Thinning of forests, generally undertaken to reduce dangers from wildfire and restore the forest to a more natural state, also can create more mountain runoff to mitigate drought effects in the central Sierra Nevada region that relies on snowpack.
In fact, researchers from the College of Agriculture, Biotechnology & Natural Resources at the ÍƼöÐÓ°ÉÔ´´ found that the quantity of additional water produced by thinned forests can be so significant that it might provide further incentive for forest managers to undertake prescribed burning or tree-removal using heavy equipment and hand crews with chainsaws.
Water yields from thinned forests can be increased by 8% to 14% during drought years, found the study undertaken by Adrian Harpold, an associate professor in the Department of Natural Resources & Environmental Science, and Elijah Boardman, a doctoral student in the Graduate Program of Hydrologic Sciences. Harpold, who also conducts research as part of the College’s , is principal investigator of the study funded by the U.S. Forest Service and recently published in .
That increased water would be particularly valuable, especially in drought years, to farmers and cities in central California and northern Nevada who rely on Sierra snowpack for much of their water supply. The new research doesn’t delve into financial considerations but focuses exclusively on the amounts of water generated by thinned forests.
Complex factors affect water yield
When forests are thinned, either through prescribed burning or mechanical removal, less rain and snow is intercepted and lost in the tree canopy, and more reaches the ground, Boardman said. Snow and rain captured by the tree canopy is likely to evaporate, while snow and rain that reaches the ground is more likely to feed water to streams.
At the same time, removal of some large trees reduces the demand for water. Fewer trees, after all, require less water, and thus runoff increases.
While forest managers long have viewed increased water yields as a side benefit of forest-thinning operations that have focused on reducing forest fire risk, the computer models applied by Harpold and Boardman present a nuanced look at the complexities involved.
For instance, Boardman noted that even though removal of trees reduces some of the demand for water, plants in the forest understory (bushes, grasses and other small plants) often thrive when larger competitors are removed. The understory, in turn, requires more water after thinning operations, although not as much as the amount required by large trees. Thus, not all the snow and rain that reaches the ground when larger trees are removed will necessarily translate into more runoff.
At the same time, the research found that large-scale flooding risks don’t increase significantly even when thinned forests yield more water. Some small infrastructure such as road culverts might be affected by greater localized runoff, but the large reservoirs such as the Oroville Dam that nearly failed from flooding in 2017 show low sensitivity to the forest restoration efforts.
The origins of the research project go back to Harpold’s involvement in the Tahoe West Project with the U.S. Forest Service, which eventually led to the opportunity to expand to a regional scale.
Harpold approached Mark Wigmosta at the Department of Energy, Pacific Northwest National Laboratory about collaborating around his hydrological simulation model. The project led to hiring Boardman who, Harpold said, ultimately “did all the work.” For example, to sort through the millions of simulations needed to make predictions in different scenarios for large river basins — the Truckee, American, Bear and Yuba — into the next 100 years of changing climate, Boardman took advantage of the Pronghorn High-Performance Computer Cluster in the University’s Office of Information Technology. Despite the technical challenges, Boardman said he is using the analytical tools used for this research in other watersheds worldwide and continuing to collaborate with the Pacific Northwest National Laboratory.
Funding source for thinning?
The new study dovetails with the work of the , a science-based effort by private, federal and state partners to restore the resilience of 2.4 million acres of forested wildlands in the Sierra Nevada. As part of that initiative, the U.S. Forest Service is assessing the potential markets for water supply, wood products and carbon storage that could financially support forest-thinning operations.
Thinner forests, in turn, are viewed by leaders of the Tahoe Central Sierra Initiative as a key element in the reduction of risks from high-severity wildfires in the many forests that are overgrown today as the result of more than a century of fire-suppression efforts.
Harpold notes that another nonprofit initiative, Sacramento-based already is using the promise of increased water yields from thinned forests to help support bonds that it sells to finance urgent forest-resilience projects.
Along with improved supplies for agriculture and municipal water customers, increased water yields from thinner forests can benefit aquatic and riparian ecosystems and potentially improve the operation of hydropower plants, Harpold and Boardman said in their study.
While the costs of large-scale thinning operations across the Central Sierra are daunting — even if water yields and other environmental benefits produce some revenue — Boardman said targeted forest thinning could be cost-effective. He explained that forest managers could use results such as his to select watersheds with especially dense forests that typically receive high amounts of precipitation (snow and rain).
The soon-to-be-minted Dr. Boardman now is putting the skills he developed as a doctoral student working with Harpold into creation of a new Nevada-based company, Mountain Hydrology LLC, that provides water forecasting and hydrological research.
