Abstract
Thermokarst waterbodies caused by permafrost thawing and degradation are ubiquitous in many subarctic and Arctic regions. They are globally important components of the biogeochemical carbon cycle and have potential feedback effects on climate. These northern waters are mostly small lakes and ponds, and although they may be mapped using very high-resolution satellites or aerial photography, these approaches are generally not suitable for monitoring purposes, due to the cost and limited availability of such images. In this study we evaluated the potential use of widely available high-resolution imagery from Sentinel-2 (S2) for the characterization of the spectral reflectance of thermokarst lakes and ponds. Specifically, we aimed to define the minimum lake area that could be reliably imaged, and to identify challenges and solutions for remote sensing of such waters in the future. The study was conducted in subarctic Canada, in the vicinity of Whapmagoostui-Kuujjuarapik (Nunavik, Québec), an area in the sporadic permafrost zone with numerous thermokarst waterbodies that vary greatly in size. Ground truthing lake reflectance data were collected using an Unmanned Aerial System (UAS) fitted with a multispectral camera that collected images at 13 cm resolution. The results were compared with reflectance from Sentinel-2 images, and the effect of lake area on the reflectance response was assessed. Our results show that Sentinel-2 imagery was suitable for waterbodies larger than 350 m2 once their boundaries were defined, which in the two test sites would allow monitoring from 11% to 30% of the waterbodies and 73% to 85% of the total lake area. Challenges for remote sensing of small lakes include the confounding effects of water reflection (both direct radiation and diffuse), wind and shadow. Given the small threshold area and frequent revisit time, Sentinel-2 provides a valuable approach towards the continuous monitoring of waterbodies, including ponds and small lakes such as those found in thermokarst landscapes. UASs provide a complementary approach for ground truthing and boundary definition.
Highlights
Climate warming and associated changes in the circumpolar north are causing accelerated permafrost thaw and mobilization of organic carbon pools that have accumulated over thousands of years [1]
Reflectance of small lakes and ponds obtained from satellite imagery, especially close to the lake margins, show a spectral mixing effect due to the presence of vegetation outside the lake, and due to shadow effects from trees and tall shrubs that project into the water body surface
The detailed comparison of water reflectance from 109 thermokarst lakes and ponds obtained from a Unmanned Aerial System (UAS) survey with Sentinel-2 data allowed the determination of a lower boundary for monitoring waterbodies using remote sensing imagery
Summary
Climate warming and associated changes in the circumpolar north are causing accelerated permafrost thaw and mobilization of organic carbon pools that have accumulated over thousands of years [1]. The degradation of ice-rich permafrost results in subsidence of local terrain and the formation of thermokarst lakes and ponds [2]. Global warming may cause the eventual disappearance of thermokarst waterbodies in the subarctic and the formation of new ones in the Arctic as the permafrost boundary continues to migrate northwards. These changes will cause modifications in GHG fluxes, and there is growing interest in the analysis of lake and pond dynamics in thermokarst landscapes [5,7,9]. The thawing landscapes are experiencing an increase in shrub and tree communities, together with peat accumulation [4,6,13,14], but this increasing terrestrial and wetland productivity may be offset by the GHG emissions from thermokarst lakes [15]
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