Modeling the Impacts of the Mountain Pine Beetle on Water and Energy fluxes in the Rocky Mountain West

The mountain pine beetle (MPB) epidemic in Western North America has generated growing concern in recent years. Increasing numbers of outbreaks have affected an estimated two million acres of forest in Colorado and Wyoming alone during 2008. Given the substantial acreage of prematurely dying forests within Colorado and Wyoming, it has been hypothesized that the effects of the MPB outbreak will be similar to those observed after forest harvesting. High tree mortality rates of recent MPB infestations have the potential to induce significant changes in forest canopy, impacting several aspects of the local water and energy cycle, including canopy interception of precipitation and radiation, snow accumulation, melt and sublimation and evapotranspiration (ET). Parflow, a variably saturated groundwater flow model was coupled with the Common Land Model (CLM) to incorporate physical processes related to energy at the land surface and used to investigate the changing hydrologic and energy regime associated with MPB infestations. Specifically, Parflow-CLM was used to model daily and annual fluctuations of ET, snow pack, groundwater storage and ground temperatures throughout the four phases of MPB attack (green, red, gray and dieback phases), at sites typical of the Rocky Mountain West. Our results demonstrate that MPB infested watersheds will experience a decrease in evapotranspiration and an increase in snow accumulation accompanied by earlier and faster snowmelt. Impacts are similar to those projected under climate change, yet with a systematically higher snowpack. These results have implications for water resource management due to higher tendencies for flooding in the spring and drought in the summer.