Past Wildfires Offer Future Roadmap for Forest Management’s Effects on Water

Forest restoration is often associated with mitigating wildfire risk and improving ecosystem health throughout the Sierra Nevada. But restoration also dramatically affects water use within forests and the amount of runoff that flows downstream.

The Sierra Nevada provides more than 60 percent of California’s water supply and sustains a globally important agricultural region. Quantifying the water-related benefits can be critical in showing the true value and cost-benefit of forest management. But until now, there hasn’t been enough locally relevant data to incentivize restoration projects.

New research from UC Merced’s Sierra Nevada Research Institute (SNRI) fills this data gap and provides a method to monetize the water-related benefits of forest thinning.

The research team, led by former SNRI postdoctoral researcher Qin Ma, used satellite imagery of Sierra Nevada and Southern Cascade Range forests from 1985 to 2017 to track changes in evapotranspiration — mainly water use by vegetation — before and after large fires. The study area covered nearly 53,000 square miles and included 14 major source-water basins.

Recently published in the Journal of Hydrology, “Wildfire Controls on Evapotranspiration in California’s Sierra Nevada,” details the team’s findings:

• On average, evapotranspiration dropped 36 percent, or 265 mm in the first year after a fire;
• Continued reductions averaged 23 percent annually in the first 15 years after a fire; and
• Changes varied in different areas depending on burn severity and forest density before the fire.

“This historical data about water yield from Sierra forests after fire are important because it acts as a guide,” explained Ma, now an assistant professor in the Mississippi State University Department of Forestry. “Understanding how water use in forests has changed after disturbances in the past can help us predict how forests — and the water supply they provide — will respond to comparable management actions such as thinning or the reintroduction of low-intensity fire in the future.”

While high-severity fires led to the greatest reductions in evapotranspiration, these events also often have detrimental impacts on human communities, wildlife habitat, carbon storage and water quality through erosion. The challenge, the researchers note, is to mitigate these negative effects while increasing runoff.

Using an analysis based on the historical data, the team found that a future moderate-severity burn, or an equivalent management action such as thinning over just 27 percent of the Sierra Nevada would result in a 9 percent reduction in evapotranspiration.

“Our data not only show that ecological restoration can simultaneously reduce drought stress in Sierra Nevada forests and increase water yield, it also demonstrates a way to quantify the benefits of forest management actions,” said study co-author Professor Martha Conklin, with UC Merced’s School of Engineering. “Because restoration projects are so often limited by cost, it’s critical that we monetize the water-related benefits to help incentivize forest management.”

The research team also included SNRI’s Professor Roger Bales and then-graduate student Joseph Rungee; research scientist Brandon Collins from UC Berkeley; and Professor Michael Goulden of UC Irvine. The research was supported by the National Science Foundation, the UC Water Security and Sustainability Research Initiative, the USDA National Institute of Food and Agriculture, and the California Strategic Growth Council through the Innovation Center for Ecosystem Climate Solutions.