Seasonal evolution of active layer thaw depth and hillslope‐stream connectivity in a permafrost watershed

Abstract

To advance our understanding of permafrost hillslope drainage dynamics and its influence on streamflow hydrochemistry, we instrumented a hillslope-stream sequence located in the headwaters of the Niaqunguk River watershed, Nunavut, Canada (63°N, 68°W). We combined high spatial resolution field measurements of water and frost tables across the hillslope, with semiweekly measurements of groundwater and streamflow chemistry to track the evolution of streamflow chemistry during active layer thaw. Interestingly, localized differential thaw patterns emerged under near saturation conditions across the instrumented hillslope, the result of a low-frequency high-magnitude summer rainfall event. Hillslope structure and uneven active layer thaw created two distinct fill-and-spill domains. A subsurface domain defined by frost table microtopography and a surface domain defined by surface topography. We observed a seasonal shift in streamflow chemistry with an increased influence of water flowing through the underlying mineral soils as the active layer thawed. As thaw progressed streamflow chemistry began to match that of the riparian groundwater, which was a mixture of hillslope surface and subsurface water. Hillslope-stream surface connections were sporadic and occurred when rainfall and saturation conditions across the lower portion of the hillslope were sufficient for water to spill out of midslope surface depressions and across a saturated riparian zone and into the stream. This research shows how hillslope structure and thaw processes influence hillslope-stream connectivity in permafrost terrain.

Impact of heat advection on the thermal regime of roads built on permafrost

“…The larger heat exchange rate can explain the relatively faster permafrost warming rate (0.08°C year −1 ) and thawing rate (0.2 m year −1 ) at our study site. The measured increasing rates of ground temperature and active layer thickness linked with subsurface water flow are consistent with observation in Arctic gully system (Fortier et al, ) and support fast permafrost thawing rate caused by water flow, based on the hydrological modelling of conceptual hillslopes in permafrost regions (Bense, Ferguson, & Kooi, ; Chiasson‐Poirier, Franssen, Lafreniere, Fortier, & Lamoureux, ; Ge, McKenzie, Voss, & Wu, ; Kurylyk et al, ; Lamontagne‐Hallé, McKenzie, Kurylyk, & Zipper, ).…”

Arctic concentration–discharge relationships for dissolved organic carbon and nitrate vary with landscape and season

“…Water chemistry monitoring at the catchment outlets is an integrative measure of the upstream aquatic and terrestrial processes that result in biogeochemical production, removal, or transformations as material is transported downstream (Creed et al 2015). Therefore, within a given flow season, we expected that increasing thaw depth and changes in terrestrial and aquatic nutrient demand would alter the C‐Q dynamics for both DOC and NO 3 − (Harms and Jones 2012; Chiasson‐Poirier et al 2020) (Fig. 2).…”

“…As noted above, soil and permafrost conditions exert a strong control on Arctic surface water chemistry. Therefore, hydrochemistry must be viewed in light of the unique natural seasonal influences of a thickening active layer and evolving subsurface flowpaths (Harms and Jones 2012; O'Connor et al 2019; Chiasson‐Poirier et al 2020), changing terrestrial and aquatic productivity and nutrient demand (Treat et al 2016), and spatial lake dynamics (Kling et al 2000). For example, in the higher productivity landscapes (Tundra and Lake‐dominated) we hypothesized that increasing organic matter availability and nutrient demand from June through late August would result in a seasonal progression of greater DOC “flushing” (↑ β , ↑FI) and NO 3 − sequestration (↓ β , ↓FI) signals (Fig.…”

Hydrological resilience to forest fire in the subarctic Canadian shield

“…This is consistent with deepening thaw enabling flow to access previously frozen (i.e., permafrost) soils, which can have elevated mineral and organic contents as a result of protection from weathering and decomposition processes, and accumulation of inorganics at the frost boundary. This kind of frozen ground mediated fill‐and‐spill runoff generation process has been documented after fire in the taiga plains of Canada's NWT (Gibson et al, 2018) and during the summer near Iqaluit, Nunavut, Canada (Chiasson‐Poirier, Franssen, Lafrenière, Fortier, & Lamoureux, 2020).…”