Multi-scale linkages between forest water use, catchment storage, and streamflow dynamics (Invited)

Forests exert an explicit control on streamflow dynamics through their use of water stored within the catchment. While the connection between stored water and stream discharge dynamics has been studied for decades, forest hydrologists continue to struggle to understand how trees modulate these fluxes on timescales of hours to years. Here we present new data from the Alsea experimental forest in Western Oregon that examines coupled ecohydrological processes from tree to headwater catchment scales. We combine experimental forest removal and water isotope (18O and 2H) analysis of tree source water with more traditional hydrometric measurements and analysis (stream gauging and groundwater hydrology) to probe forest-catchment coupling mechanisms. Preliminary results show that removal of riparian zone Douglas-fir and red alder forest from first-order channels had no discernable effect on diel fluctuations in stream discharge. In contrast to this tightly coupled behavior, analysis of precipitation and stream water isotope signals (weekly sampling) indicate that the mean transit time of our stream water is in excess of 2.5 years. In between the extremes of diel streamflow fluctuations and multi-annual particle flux through the subsurface, water levels in shallow (5-8 m depth) and deep (37 m) fractured bedrock groundwater systems show evidence of storm rainfall response, but only after a storage deficit has been met through the course of the Pacific Northwest’s seasonal wetting cycle. Once activated, event-based groundwater level response was as much as 0.5 m and typically lagged the precipitation center of mass on the order of minutes to hours. During the same wetting period, streamflow runoff ratios increased from 1-5% under dry catchment conditions and plateaued at 65-85% when events occurred during moderately wet to very wet catchment conditions. Overall, our results suggest that while forests modulate small daily fluctuations in summer baseflow, they also set the threshold for full catchment hydrological activation; which is a prerequisite to produce the high runoff ratios observed during winter storm events. Our findings help define the temporal and spatial importance of forest vegetation and its interaction with the subsurface hydrological system for improved rainfall-runoff process conceptualization of forested catchments.