Unraveling the non-linear relationship between surface deformation and active layer thickness in the Qinghai-Tibetan permafrost region

Abstract

Seasonal freezing and thawing of the active layer in permafrost regions generally induce surface deformation that interferometric synthetic aperture radar (InSAR) can monitor on regional scales. InSAR has been widely used for detecting changes in active-layer thickness (ALT), an indicator of permafrost thaw. Previous studies have shown that the depth of the active layer and its soil water content have a strong influence on the magnitude of the surface deformation, and a linear relationship between ALT and surface deformation is often assumed in poorly drained Arctic soils. However, in dry areas such as the Tibetan Plateau permafrost region, the relationship between ALT and deformation is more complex and challenging to elucidate.   To examine the relationship in the Qinghai-Tibetan permafrost region, this study synthesizes InSAR-derived surface deformation data, multispectral measurements from unmanned aerial vehicle (UAV) sensors, and in-situ soil temperature and moisture data along a ~930-km transect. Our analyses reveal that seasonal deformation generally increases with ALT (R=0.74, p=0.26) at sites with moderate to dense vegetation cover (summer NDVI>0.5), aligning with previous research. However, at sites with sparse vegetation (summer NDVI<0.5), a strong negative correlation was found between the seasonal deformation and ALT (R=-0.83, p=0.01). Those areas are generally associated with deep active layers and low surface soil moisture. Among all sites, seasonal deformation shows a stronger correlation with the soil moisture content of the lower portion of the active layer (R=0.77, p=0.006), but a weak correlation with either surface or profile soil water content (R<0.20，p>0.60). This study provides insights into the non-linear relationship between deformation and ALT in arid and semi-arid permafrost regions such as the Tibetan Plateau, allowing for accurate active-layer estimates and soil water dynamics in this region.