A –Streams of Thought– contribution by Matthew Morison.

A little background: recently, the Canadian branch of the Young Hydrologic Society was formally recognized as a committee of the Canadian Geophysical Union Hydrology Section. As an international member of the diverse global YHS community, we are so excited to be apart the next generation of hydrological research and to have new links to so many different regions and countries! In this spirit, this article strives to shed some light on some research which is not uniquely Canadian (in fact, far from it), but remains a large research focus in Canada – permafrost hydrology. 

Permafrost is the state of a soil remaining below 0⁰ C for a period of at least two years – so permafrost itself is a state or condition of a soil, not a specific textural or chemical makeup of a soil. Over half of Canada is underlain by some form of permafrost, either continuous (>90% spatial coverage), discontinuous (50 – 90%), or sporadic (10 – 50%). The spatial distribution of permafrost is driven by elevation, continentality, and latitude, with increasing coverage of permafrost poleward, in the heart of North America and Eurasia, and in alpine environments. Permafrost usually begins anywhere from a couple centimeters to hundreds of meters below the surface. The overlying soil, which is seasonally thawed and seasonally frozen, is termed the active layer. Permafrost layers then can extend further hundreds of meters below the surface until underlying geothermal heat brings the material back above 0⁰ C.

Indigenous peoples and settlers of Canada have long cultural histories of working and living with permafrost, whether burying into it to employ it as a freezer for food in the summer months, or trying to adjust to the challenges of building infrastructure on soil which is subject to cryoturbation (frost heaving). Of more recent scientific concern is that much work has quantified the great carbon stores in permafrost soils, which, while they remain frozen, are more readily sheltered from liberation to the atmosphere as greenhouse gasses carbon dioxide and methane.

Permafrost has thawed extensively in the past century (Payette et al., 2004; Jorgenson et al., 2006) and is projected to continue to do so. Within the next century, up to 70% of existing terrestrial surface permafrost may be lost to different modes of degradation due to these climatic changes (Schaefer et al., 2011; Schuur et al., 2013), including the elimination of all sporadic and discontinuous permafrost in central Canada by 2100 (Camill, 2005). So, as hydrologists in a country which is half underlain by this permanently frozen soil which is quickly thawing, naturally a lot of exciting research over the years has moved towards an understanding of how this frozen ground influences (and is influenced by) the movement of water and solutes through soil, atmospheric exchange of moisture and energy, and the implications of these things for the hydroecology and water resources management of these systems.

In permafrost environments, groundwater flows, plant root water access, and evapotranspiration are typically restricted to water seasonally housed in the small suprapermafrost zone (Woo and Winter 1993). In seasonally frozen landscapes, interactions with deeper groundwater is facilitated by the annual downward migration of the frost table to a state of complete thaw. These conditions can govern the existence of longevity of wetlands in permafrost environments compared to seasonally frozen ground (Woo and Young, 2003). Interactions between the active layer and subpermafrost water are rare and the lack thereof are among the central distinguishing features of the hydrology of permafrost systems (Woo, 2012). This segregation of zones occurs from the blocking of pores in saturated media by ice, which lowers the effective conductivity of the media by several orders of magnitudes (McCauley et al., 2002). This leads the community to conceptualize frozen ground as an “impermeable barrier” (Walvoord and Kurylyk, 2016) that inhibits flow, but unsaturated conditions, despite being below 0 degrees, may allow for considerable flow until the infiltrating water freezes. Another common conduit for liquid water in these areas are unfrozen zones known as “taliks”, pockets of unfrozen ground which are bookended by permafrost laterally, but can be connected to unfrozen ground above and below (an “open” talik; Figure 1).

Figure 1: The hydrology of permafrost landscapes until (a) the present climate and (b) under a warming climate (from Walvoord and Kurylyk, 2016)

Freshwater export from permafrost catchments can influence the global climate system through the total quantity and quality of water contributed to the Arctic Ocean (Rouse et al., 1997), as these fluxes impact the formation and longevity of sea ice (with implications for arctic albedo) and ecological trophic structure. Increased freshwater and heat flux to the ocean from riverine transport will result in decreased mixing of nutrients and a resulting decline in phytoplankton communities with cascade effects to higher trophic levels (Li et al., 2009).  Changes to discharge from arctic and subarctic catchments are highly variable through space. Work has shown increasing discharge from Eurasian rivers (Peterson et al., 2002), decreasing flow from central North America (Déry et al., 2005), and unchanged flow from northwestern Canada (Déry and Wood, 2005). Coupled climate-hydrological models and long term observations show wide variability in changes to annual discharge (Milly et al., 2005; Aerts et al., 2006; Moore et al., 2002; Figure 2), although seasonal variations between flow regimes show varying response, where autumn to spring discharge will increase and summer flow will decrease (Thorne, 2011).

Figure 2: Annual discharge of rivers in km³ through basins with a high concentration of permafrost (from Arctic Climate Impact Assessment, 2004)

The mechanisms of these changing flow regimes across space and time start leading to many of the open research questions in permafrost hydrology. How do the different modes of permafrost disturbance change a catchment’s ability to generate and transmit runoff? As frost-driven geomorphic structures collapse (a process analogous to karst formation, known as thermokarst) how will the hydrologic function of these new collapse structures differ? As permafrost thaws, what new nutrient and mineral constituents will flow paths pass through, and will this change the chemistry of water being exported through lakes, wetlands, rivers, and oceans? How will these processes interact with changing ground heat flux and the portioning of energy into sensible and latent heat fractions? Although an interesting problem for Canadian hydrologists, this is a global scientific effort which is an exciting and growing element of future hydrological research, with active research basins in Canada, Siberia, Tibet, Antarctica, Northern Europe, the United States, Peru, Patagonia, New Zealand, and many other countries collaborating on exciting new projects.

References
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Citation: Morison, M. (2017), Permafrost hydrology: the urgency for understanding in a thawing world, Streams of Thought (Young Hydrologic Society), Published February 2017.

About the author
Matthew Morison (@MatthewQmar) is a PhD student at University of Waterloo, Ontario, Canada and active member of the Canadian branch of the Young Hydrological Society (@Canadianyhs).