How a dry year affects spatial variability of ground thaw and changes the hydrology of a small Arctic watershed

Abstract

The summer of 2021 in the Inuvik area, NWT was warm and dry. As recorded in Siksik Creek, a sub-catchment of Trail Valley Creek located 50 km north-east of Inuvik, this was the 7th warmest summer and driest July recorded to date. This presented a unique opportunity to study the drying phenomena of Arctic ecosystems. This is pertinent to the study of permafrost degradation, as the drying phenomena is still vastly understudied and there are few datasets available that record abnormally dry conditions in Arctic catchments. These data sets are needed to properly show the influence that this has on active layer thicknesses. It is unknown whether these conditions may pose a risk to permafrost, if this is spatially variable, and what other processes might amplify or hinder this. The main objective of this thesis is to explore how a dry year affects active layer thaw and the hydrology of Siksik Creek so that we may better understand how catchments such as Siksik will respond to ongoing climate change. To do this a mix of field results and modelling was used to show and quantify how these may affect active layer thaw as well as water balance components. The three main research chapters of this thesis divide this by analyzing active layer thaw as physically measured in the catchment to previous years, by using the model GEOtop to assess how this affects water balance components, and then by simulating wetter conditions to show the affect soil moisture has. Field data were collected from May 25th to August 29th in Siksik Creek during the summer of 2021, where the data collected included active layer thicknesses, depth to the water table, as well as stratigraphy and soil thicknesses across a variety of terrain type throughout the entirety of the catchment. This study specifically focused on measuring these data across hummocks and inter-hummocks throughout the catchment, as these features are ubiquitous in the Mackenzie uplands. In addition to analyzing the 2021 field data, the GEOtop physically based hydrology model was used to explore the processes controlling active layer thicknesses, water table depths, and various water balance components over the course of the summer of 2021. GEOtop is designed to handle microtopographies, such as hummocks and other terrain features. Further, we compare the physical and simulated results from the summer of 2021 to a more normal and wetter year (2016) to assess the differences that soil moisture has on the hydrology and active layer thaw of Siksik Creek. We explored how the movement of water impacted thaw depths in these landscapes, how spatial variability of thaw is influenced by soil moisture, and by the terrain features that control this. We found that peat thicknesses in this area are controlled by the presence of hummocks, where peat is thickest between hummock mounds in an area called the inter-hummock zone. This variability of peat thicknesses directly controls the spatial variability of soil moisture, and this had implications on thaw. In Chapter Two it was found that thaw for the summer of 2021 was shallower than expected in the inter-hummock zones by as much as 20cm compared to similar studies in Siksik and in similar landscapes in Alaska. This chapter also showed that the overlying vegetation, specifically lichens and mosses, were statistically linked to peat thicknesses representative of hummocks and inter-hummocks - where lichens tended to be overtop of hummocks, and mosses overtop of inter-hummocks. This correlation was then used with UAV imagery, taken in mid-June, to map mosses and lichens across the catchment and by proxy the locations of hummocks and inter-hummocks. This map was built into the model GEOtop to simulate Siksik Creek for the summer of 2021. Starting in Chapter Three, the modelled portion of this thesis covered a wide aspect of simulations, where the influence of hummocks was assessed, as well as shrubs and snow when they were added to the model, and finally assessing the role soil moisture plays within all of these various processes. It was found that hummocks, when they were specifically discretized and compared to a simple soil column, reached freshet and max discharge sooner by as much as two days, a lag that existed in the evapotranspiration outputs as well. However, these results were relatively consistent and the only major difference between these two simulations was seen in the 2d active layer depth maps. It was found that microtopography by the end of the summer seemed to influence local patterns of thaw more than larger topographical features such as natural depressions in the landscape did. When shrubs and snow were added to the model domain and simulated it was found that the presence of snow or lack thereof was the main component of difference in the discharge and evapotranspiration data. The active layer depth maps changed between simulations, with the shrub only simulations having the lowest degree of thaw, with a degree of variability seen between simulations. In comparison, the water table depths hardly changed between simulations, and it was hypothesized that the dryness of the summer and the lack of soil moisture was the main culprit for this. To test if this was the case, in Chapter Four, precipitation and snow water-equivalent data was taken from a wetter year (2016) and replaced the values for the dry summer of 2021. This was done so that only moisture available to the system was changed. It was found that soil moisture was in fact the main cause for this lack of variability. The wetter simulations because of this had deeper thaw throughout the catchment, with both the extent, max thaw depths, and min thaw depths increasing. The water table depths on the other hand became shallower and the ground surface was much more inundated with water. The spatial variability in the water table depth maps was found to match where the presence of taller shrubs in the catchment exist, where these areas had the least amount of soil moisture and the shallowest thaw depths. Whereas the areas with the highest amount of soil moisture content had the deepest thaw depths in the catchment. Overall, this thesis helps to improve our understanding of how peat catchments similar to Siksik Creek might respond to either the wetting or drying of the Arctic. This thesis also advances our understanding on the controls of soil moisture variability and ground thaw, as well as its spatial variability. One can infer from this study that for posterity it is the warm and wet summers rather than the warm and dry summers that pose the largest risk to permafrost and its degradation.