Near-Surface Drying of a Continuous Permafrost Hillslope With Water Tracks Following Ground Collapse

Abstract

Increasing air temperatures in the Arctic cause permafrost to thaw, releasing carbon dioxide and methane into the atmosphere. Carbon in thawing permafrost is released approximately three times more readily when soils are unsaturated versus saturated. Therefore, understanding if the Arctic is wetting or drying as permafrost thaws is crucial to predicting soil carbon emissions. In upland permafrost regions, near-surface soil moisture is influenced by unchannelized curvilinear zones of enhanced saturation known as water tracks. The ground underneath water tracks can collapse into thermoerosional gullies, altering their thaw depth and seasonal saturation. Water tracks and thermoerosional gullies frequently occur together on upland hillslopes but exhibit heterogeneous saturation dynamics. Thus, understanding saturation states in water tracks and gullies is crucial to predicting soil carbon emissions. In this study, we quantify saturation across water tracks and a gully and examine changes in near-surface saturation metrics over time by leveraging ~30 years of meteorological data and remotely sensed wetness indices from Landsat (1994–2023) and PlanetScope (2017–2023) imagery for a permafrost hillslope on the North Slope of Alaska, USA. Results suggest that the studied water tracks are drying following the ground collapse event, decreasing the overall saturated area proximal to the collapse, but that the water tracks still have relatively high mean Normalised Difference Water Index (NDWI) values for all rainfall magnitudes. Given the importance of soil saturation for predicting carbon emissions, the results of this work may provide tools for improving estimates of carbon release from thawing continuous permafrost hillslopes.