Changing stream water sources from climate-exacerbated insect infestation in forested watersheds: a combined field and multi-scale modeling analysis.

Abstract:

Changing climate, including warmer temperatures and drought conditions, has intensified mountain pine beetle infestation in the Rocky Mountains of North America, resulting in tree death over the last decade that is unprecedented in recorded history. The subsequent perturbation to tree-scale water budget processes such as interception, transpiration, and evaporation often combine non-uniformly and produce variable catchment-scale responses. Potentially offsetting perturbations such as decreased transpiration with tree death and increased exposure and evaporation with needle fall can produce changes in peak streamflow and water yield that are undetectable above typical interannual variability. These combined perturbations, however, may change streamflow generating processes and water sources that impact water quality in important mountain headwater streams. To determine the potential impact of widespread land cover change on catchment contributions to streamflow, this study combines a chemical and isotopic separation analysis using paired watersheds and pre-infestation controls with a multi-scale modeling approach (from hillslope to headwaters systems) that determines changes in water stores and fluxes between canopy, land-surface, groundwater and streamflow. Field observations and chemical hydrograph separation analysis suggest that groundwater contributions to streamflow increase with recent insect infestation, as transpiration ceases to remove water from the subsurface but potentially increased ground evaporation removes water from the land and subsurface with relative uniformity. Comparing these field observations to hillslope models provides additional spatial and temporal controls on inherently challenging field heterogeneities as well as a way of testing the influence of natural properties such as precipitation and topography on perturbations to streamflow partitioning from insect infestation. At larger scales, watershed models demonstrate how other factors may compensate to mute these responses between stand, hillslope and watershed scale. Ultimately, identifying these changes in stream water sources provides needed insight for water resource management in MPB-infested watersheds and for changing forested landscapes throughout the region.