Mountain watersheds are crucial sources of fresh water across western North America, where the year-to-year availability of water depends in large part on variations in vegetation cover and climate. For much of the past century, fire suppression efforts in many areas have led to denser forests with growing water demands that have contributed to increased fire risk and water scarcity.
Researchers have previously hypothesized that reintroducing natural fire regimes to these landscapes—essentially letting natural patterns of lightning-ignited fires play out absent suppression efforts—should help mitigate water stress and decrease fire hazard, but in practice few studies have examined how restoring natural wildfire conditions affects a forest’s long-term water balance.
Boisramé et al. have helped fill this knowledge gap by evaluating the effects of changing fire management on Yosemite National Park’s Illilouette Creek Basin, a snowmelt-dominated catchment in California’s Sierra Nevada. Following a century of suppressing fires in this basin, park managers enacted a new management policy in the early 1970s that allows most lightning-sparked fires to burn out naturally. Although researchers have observed that the ensuing fires have reduced the basin’s forest cover by about 25%, few data are available to quantify corresponding changes in the catchment’s water balance.
To address this deficiency, the researchers utilized the Regional Hydro-Ecological Simulation System (RHESSys) model to conduct two experiments: one that assesses the water balance changes that have occurred since wildfires were reintroduced to the Illilouette Basin and a second that evaluates how the catchment’s water balance would have varied had its wildfires continued to be suppressed until 2017, the end of the study period. The results indicate that restoring a natural fire regime in the Illilouette Basin has reduced forest water stress and increased the availability of water downstream. The simulations suggest these water balance changes have been most pronounced during wet years and are due to small increases in snowpack and belowground water storage, coupled with reductions in evapotranspiration, that collectively resulted in earlier snowmelt runoff and a roughly 5% increase in annual streamflow.
The new study capitalized on the unique data set available from the Illilouette Basin, which is the only watershed in the U.S. West with a restored fire regime where enough hydrological data have been collected to allow model validation. The results demonstrate how large-scale forest restorations may affect water resources, a topic of considerable interest across much of the region. (Water Resources Research, https://doi.org/10.1029/2018WR024098, 2019)
—Terri Cook, Freelance Writer