Forest thinning may increase water yield from the Sierra Nevada

Research news Forest thinning may increase water yield from the Sierra Nevada D iscussions of California’s limited water supplies often leave out the biggest water users of all — forests. Elena Zhukova About 60% of California’s developed water supply originates in the Sierra Nevada. But an average of roughly 50% of the precipita- tion that falls in the Sierra Nevada never makes it to the state’s rivers, groundwater basins, reservoirs and aqueducts. Instead, it sustains trees and other vegeta- tion, or evaporates. A team of UC researchers is investigating how thinning forests could increase the water yield of the Sierra Nevada, which supplies about 60% of the sur- face water used by California’s cities and farms. The Sierra Nevada Watershed Ecosystem Enhancement Project (SWEEP) builds on prior re- search that indicates that forest thinning — removing a fraction of trees and other vegetation — can improve the health of remaining trees and reduce the risk of severe wildfire (Bales et al. 2011; Collins et al. 2014). The research team aims to quantify how invest- ments in forest management translate to increases in the amount of runoff from a given watershed. While research from a variety of sites around the world shows that thinning can increase water yield, the CALIFORNIA AGRICULTURE • VOLUME 69 , NUMBER 1 magnitude of that effect varies widely depending on climate, elevation and other variables, and it isn’t well studied in the Sierra Nevada. Better data on the benefits of vegetation manage- ment — for water availability as well as forest health, fire vulnerability, wildlife habitat and other ecosystem services — could form the basis for deals creating new sources of funding for forest management. Owners of hydroelectric dams, for instance, might chip in for watershed management if it was clearly established that a healthier forest would mean more water flowing through turbines downstream. But such transactions would need to be based on solid research. “You need good data if you’re going to ask people to pay for it,” said Roger Bales, a hydrologist and pro- fessor of engineering at UC Merced and a lead SWEEP researcher. Marketing the ecosystem benefits of forest thinning could help address a key problem in forest manage- ment: While there is wide agreement that tree density is higher than desirable on much California forest- land, funding is scarce for thinning, which typically costs $300 to $500 per acre. Preliminary estimates suggest that reducing forest cover by 40% could be ex- pected to increase water yield about 9%. Bill Stewart, a UC Cooperative Extension special- ist who is studying the economic aspects of SWEEP,