Abstract
Active rock glaciers (ARGs) are important permafrost landforms in alpine regions. Identifying ARGs has mainly relied on visual interpretation of their geomorphic characteristics with optical remote sensing images, while mapping ARGs from their kinematic features has also become popular in recent years. However, a thorough comparison of geomorphic- and kinematic-based inventories of ARGs has not been carried out. In this study, we employed a multi-temporal interferometric synthetic aperture radar (InSAR) technique to derive the mean annual surface displacement velocity over the Daxue Shan, Southeast Tibet Plateau. We then compiled a rock glacier inventory by synergistically interpreting the InSAR-derived surface displacements and geomorphic features based on Google Earth images. Our InSAR-assist kinematic-based inventory (KBI) was further compared with a pre-existing geomorphic-based inventory (GBI) of rock glaciers in Daxue Shan. The results show that our InSAR-assist inventory consists of 344 ARGs, 36% (i.e., 125) more than that derived from the geomorphic-based method (i.e., 251). Only 32 ARGs in the GBI are not included in the KBI. Among the 219 ARGs detected by both approaches, the ones with area differences of more than 20% account for about 32% (i.e., 70 ARGs). The mean downslope velocities of ARGs calculated from InSAR are between 2.8 and 107.4 mm∙a−1. Our comparative analyses show that ARGs mapping from the InSAR-based kinematic approach is more efficient and accurate than the geomorphic-based approach. Nonetheless, the completeness of the InSAR-assist KBI is affected by the SAR data acquisition time, signal decorrelation, geometric distortion of SAR images, and the sensitivity of the InSAR measurement to ground deformation. We suggest that the kinematic-based approach should be utilized in future ARGs-based studies such as regional permafrost distribution assessment and water storage estimates.
Highlights
The gray dots represent the relict rock glaciers inventoried from the from the geomorphic-based approach. (c) The outlines of Active rock glaciers (ARGs) from kinematic-based inventory (KBI) shown on the mean annual surface velocity geomorphic-based approach. (c) The outlines of ARGs from KBI shown on the mean annual surface velocity map derived map derived from multi-temporal interferometric synthetic aperture radar (InSAR)
Results were summarized into four cases: ARGs sharing the common boundary in KBI
Considering the relative subjectivity in mapping rock glacier outlines, the so-called common boundary of an ARG means that the polygons from two inventories have the same geomorphological structure, and the area difference is less than 20%
Summary
KBI generally works by quantifying ground surface motion of alpine regions using geodetic techniques, from which active rock glaciers (ARGs), including the transitional ones, are detected [9] Geodetic techniques such as cross-correlation of optical images, interferometric synthetic aperture radar (InSAR), and differential laser scanning have been used to detect and map ARGs [6,26,27]. Current studies of mapping ARGs based on the InSAR-assisted approach mostly use one or a limited number of SAR image pairs, which may generate incorrect identification of some ARGs [32] This is mainly because the surface displacement rate detected from a SAR interferogram is confined to a certain range. With a velocity range from 0.88 to 5.26 cm·a−1 , demonstrating the strength of multi-temporal InSAR for ARG mapping Both kinematic- and geomorphic-based rock glacier inventories have their effectiveness. We discuss the possible influences of different approaches used to inventory rock glaciers on future rock glacier-based studies such as permafrost distribution assessment and water storage estimates
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