Numerical experiments to explain multiscale hydrological responses to mountain pine beetle tree mortality in a headwater watershed

Abstract

Abstract The effects of mountain pine beetle (MPB)‐induced tree mortality on a headwater hydrologic system were investigated using an integrated physical modeling framework with a high‐resolution computational grid. Simulations of MPB‐affected and unaffected conditions, each with identical atmospheric forcing for a normal water year, were compared at multiple scales to evaluate the effects of scale on MPB‐affected hydrologic systems. Individual locations within the larger model were shown to maintain hillslope‐scale processes affecting snowpack dynamics, total evapotranspiration, and soil moisture that are comparable to several field‐based studies and previous modeling work. Hillslope‐scale analyses also highlight the influence of compensating changes in evapotranspiration and snow processes. Reduced transpiration in the Grey Phase of MPB‐induced tree mortality was offset by increased late‐summer evaporation, while overall snowpack dynamics were more dependent on elevation effects than MPB‐induced tree mortality. At the watershed scale, unaffected areas obscured the magnitude of MPB effects. Annual water yield from the watershed increased during Grey Phase simulations by 11 percent; a difference that would be difficult to diagnose with long‐term gage observations that are complicated by inter‐annual climate variability. The effects on hydrology observed and simulated at the hillslope scale can be further damped at the watershed scale, which spans more life zones and a broader range of landscape properties. These scaling effects may change under extreme conditions, e.g., increased total MPB‐affected area or a water year with above average snowpack.