2:30 PM - 2:45 PM
[ACG43-15] Spatio-temporal heterogeneity of post-fire permafrost thaw process around Batagay detected by L-band InSAR and on-site observation
Keywords:InSAR, ALOS-2, wildfire, permafrost, abrupt thaw, Batagay
Permafrost thaw due to global warming leads to microbial decomposition of soil organic carbon and releases greenhouse gases. Especially, the abrupt permafrost thaw, which is a deep and drastic thaw in burned and deforested areas, could be responsible for even more efficient carbon release than the gradual thaw caused by temperature rising. In addition, global warming will lead to an increase in lightning in the Arctic. As a result, it is also concerned that increases in fires and associated abrupt thaw will further accelerate temperature rise in the future. However, as abrupt thaw occurs locally and ununiformly, it is more challenging to estimate the thaw amount than the case of the uniform gradual thaw. Therefore, the abrupt thaw effect is not fully considered in current earth system models. Accordingly, it is necessary to elucidate the abrupt thaw process and observe the thawing amount globally.
This study reports post-fire permafrost thaw around Batagay, Sakha Republic, Northeastern Siberia. Two wildfires burned 6.8km² areas in 2018 and 2019 adjacent to a retrogressive thaw slump called Batagaika Megaslump, located about 10 km southeast of the village. The melting of ground ice will cause irreversible ground subsidence and can trigger a second mega-slump. Therefore, we applied Interferometric Synthetic Aperture Radar (InSAR) and time series analysis using the SBAS method to analyze the amount of seasonal and annual ground deformation. SAR data are from ALOS-2, JAXA's L-band SAR satellite, and we mainly used the data of SM1 mode, which has higher spatial resolution than our previous studies in Batagay. As a result, we detected spatial heterogeneity of ground deformation signals inside the fire scars. In September 2021, we conducted an on-site observation measuring thaw depth and soil water content to verify the heterogeneity of the signals based on the thaw conditions. In addition, we discussed the causes of the heterogeneity by comparing elevation, slope undulations, and the indexes of vegetation and burn severity from optical images.
In most parts of the 2019 scar, we detected seasonal deformation signals of ~6 cm in the satellite line-of-sight direction, but there was no annual deformation signal despite only two years since the fire. The deformation pattern was spatially different around the gully, and neither seasonal nor annual deformation signals were detected on the gully's flank. We also detected deformation signals on the east and west parts of the 2018 fire scar. The amplitudes of the signals were ~10cm in seasonal and ~8 cm in annual. On the other hand, neither seasonal nor annual deformation signals were detected in the central part of the 2018 scar. We measured thaw depth and soil water content along the transect across the deformed and non-deformed areas. The thaw depth values were ~150cm and spatially uniform, while the values of soil water content tended to decrease by ~20% in the non-deformed areas. The soil water distribution did not correlate with vegetation or burn severity but instead with gully topography and slope undulations of about 2~3m. The increasing seasonal subsidence signal seems to correspond to the thaw depth when roughly comparing a burned area with an unburned area. However, the uniform thaw depth values could not explain the spatial heterogeneity within the fire scar. Instead, the heterogeneity correlated with the soil water distribution corresponding to the topography. This correlation qualitatively indicates that seasonal deformation depends on the amount of ice lens formation. Furthermore, it suggests that a simple algorithm for estimating a thaw depth from a seasonal subsidence value would be inappropriate.
This study reports post-fire permafrost thaw around Batagay, Sakha Republic, Northeastern Siberia. Two wildfires burned 6.8km² areas in 2018 and 2019 adjacent to a retrogressive thaw slump called Batagaika Megaslump, located about 10 km southeast of the village. The melting of ground ice will cause irreversible ground subsidence and can trigger a second mega-slump. Therefore, we applied Interferometric Synthetic Aperture Radar (InSAR) and time series analysis using the SBAS method to analyze the amount of seasonal and annual ground deformation. SAR data are from ALOS-2, JAXA's L-band SAR satellite, and we mainly used the data of SM1 mode, which has higher spatial resolution than our previous studies in Batagay. As a result, we detected spatial heterogeneity of ground deformation signals inside the fire scars. In September 2021, we conducted an on-site observation measuring thaw depth and soil water content to verify the heterogeneity of the signals based on the thaw conditions. In addition, we discussed the causes of the heterogeneity by comparing elevation, slope undulations, and the indexes of vegetation and burn severity from optical images.
In most parts of the 2019 scar, we detected seasonal deformation signals of ~6 cm in the satellite line-of-sight direction, but there was no annual deformation signal despite only two years since the fire. The deformation pattern was spatially different around the gully, and neither seasonal nor annual deformation signals were detected on the gully's flank. We also detected deformation signals on the east and west parts of the 2018 fire scar. The amplitudes of the signals were ~10cm in seasonal and ~8 cm in annual. On the other hand, neither seasonal nor annual deformation signals were detected in the central part of the 2018 scar. We measured thaw depth and soil water content along the transect across the deformed and non-deformed areas. The thaw depth values were ~150cm and spatially uniform, while the values of soil water content tended to decrease by ~20% in the non-deformed areas. The soil water distribution did not correlate with vegetation or burn severity but instead with gully topography and slope undulations of about 2~3m. The increasing seasonal subsidence signal seems to correspond to the thaw depth when roughly comparing a burned area with an unburned area. However, the uniform thaw depth values could not explain the spatial heterogeneity within the fire scar. Instead, the heterogeneity correlated with the soil water distribution corresponding to the topography. This correlation qualitatively indicates that seasonal deformation depends on the amount of ice lens formation. Furthermore, it suggests that a simple algorithm for estimating a thaw depth from a seasonal subsidence value would be inappropriate.