarily via snowmelt, and the conifers of the Sierra are where many of the most severe fires are burning.
As the trees burn, those land cover changes affect the hydrologic cycle. Previous studies involving experiments on changes in runoff and streamflow, evapotranspiration, soil moisture and infiltration, and snow dynamics have indicated that all of these factors would be somewhat affected after a fire. But until now, scientists haven’t put it all together, said Fadji Zaouna Maina, a hydrologist at Lawrence Berkeley National Laboratory (LBNL) and lead author of one of the new papers.
Maina and her LBNL colleague Erica R. Siirila-Woodburn devised a large-scale modeling effort to understand how “postfire perturbations” affect hydrologic dynamics in the Cosumnes watershed, a vast and complex watershed that spans the Sierra Nevada and the Central Valley. The watershed includes 2,000 meters in elevation change from the headwaters to the valley, irrigated areas as well as forestland (more than half the watershed is conifer forests), and variegated geology, from low-permeability volcanic rocks to highly permeable sands and gravels in the valley. Most precipitation falls as snow. It’s “highly representative” of most watersheds in California, Maina said.
Maina and Siirila-Woodburn ran simulations based on fires occurring in the upper mountainous part of the watershed, in the intermediate area of the watershed, or in the Central Valley downstream. They modeled hydrologic changes based on one of the driest years on record (2015) and the wettest year on record (2017).
Land Cover Changes
Maina and Siirila-Woodburn found that land cover changes were the primary factor controlling hydrodynamics in the watershed.
It’s counterintuitive, Maina said, but snow accumulations increase, and evapotranspiration decreases regardless of whether the fire is followed by a wet or dry year.
Whether there’s a lot of precipitation or a little, snowpack is larger after a fire, which then means that runoff is larger, explained research hydrologist Dennis Hallema, who wasn’t involved in either of the new studies. That’s because as snow falls on an unburned tree canopy, the canopy intercepts much of the snow, which is then lost to sublimation rather than falling to the ground, melting, and recharging aquifers or running off into streams.
The research presents “an interesting pattern,” Hallema said. Burned mountainous watersheds, which researchers found had the most impact, produce higher streamflows downstream than expected, even in a drought. However, he said, “the extra water that comes downstream after a fire is not necessarily beneficial for municipal water supplies because of water quality issues” such as higher phosphorus levels and more sediment.
Despite the increasing streamflows downstream, he added, “I would not recommend burning down your watershed to have a bit more water.”
Soil Property Changes
In the other Hydrological Processes paper, Jingjing Chen of the Virginia Polytechnic Institute and State University and colleagues looked specifically at the issue of soil repellency and infiltration after fires. They compared soils in burned and unburned areas in Virginia and North Carolina and confirmed that water repellency is increased in soils after fires. The depth of the most water repellent soil varied across the sites.
Chen said the factors of fire-induced soil water repellency include fire temperature, duration, and intensity; soil water content; and organic matter content and its composition derived from plants and microorganisms. Soil texture (including compaction), clay content, and even clay mineralogy all influence the soil water repellency degree, she said, and “the rainfall amount, frequency, and intensity may also influence the persistence of fire-induced soil water repellency.”
“The effect of depth is interesting,” Hallema said. But the fact that Chen and her colleagues found these changes were still significant more than a year after the fires lends credence to the idea that differences in water repellency might be more related to physical properties in the soils than the fires, Hallema said.
Chen, however, says that the findings indicate that hydrologic processes take longer to recover than previously thought. “If the fire-induced repellency disappears,” she said, that would mean the hydrologic processes reverted to normal, which would have a positive influence on the recovery of plants and ecosystems.
It will be important to see, she added, whether soils in the West respond to fires like the soils her team studied—and that is their next step, along with identifying the mechanisms driving the repellencies.
Fires are hard to prepare for. About the best scientists can do, Hallema said, is develop better models so water and forest managers can make better decisions.
—Megan Sever (@MeganSever4), Science Journalist