Controls of Wetland Hydrology in the Zone of Discontinuous Permafrost, Northwest Territories, Canada

Mean annual air temperatures are increasing rapidly in northwestern Canada, which is among the most rapidly warming regions on Earth (Box et al., 2019). Consequently, permafrost thaw within northwestern Canada is proceeding at a rate not found in the historical record (Pelletier et al., 2017), with areal thaw rates especially high in the peatland-dominated southern margin of discontinuous permafrost (Quinton et al., 2019). Permafrost thaw has caused vast land cover changes and transformed the permafrost underlain forest into permafrost free wetlands over much of Taiga Plains (Carpino et al., 2018; Jorgenson & Osterkamp, 2005). This land cover change has altered how water is stored and cycled on the landscape (Haynes et al., 2018). These changes were driven by the expansion of runoff contributing areas, as permafrost “barriers” are removed with thaw (Connon et al., 2014). This process effectively taps water stored in the interior of plateau-wetland complexes. Considering the changes that occurred within these wetland complexes, an updated wetland basin area found that not only had the contributing area has expanded but the wetland basin had grown with the inclusion of additional wetlands to the wetland cascade. However, proportionally the forest to wetland ratio has remained the same at almost equal proportions. This study examined the inter-annual variability of water balance components for each year from 2013 to 2022 for a 11.2 hectare plateau-wetland complex. Further narrowing into a detailed wetland-based water balance for 2022, examining the intra- annual variation of water storage within the collapse scar wetland W-3. The summer of 2022 (April 1 – September 30) was characterized with an extremely low rainfall (103 mm) and deep late winter snowpack (240 mm). Subsurface hillslope runoff from the surrounding forest was the greatest flux of water into the wetland (249 mm). Although subsurface runoff varies over time, a conceptual model is presented with the inclusion of the capillary fringe as an area of intermediary water storage and transport. Overland flow between wetland was the greatest water loss mechanism for the wetland (487 mm) and was greatest during the spring freshet with 81% of the total overland flow occurring during this period, although discharge was greatly reduced post-freshet. Evapotranspiration was the second largest source of water loss within the wetland (294 mm), by June 1 evapotranspiration was the predominant source of water loss. This led to a large wetland water deficit by the end of September (-177 mm). The extreme hydrological events of 2022 are used to identify the critical thresholds of the water balance components. This study furthered our understanding on how water movement and storage within wetlands will change with continued permafrost loss. When the critical thresholds of water balance components are exceeded, the integrity of wetlands within the region is threatened.

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