Groundwater contribution to winter streamflow in the Canadian Rockies

Alpine watersheds represent an important source of fresh water in western Canada. Understanding of hydrological processes in headwater environments is crucial to effectively manage and allocate alpine water resources. Since rainfall, snowmelt and glacier melt make minimal contributions to streamflow during winter months, essentially all winter flow in unregulated streams is provided by groundwater discharge. Therefore, the analysis of winter flow provides critical information regarding the magnitude of groundwater discharge and its relation with the physiographical characteristics of watersheds such as climate, geology and topography. Streamflow records of 18 watersheds (21-3900 km(2)) in the Canadian Rocky and Columbia mountain ranges were analyzed. Winter flows were in a narrow range (0.2-0.6 mm d(-1)) throughout the study area, and had a relatively small inter-annual variability, while the flows during spring, summer and fall had a large inter-annual variability. This suggests that the groundwater storage is filled up to the maximum capacity every year, and the groundwater discharge in winter is mostly controlled by the stationary factors such as the spatial variability of geology, topography and climatic variables. Precipitation inputs, reflected in the long-term mean annual flow, had a strong correlation with the long-term mean winter flow, indicating the dominant effect of the climatic gradient. Bedrock geology influenced winter flows, where the watersheds underlain by younger rocks having generally higher permeability and porosity had higher winter flow, while the topographic effects were not discernible. These suggest that the local effect of topography on the hydraulic gradient is averaged out, but the geological influence on the aquifer hydraulic property operates at a scale comparable to the size of the study watersheds.