Measuring Seasonal Variations of East Siberian Thermokarst Lake Areas using PALSAR Data

Degradation of permafrost owing to climate warming is expected to have profound impacts on Arctic hydrology and biogeochemical cycling. Thermokarst lakes, or thaw lakes, are major features of Arctic landscapes and are important modulators of biogeochemical activity in permafrost-affected regions. Recent studies of thaw lake change in eastern Siberia using remotely-sensed optical images acquired several decades apart suggest that thaw lakes have increased in number and extent in areas of continuous permafrost. However, such studies are limited by their inability to account for seasonal variations in lake extent associated with summer snowmelt. The objective of this study was to quantify changes in thaw lake area throughout the summer melt season within a ~2500 km² area of the Kolyma River drainage basin in eastern Siberia. The analysis was based on 9 scenes acquired by the Phased Array L-band Synthetic Aperture Radar (PALSAR) instrument during the summers of 2007, 2008 and 2010. The unique backscatter signature of open water in L-band SAR data allows for straightforward delineation of open water. Lakes were classified by applying a histogram threshold to cross-polarized radar backscatter images using a minimum spatial threshold of 0.26 hectares (16 contiguous pixels). The results of this analysis indicate that the total extent of thaw lakes can decrease by as much as ∼25% during the period from one week after freshet in early June to early September. These results suggest that estimates of interannual change in thaw lake extent based on remotely-sensed images may be sensitive to the time of year when the data were captured and that higher temporal resolution image data are needed to draw firm conclusions about trends in thaw lake extent in areas of continuous permafrost.