Permafrost thaw induced changes to runoff generation and hydrologic connectivity in low-relief, discontinuous permafrost terrains

Recent climate warming in northwestern Canada is occurring at an unprecedented rate in recorded history and has resulted in the widespread thaw of permafrost. Where present, permafrost exerts a significant control on local hydrology, and disappearance of permafrost threatens to change the hydrology of northern basins. In the peatlands that characterise the southern distribution of permafrost in low relief terrain, permafrost takes the form of forested peat plateaus and is interspersed by permafrost-free wetlands (i.e. channel fens and flat bogs). Previous field studies have found that channel fens serve as the drainage network and route water to the basin outlet, whereas flat bogs have been viewed primarily as storage features. Wetland expansion in response to permafrost thaw can transform the primary hydrologic function of flat bogs from storage units to runoff-producing units by removing the relatively impermeable permafrost barrier that encompasses them. As a result, permafrost thaw has the potential to greatly increase the runoff contributing area when large storage features form hydrological connections with the basin drainage network. It has been well documented that permafrost thaw in this region results in the loss of forest and a concomitant expansion of wetlands, however the hydrologic response of these changes is poorly understood.\nStream flow records in four Water Survey of Canada gauged basins (152 - 2050 km2)in the lower Liard River valley were analyzed to determine the impact of permafrost thaw-induced land cover change on basin runoff. Annual runoff between 1996 and 2012 increased by between 112 mm and 160 mm and these changes were significant in all four basins (p>0.05). Changes to stream flow were assessed using the Mann-Kendall non-parametric test and the Kendall-Theil robust line. Permafrost thaw between 1977 and 2010 was quantified by comparison of historical aerial photographs and high-resolution satellite imagery (World View 2) over a 6 km2 area of interest, where changes in tree-covered terrain were used as a proxy for permafrost loss. It was found that land cover change from forest to wetland was the most important factor contributing to the increases in runoff (37 -61 mm), and that basins with a relatively high cover of flat bogs were subject to the largest increases in runoff.\nThis analysis examined increases in runoff contributing area when a direct connection was formed between wetlands. Field studies have indicated the presence of ephemeral drainage channels connecting flat bogs within a peat plateau-bog complex. These drainage channels cut through the peat plateaus and create a series of cascading bogs that ultimately discharges into the channel fen. The bog cascades can greatly increase the runoff contributing area of a basin when the cascade is hydrologically active. To investigate the transport of water through these features, two bog cascades were instrumented with sharp crested v-notch weirs and cut-throat flume boxes in 2013 and 2014. Within the peat plateau-bog complex, the two cascades had markedly different plateau:bog ratios, and therefore different contributing areas. Runoff between the two cascades varied significantly with one cascade producing 125 mm of runoff over the two year period and the other producing only 25 mm. Both cascades were active during the snowmelt period of each year, however only the cascade with the higher plateau:bog ratio produced runoff in response to rain events. It is proposed that the bog cascades operate under an “element threshold concept’, whereby in order for water to be transmitted through a bog, the depression storage capacity of that bog must first be satisfied. This work suggests that neglecting to represent these cascades of connected bogs in numerical models can underestimate basin stream flow by between 10 and 15%.\nAt the southern distribution of discontinuous permafrost, the rate of permafrost thaw has increased in recent years. This research demonstrates a mechanism whereby permafrost thaw may show a non-linear response to warming air temperatures. Measurements of active layer thickness (ALT) are typically taken at the end of summer and inherently assumed to be analogous to maximum thaw depth. By definition, the active layer is the layer above permafrost that thaws in the summer and freezes again in winter. In Subarctic Canada, field measurements conducted at the end of winter found that the entire thickness of ground atop permafrost does not entirely re-freeze. This results in the formation of a thin talik between the frozen active layer and permafrost, and indicates that ALT must be measured by the depth of re-freeze. As talik thickness increases at the expense of the underlying permafrost, ALT is shown to simultaneously decrease. This suggests that the active layer has a maximum thickness that is controlled by the amount of energy lost from the ground to the atmosphere during winter. Vertical permafrost thaw was found to be significantly greater in areas with taliks (0.07 m year-1) than without (0.01 m year-1). Furthermore, the spatial distribution of areas with taliks increased between 2011 and 2015 from 20% to 48%, a phenomenon likely caused by an anomalously large ground heat flux input in 2012. Wide-spread talik development can therefore expedite permafrost thaw and add further complexity to the observed changes to basin hydrology.

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