ALPINE SHRUB TUNDRA WATER STORAGE AND RUNOFF DYNAMICS IN THE MACKENZIE MOUNTAINS, SAHTÚ TERRITORY, NT

Alpine regions receive large volumes of precipitation and are important to local and regional water balances, particularly during baseflow periods of winter cold and summer drought when the larger basin area is frozen and/or water limited. Alpine headwaters in western Canada are expected to warm and receive more precipitation during the coming decades, with implications for groundwater recharge and streamflow generation within these systems and the regional river networks to which they contribute. Throughout the North, thawing peat plateaus and other ice-rich permafrost features are resulting in an increased extent of thermokarst and wetland land cover. This transition places infrastructure and water resources at risk as the structural integrity and reliable flow paths previously maintained by the frozen soils become compromised. Alpine systems are particularly susceptible to hydrological change due to the amplification of climate warming with both latitude and elevation. The inherent spatial heterogeneity of these same systems makes attempts to quantify the impacts of climate change on current and future basin water balance even more challenging, yet few field studies of alpine hydrology have been conducted in northern Canada. Specifically, no hydrological field studies have previously occurred within alpine shrub tundra terrain overlapping the Taiga Cordilleran Ecozone and/or the Mackenzie River basin.\nThe objective of this dissertation is to characterize the spatial and temporal variability in hydrological processes controlling the water balance of an alpine shrub tundra basin. Chapter Two presents five cover classes that are hydrologically distinct based on physiographic, surface, and subsurface characteristics. Glaciofluvial uplands are isolated from the channel network, routing all inputs to aquifer recharge. Peat plateaus have ice-rich permafrost at depth, resulting in limited storage and efficient subsurface runoff to neighbouring fens. Fen and riparian swamp iii cover classes both act as primary contributors to the channel network, although some fen areas may be isolated thermokarst features. These thermokarst features lose water via taliks recharging aquifers and/or evaporative loss from surface ponds. In the context of climate change, permafrost thaw will result in the replacement of peat plateaus with fens, such that both storage capacity and groundwater connections will expand. A conceptual model presents the basin storage compartments and expected flow paths linking the cover classes to each other and the larger area beyond the topographical extent of the study basin.\nChapter Three utilizes the land cover classification established in Chapter Two to investigate temporal differences in 2019 open water season basin water balance. During the freshet, a large volume of snowmelt was received, and storage capacity was limited by shallow frost tables and bedfast ice. As a result, runoff generation was highly efficient and streamflow volumes large. The exception to this is the glaciofluvial upland, which channeled all snowmelt to aquifer recharge. As the freshet transitioned to summer, small magnitude rain events began to occur, and evapotranspiration became the primary means of basin water loss. Furthermore, groundwater exchange became more important to the basin water balance, with groundwater discharge from springs in the headwaters sustaining streamflow and channel bed infiltration becoming more prominent as bedfast ice and channel banks thawed. As the summer progressed, cumulative storage, streamflow, and evapotranspiration rates declined as groundwater discharge became the primary input and groundwater recharge the primary output. As climate change continues, a greater proportion of precipitation will be received as rain and the open water season will extend, resulting in a greater proportion of total annual basin outputs occurring via aquifer recharge, although shrubification and permafrost thaw may result in greater influence of evapotranspiration.\nChapter Four assesses the basin runoff response following discrete precipitation events and utilizes stable isotope analysis to establish seasonally distinct source water contributions, evaporative influence, and subsurface flow paths during the 2019 open water season. The large volume of snowmelt received during the freshet caused peak streamflow rates, but only 8 % of total freshet discharge was isotopically designated as event water at the main basin outlet. In comparison, the maximum daily and total freshet event water fraction was reduced at the headwater subbasin outlet, where spring sources of groundwater discharge were more influential on streamflow. During the summer months, headwater subbasin streamflow was volumetrically and isotopically unresponsive to rain events and groundwater discharge continued to dominate. At the main outlet, early summer runoff response volumes and event water contributions following precipitation events were greatly reduced, in part due to the smaller magnitude of rain input volumes compared to snowmelt, but also due to the increase in fen storage capacity. By the late summer, the frost table also reached the mineral substrates at depth in the riparian swamp, extending the flow path for rain received by this cover class. As a result, late summer streamflow following rain was composed of even less event water and the hydrograph response was characterized by a lower peak and extended recession limb compared to the early summer event.\nThis dissertation greatly enhances our understanding of the hydrological role alpine tundra plays in sustaining regional river systems via both surface streamflow and aquifer recharge. These findings provide the model structure and parameter values necessary for future hydrological modelling efforts that seek to better represent the contribution of these headwater subbasins to larger regional river systems under current and future climatic conditions.

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