Stream thermal regimes within an alpine discontinuous permafrost catchment, southern Yukon Territory

Abstract

Stream temperature, which influences many biogeochemical processes, is controlled by the exchange of water and energy across the stream surface, banks, and bed. A stream’s thermal sensitivity is its sensitivity to changes in air temperature and is increasingly important with projected warming in northern regions. As the thermal dynamics in permafrost underlain headwater streams are poorly documented, this study examines the thermal signals of two neighboring alpine streams (Granger Creek, GC and Buckbrush Creek, BB) within the Wolf Creek Research Basin (WCRB), Yukon Territory, and quantifies the dominant energy fluxes through an energy budget approach. A 1950 m study reach was established in each stream where water temperature, vertical streambed temperature, and stream discharge were measured. Distinct thermal heterogeneity is observed in each stream, with areas of persistent temperature decrease along their lengths, minimal downstream increase in stream temperature in GC, and an overall decrease in downstream temperature in BB. These observations are indicative of focused groundwater upwelling, low thermal sensitivity, and an increasing downstream influence of groundwater. This is inferred from increases in specific conductivity (2 μS/cm – 20 μS/cm) at locations of temperature decrease, and patterns in low (high) mean air-water linear regression slope (intercept) values. With distance downstream, regression slope (intercept) in GC went from 0.37 (1.2 ̊C) upstream to 0.33 (2.6 ̊C) at the outlet. Similarly, in BB regression slope (intercept) went from 0.37 (2.1 ̊C) upstream to 0.24 (3.3 ̊C) downstream. These patterns also indicate an overall lower thermal sensitivity and greater groundwater influence across the study reach of BB. Downwelling is observed at locations of vertical temperature profiles in each stream through efficient downward propagation of the diel temperature signal into the streambed, indicating hyporheic exchange. The energy balance indicates a large unaccounted for sink in BB, further denoting a larger groundwater influence in its study reach. These findings highlight the importance for continued study of thermal regimes and the complicated interconnections between heat exchange processes in alpine catchments in permafrost regions.