Restoring a Natural Fire Regime Alters Streamflow, Snowpack, and Storage in a Sierra Nevada Catchment

Lightning-ignited wildfires are naturally-occurring processes of landscape change in the mountains of the western USA. Due to fire suppression, however, many watersheds in this region now contain dense forests with high water demands. Restoring natural wildfire regimes to these forests could affect hydrology by changing vegetation composition and structure, but little is known about the specific effects on water balance. Mountain watersheds supply water to much of the western USA, so understanding the relationship between fire regime and water yield is essential to inform management.

The Illilouette Creek Basin (ICB) in Yosemite National Park represents a unique experiment in the Sierra Nevada, in which managers have moved from fire suppression to allowing a near-natural fire regime to prevail since 1972. This is the only watershed in California with the right combination of fire history, hydrological records, and meteorological data, to allow hydrological modeling of a frequent fire regime that can be run and validated using observed data. We used the RHESSys model to quantify hydrologic response to observed fire-related changes in vegetation cover within the ICB. Over the past 45 years, as successive fires reduced the ICB's forest cover approximately 25%, model results show that annual streamflow, subsurface water storage, and peak snowpack increased relative to a fire-suppressed control, while evapotranspiration and climatic water deficit decreased. Spring snowmelt runoff was higher under burned conditions, while summer baseflow was relatively unaffected. Model experiments also showed that wet years experienced greater fire-related reductions in evapotranspiration and increases in streamflow, while reductions in climatic water deficit were greater in dry years. These model results are consistent with field measurements and remote sensing observations within the same watershed.