Modeling Wildfire-Induced Permafrost Deformation in an Alaskan Boreal Forest Using InSAR Observations

Abstract

Abstract:The discontinuous permafrost zone is one of the world's most sensitive areas to climate change. Alaskan boreal forest is underlain by discontinuous permafrost, and wildfires are one of the most influential agents negatively impacting the condition of permafrost in the arctic region. Using interferometric synthetic aperture radar (InSAR) of Advanced Land Observation Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) images, we mapped extensive permafrost degradation over interior Alaskan boreal forest in Yukon Flats, induced by the 2009 Big Creek wildfire. Our analyses showed that fire-induced permafrost degradation in the second post-fire thawing season contributed up to 20 cm of ground surface subsidence. We generated post-fire deformation time series and introduced a model that exploited the deformation time series to estimate fire-induced permafrost degradation and changes in active layer thickness. The model showed a wildfire-induced increase of up to 80 cm in active layer thickness in the second post-fire year due to pore-ice permafrost thawing. The model also showed up to 15 cm of permafrost degradation due to excess-ice thawing with little or no increase in active layer thickness. The uncertainties of the estimated change in active layer thickness and the thickness of thawed excess ice permafrost are 27.77 and 1.50 cm, respectively. Our results demonstrate that InSAR-derived deformation measurements along with physics models are capable of quantifying fire-induced permafrost degradation in Alaskan boreal forests underlain by discontinuous permafrost. Our results also have illustrated that fire-induced increase of active layer thickness and excess ice thawing contributed to ground surface subsidence.

Post‐Wildfire Surface Deformation Near Batagay, Eastern Siberia, Detected by L‐Band and C‐Band InSAR

“…While ALOS2 has only imaged the area since 2014 its data acquisition interval is much longer than that of Sentinel-1 ( Figure 2). Previous studies demonstrate that it is not possible to infer the total subsidence using pre-and post-wildfire SAR images, as the drastic changes in land cover cause low interferometric coherence (Liu et al, 2014;Molan et al, 2018). Additionally, JAXA changed the carrier frequency of PALSAR-2 in June 2015 ( Figure 2).…”

“…Journal of Geophysical Research: Earth Surface thaw-related landforms in permafrost areas (e.g., Antonova et al, 2018;Chen, Günther et al, 2018;Liu et al, 2014Liu et al, , 2015Liu et al, , 2010Molan et al, 2018;Short et al, 2011;Strozzi et al, 2018). Although subsidence signals as a result of thermokarst associated with Alaskan wildfires have been detected using InSAR (Iwahana, Harada, et al, 2016;Liu et al, 2014;Michaelides et al, 2019;Molan et al, 2018), no such studies have been conducted on Siberian fires, to our knowledge. Also, all previous InSAR-based post-wildfire deformation mapping has been performed over relatively flat terrains, but no reports over hillslopes have been shown.…”

“…Our second objective was to estimate the cumulative spatial distribution of subsidence, which allows us to estimate the thawed ice volume. Surface deformation signals over permafrost areas have been interpreted as being caused by two major processes: (1) irreversible subsidence due to thawing of ice-rich permafrost or excess ice and (2) seasonally cyclic subsidence and uplift (Liu et al, 2014(Liu et al, , 2015Molan et al, 2018). In these previous reports, however, quality interferograms (InSAR images) were limited in terms of both the temporal coverage and resolution.…”

“…For instance, Liu et al (2015) assumed a simple linear subsidence trend in their inversion, probably because of the limitation in temporal coverage. Moreover, the 1.5-year temporal coverage in Molan et al (2018) would be not long enough to resolve the detailed temporal evolution. Hence, the total thawed ice volume estimates were uncertain.…”

“…Post-wildfire deformation over a permafrost area presumably consists of two contributions: (1) irreversible subsidence due to melting of ice-rich permafrost below the active layer and (2) seasonally cyclic subsidence and uplift due to freeze-thaw of the active layer (Liu et al, 2014(Liu et al, , 2015Molan et al, 2018). In order to separate the two processes from the observed deformation data, Liu et al (2014) used independent ground-measured ALT data to predict the ALT contribution to total subsidence.…”