An Observational and Modeling Study of Impacts of Beetle-kill on Surface Energy and Hydrological Cycles

Observations obtained at the Glacier Lakes Ecosystem Experiments Site (GLEES) from 2005-2007 (pre-beetle) and from 2009-2010 (post-beetle) are combined with Noah-MP model simulations to assess surface energy and hydrological impacts of red-phase mountain spruce-beetle attack. Increased albedo during red-phase results in more reflected solar radiation and less net radiation, but the magnitude of these post-beetle variations are smaller than or comparable to their annual variability. The most dominant and greater than annual variability signals are large reduction (27%) in summer daytime evaporation and companion large increase in sensible heat fluxes (25%), leading to increased Bowen ratio. While reduced canopy interception and net radiation may favor winter snow accumulation, larger downward turbulent and less longwave-radiation loss seems to increase the rate of spring snowmelt, resulting in a 5-day shorter snow season. Model simulations show that more spring snowmelt and less spring-summer transpiration produce wetter soils and more runoff (18%). This trend is similar to runoff change in harvested forests where reduced forest density and cover results in more spring snowmelt and annual water yields. This study highlights the need for taking into account the transient effects of forest disturbances in modeling land-atmospheric interactions and their potential impacts on regional weather and climate. We also combine beetle-kill trajectory and land-surface models to perform a reanalysis of surface conditions (fluxes, soil moisture, runoff, evaporation) for 2000-2010 to extend this study to watershed or regional scales. Moreover, this study highlights the need for taking into account the transient effects of forest disturbances in modeling land-atmospheric interactions and their potential impacts on regional weather and climate.