Verifying the water impacts of vegetation management in heterogeneous, mixed-conifer Sierra Nevada forests
Sierra Nevada mixed-conifer forests populate the precipitation phase transition from lower, rain-dominated elevations to higher, snow-dominated elevations. Vegetation structure plays a critical role in determining ecosystem energy and water fluxes along this transition through regulation of snowmelt and evapotranspiration rates. Based on pre-fire suppression surveys and photographs, density of vegetation in this region is approximately 50% higher than it was in 1900. This additional biomass has increased water loss through transpiration, but verifying the impact of vegetation density on water is challenging because changes in evapotranspiration do not translate into linear changes of soil storage or water yield. We have developed a strategy for assessment and verification of the water balance in these forested catchments using spatially distributed embedded sensor networks and high-resolution vegetation mapping. These observed measurements were then used to constrain parameterization of a hydro-ecological model, the Regional Hydro-Ecological Simulation System (RHESSys), applied to 8 headwater catchments at the rain-snow transition elevation (1500-2100 m). The headwater model was then scaled up to a 40-km2 catchment using geological and hydrological similarities. Using the calibrated model, the water balance effects of reduced vegetation through forest management and from wildfire were tested.
The hydrologic model was successfully calibrated to each headwater catchment (NSE=0.50-0.81) using snow, soil moisture, and streamflow observations. Simulated projections of vegetation reduction resulted in water yield differences greater than 10% between forest thinning and wildfire for these forest, climate, and watershed characteristics. The differences in response can largely be attributed to the spatial distribution of vegetation reductions, which affected the localized water and energy balances. Managing vegetation for water as an ecosystem service can be done in combination with management for forest health, wildfire risk, and habitat diversity. Given the economic importance of this relationship to hydropower, water supply and other ecosystem services, further field measurement programs to reduce uncertainty are being planned. These programs represent a strategic partnership between researchers and stakeholders that is needed to support both immediate and longer-term policies and decision making.