Forest Fuel Reduction and Wildfire Effects on Runoff and Evapotranspiration in Sierra Nevada Mixed-Conifer Forest

Abstract:

Large, high-intensity wildfire risk in the western United States is growing, fueled by increasing vegetation density from a century of fire suppression and climatic shifts resulting in extended dry seasons. Strategically Placed Landscape Treatments (SPLATs) are a fuel reduction method designed to reduce fire risk on the entire landscape by treating only a fraction of the area. During 2011 and 2012, SPLATs were implemented in the mixed-conifer zone of the Tahoe (Last Chance study area, American River Basin) and Sierra (Sugar Pine study area, Merced River Basin) National Forests. Wildfire events were then simulated for both treated and untreated conditions. We integrated the vegetation changes with the Regional Hydro-Ecological Simulation System (RHESSys) to project impacts of fuel treatments and wildfire on runoff and evapotranspiration for the period of observed data, water years 2010-2013. Results from the model simulations show that vegetation treatments in the Last Chance study area, which removed 8.0% of the total biomass by treating 25% of the area, increased mean annual runoff by 12.0% and decreased mean annual evapotranspiration by 4.1%. Vegetation treatments in the Sugar Pine study area, which removed 7.5% of the total biomass by treating 33% of the area, increased runoff by 2.7% and decreased ET by 0.5%. Compared to pre-treatment conditions, wildfire simulations in Last Chance reduced total biomass by 38-50% when fuel treatments were not applied, resulting in a 55-67% runoff increase and a 19-23% evapotranspiration decrease. In Sugar Pine, fire simulation reduced biomass 39-43%, increasing runoff and decreasing ET by 13-15% and 1.8-2.7% respectively. Applying the same magnitude of biomass reductions equally over the entire watershed, in contrast to the localized areas of vegetation reductions due to treatment or fire, resulted in smaller impacts on runoff and evapotranspiration rates. Vegetation effects on hydrologic fluxes are greater in Last Chance than in Sugar Pine, and can be attributed to the higher precipitation rates, suggesting vegetation in the Sugar Pine area is more water-limited. These model results also indicate that strategic fuel treatments implemented to reduce the risk of high-intensity fire have a potential benefit of producing additional water yield.