How Much Does Wildfire Reduce Forest Water Use and Increase Runoff

Wildfire is reshaping the forests of California's Sierra Nevada and affecting the water those forests supply to downstream users.

A paper published in Hydrology and Earth System Science reports that the effect of wildfire on potential runoff from the state's main water-supply watersheds has increased in the period 1985-2023, most markedly since 2019.

To measure how much water forests use, researchers from UC Merced, UC Berkeley and UC Irvine used data on evapotranspiration, the measure of water evaporating from soil and released by plants. The data drew from satellite imagery, ground measurements and climate records. Annual potential runoff was calculated by subtracting these evapotranspiration values from annual precipitation.

They verified their findings with full-natural-flow values for the study area, which includes the main watersheds serving California's State Water Project and Central Valley Project. Full-natural flow is calculated as the runoff that would occur at a stream-gauging station in the absence of upstream diversions and changes in reservoir storage.

The researchers found that by 2023, the cumulative burned area, about one-fifth of the total watershed, resulted in a potential runoff increase of about 740,000 acre-feet per year, or about 5.4 percent of the combined average full-natural flow from these basins.

"The annual potential runoff after a wildfire is a balance between vegetation mortality and regrowth, which is accounted for by the satellite data used in developing evapotranspiration," said lead author Ziying Han, who carried out the analysis while a graduate student at Berkeley.

"While wildfire can increase potential runoff, it can also have devastating effects on communities and ecosystems in the fire area," said co-author Han Guo, a postdoctoral researcher at UC Merced. "Forest restoration using mechanical thinning and prescribed fire can produce similar water benefits, while also improving forest health, recreation in the forest, carbon storage and other ecosystem services."

Developing quantitative estimates of water and related benefits is central to advancing forest restoration. While fire is an important part of these ecosystems, lower-severity fire can have significantly more net benefits compared to the stand-replacing wildfires so apparent in recent years.

"Linking benefits of fuels treatments to beneficiaries and reducing uncertainty in projected benefits, are central to financing the multi-billion dollar per year need for forest restoration," said engineering Professor Emeritus Roger Bales, a co-author of the study. "Reducing projected wildfire severity through fuels treatment has enormous co-benefits to public health, timber production, erosion reduction, biodiversity, community protection and much more."

The findings support ongoing efforts to restore and protect the region's forests.

"This research strengthens the case that fuels treatment isn't just wildfire risk reduction - it's a water security investment," said Gregg Verutes, director of conservation science at the nonprofit Blue Forest. "That's the kind of evidence that helps us bring new capital to this work."