Hillslope response to insect-induced land-cover change: An integrated model of end-member mixing

Current understanding of streamflow composition in mountain watersheds is often limited by inherent uncertainties and collection limitations in field data and assumptions associated with modelling techniques. Additional complexity arises in catchments experiencing land-cover change. Here, a hillslope model with fully integrated processes from the subsurface through the canopy is combined with Lagrangian particle tracking through the surface and subsurface domains to understand changes in flow paths and source waters with insect-induced tree death. This approach explicitly simulates end-member mixing by tracking parcels of water tagged as rain, snow, and pre-simulation ('old') groundwater and provides a method of separating outflows from these sources. Model simulations identify increases in subsurface water availability resulting from transpiration loss and altered canopy processes that increase throughfall and land-surface energy. Combined with changes in snowmelt timing, the shallower depth to saturation associated with tree death results in increased old groundwater contributions to streamflow. This shift in the source of outflow is consistent with prior field analysis of changing streamflow contributions with tree mortality from widespread insect infestation in the Rocky Mountains of North America. Model results also highlight mixing of old water and new precipitation within the groundwater end-member. Mixed hillslope outflows indicate that combinations of topography and precipitation can drive a range of signatures in groundwater inputs over meaningful time periods. Ultimately, this work and analysis of field observations provide insight into hillslope hydrologic processes and can serve as a platform for more complex simulations of land-cover perturbations to streamflow source partitioning.