Pol-InSAR and PolSAR based Inversion Modelling for Snow Depth and SWE Estimation in the Northwestern Himalayan Watershed

Abstract

Snow depth (SD) and Snow Water Equivalent (SWE) constitute essential physical properties of snow and find extensive usage in the hydrological modelling domain. However, the prominent impact of the hydrometeorological conditions and difficult terrain conditions inhibit accurate measurement of the SD and SWE— an ongoing research problem in the cryosphere paradigm. In this context, spaceborne synthetic aperture radar (SAR) systems benefit from global coverage at sufficiently high spatial resolutions. The copolar phase difference (CPD) method based on the X-band polarimetric SAR (PolSAR) technique has displayed promising results regarding the fresh snow depth (FSD) estimation in the literature. Still, this FSD inversion model has not been tested in the presence of extreme topographically varying conditions, such as the northwestern Himalayan belt. It is also susceptible to high volume scattering at X-band occurring from the increased snow grain sizes as a result of the standing (or old) snow formation driven by the temperature induced snow metamorphosis process. Hence, to model this volume decorrelation, the polarimetric SAR interferometry (Pol-InSAR) technique can be applied. In this work, the FSD and standing snow depth (SSD) are computed using the ∗Corresponding author Email address: [email protected] ( Sayantan Majumdar ) Preprint submitted to Int J Appl Earth Obs Geoinformation June 23, 2019 PolSAR CPD method and the single-baseline Pol-InSAR based hybrid Digital Elevation Model (DEM) differencing and coherence amplitude inversion model. To achieve these, the TerraSAR-X, TanDEM-X Coregistered Single look Slant range Complex (CoSSC) bistatic acquisition over Dhundi (situated in the Beas watershed, northwestern Himalayas, India) on January 8, 2016, is used. Due to the associated problems of model parameter tuning, complex topographical conditions, and limited ground-truth measurements, appropriate sensitivity analyses have been carried out for the parameter optimisation. Furthermore, the uncertainty sources are identified by performing a summer (June 8, 2017) and wintertime (January 8, 2016) comparative analysis of the study area which quantitatively highlights the changes in the percentages of the surface and volume scatterings. Evidently, the improved models display sufficiently high FSD and SSD accuracies of 94.83% and 99.53% respectively with the corresponding fresh SWE (FSWE) and standing SWE (SSWE) accuracies of 94.84% and 99.48% (measured using a 3×3 window at Dhundi). These results demonstrate the practicability of the PolSAR and Pol-InSAR models in the context of the SD estimation over rugged terrains.