Impact of lateral groundwater flow on hydrothermal conditions of the active layer in a high-Arctic hillslope setting

Alexandra Hamm,Andrew Frampton

doi:10.5194/tc-15-4853-2021

Alexandra Hamm, Andrew Frampton

Open Access

https://doi.org/10.5194/tc-15-4853-2021

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Abstract

Abstract. Modeling the physical state of permafrost landscapes is a crucial addition to field observations in order to understand the feedback mechanisms between permafrost and the atmosphere within a warming climate. A common hypothesis in permafrost modeling is that vertical heat conduction is most relevant to derive subsurface temperatures. While this approach is mostly applicable to flat landscapes with little topography, landscapes with more topography are subject to lateral flow processes as well. With our study, we contribute to the growing body of evidence that lateral surface and subsurface processes can have a significant impact on permafrost temperatures and active layer properties. We use a numerical model to simulate two idealized hillslopes (a steep and a medium case) with inclinations that can be found in Adventdalen, Svalbard, and compare them to a flat control case. We find that ground temperatures within the active layer uphill are generally warmer than downhill in both slopes (with a difference of up to ∼0.8 ∘C in the steep and ∼0.6 ∘C in the medium slope). Further, the slopes are found to be warmer in the uphill section and colder in the base of the slopes compared to the flat control case. As a result, maximum thaw depth increases by about 5 cm from the flat (0.98 m) to the medium (1.03 m) and the steep slope (1.03 m). Uphill warming on the slopes is explained by overall lower heat capacity, additional energy gain through infiltration, and lower evaporation rates due to drier conditions caused by subsurface runoff. The major governing process causing the cooling on the downslope side is heat loss to the atmosphere through evaporation in summer and enhanced heat loss in winter due to wetter conditions and resulting increased thermal conductivity. On a catchment scale, these results suggest that temperature distributions in sloped terrain can vary considerably compared to flat terrain, which might impact the response of subsurface hydrothermal conditions to ongoing climate change.

Highlights

Permafrost is defined as ground that remains below 0 ◦C for at least 2 consecutive years
We investigate the role of hydrology on two idealized, 50 m long, high-Arctic hillslopes and its effects on the active layer and ground temperatures, using a two-dimensional physically based numerical model
There is variability in these temperature differences over the year, with most pronounced differences occurring during the warm season, typically including a peak just after the thaw period and another peak after freeze-up, indicating the greatest differences occurring during these times

Summary

Introduction

Permafrost is defined as ground that remains below 0 ◦C for at least 2 consecutive years It covers approximately 24 % of the exposed land area in the Northern Hemisphere (Zhang et al, 1999) and stores about 1030 Pg of organic carbon in the upper 3 m of soil (Hugelius et al, 2014). How much carbon gets released from the permafrost is strongly influenced by the depth of the active layer, the part of the soil that seasonally thaws out (e.g., Biskaborn et al, 2019). The correlation between increasing air temperature and depth of the active layer is well established (e.g., Zhang et al, 1997; Isaksen et al, 2007; Frauenfeld et al, 2004).

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