InSAR Estimates of Active Layer Thickness in the Entire Continuous Permafrost Regions of Alaska

Abstract:

Monitoring active layer thickness (ALT) provides a crucial indicator of how the permafrost system changes over time. Using synthetic aperture radar interferometry (InSAR), ground subsidence pertaining to the active layers' seasonal thaw can be detected over large areas. Measuring such subtle changes of the ground subsidence, when translated into the extent of thaw depth, can reflect on the heterogenous distribution of ALT across regions with different land covers. We processed ascending and descending Sentinel 1A/B InSAR time series over the entire North Slope of Alaska between 2017 and 2020 using the ArcticDEM. Coherent interferograms are observed for the pairs within and between years for the months from June to October. The study area covers three major ecoregions of coastal plains, foothills region, and the Brooks Range area with predominant land covers of wet sedges, tussock tundra, and dwarf shrub dryas, respectively. The estimated inter-annual subsidence rate – a measure of year-to-year loss of permafrost due to melting – ranged between 1 to 6 mm/year across the coastal plains. We estimated the maximum seasonal thaw depth at 1-4 cm, considering accumulated degree days for thawing. Then using a uniform mixed-soil (organic and mineral contents) water content model, we derived the ALT and its uncertainties. Our ALT estimates ranged between 20 to 75 cm across the coastal plains with lower values on sites with predominantly poorly drained tussock tundra while higher values on sites with mesic sedge- dwarf shrub. Due to simple assumptions made on the soil moisture profile across depth and the type of land cover, our ALT estimates deviate slightly (less than 10 cm) from the in-situ measurements. However, large-scale ALT estimates with a spatial resolution of about 30 m can indicate areas that are vulnerable to abrupt thaws.