Reexamining Mountain Pine Beetle (MPB) Influence on Snow Water Availability Utilizing a Multilayer Higher Order Turbulence Closure Plant Canopy Model

In the Western U.S., where the majority of surface water is supplied from snowmelt from forested watersheds, future water supply is uncertain due to climate warming and environmental disturbances such as the Mountain Pine Beetle. Although numerous studies have looked at climate change impacts on snowpack, many do not take the forest canopy into account when estimating snow mass or energy balance changes. The Mountain Pine Beetle (MPB) has killed over hundreds of thousands of square kilometers of forest; completely altering the forest structure and therefore the energy and water budget of these systems. In this study we re-evaluate previous MPB studies utilizing a vertical resolved plant canopy model, the Advanced Canopy-Atmosphere-Soil-Algorithm (ACASA). ACASA is a multilayer soil-vegetation-atmosphere numerical model based on higher-order closure of turbulence equations. The higher-order turbulence closure scheme of ACASA allows for the detailed simulation of turbulent fluxes of heat and water vapor as well as the CO2 exchange of several layers within the canopy. The model is used to assess how the water and energy balance is changing within a dead forest canopy compared to a healthy canopy. The results show the increased vertical resolution allows for greater understanding of snow-forest processes. In comparison with previous single layer model simulations, the multilayer model simulations yield different results regarding the importance of LAI and other canopy variables to energy and hydrologic processes. Sensitivity tests examine the importance of model parameterizations to these differences.