Characterizing seasonal differences in hydrological flow paths and source water contributions to alpine tundra streamflow

Alpine headwaters collect larger volumes of precipitation per unit area than neighbouring lowlands, recharge regional aquifers, and generate a greater proportion of river discharge than their limited extent would suggest. Despite the importance of alpine headwaters, field observations and assessments of source water contributions to streamflow in alpine tundra settings are sparse throughout the subarctic and absent in the Taiga Cordillera ecozone specifically. As such, it remains uncertain how changes to seasonally specific hydrological processes control discharge of the larger rivers to which these alpine headwaters contribute. This study quantified variability in source water contributions and flow paths during the 2019 open water season within a Mackenzie Mountain alpine tundra basin based on measurements of stable water isotopes, specific conductivity (SPC), and water volumes during runoff generating events. During the freshet, large daily snowmelt volumes resulted in the greatest volume of streamflow, which was composed mainly of pre-event water (similar to 92%). As the summer progressed, evapotranspiration increased, and groundwater flow paths extended, resulting in reduced event water fractions and hydrograph amplitude, and an extended duration of streamflow response. A headwater subbasin within the larger study basin was both hydrologically and isotopically unresponsive to summer rains, and instead was characterized by a delayed hydrograph response and reduced event water fraction. Results indicate this portion of the catchment was regulated by discharge from groundwater springs capable of sustaining streamflow before snowmelt commenced and during the dry summer months. As climate change continues, greater precipitation volumes and a longer open water season will likely result in reduced runoff and stream discharge from alpine basins as greater evapotranspiration and channel bed infiltration occur. This study provides a valuable data set and observations of seasonally distinct runoff generation processes to inform prediction of changes in northern alpine tundra hydrology in response to a warming climate.