Snow Water Equivalent Retrieval and Analysis Over Altay Using 12-Day Repeat-Pass Sentinel-1 Interferometry

Abstract. Accurate Snow Water Equivalent (SWE) estimation is significant for understanding global climate change, surface energy balance, and regional water cycles. However, although there have been many studies on the inversion of SWE using active and passive microwave remote sensing, it remains challenging to assess the global distribution of SWE with sufficient temporal and spatial resolution and accuracy. Interferometric Synthetic Aperture Radar (InSAR) has become a promising technique for SWE change estimation, which is limited by the optimal radar frequencies and revisit intervals that have not been available until recently. In this study, 12-day Sentinel-1 C-band InSAR data from 2019 to 2021 are used to retrieve ΔSWE (SWE changes in one InSAR pair) and cumulative SWE in the Altay region of Xinjiang, China. The correlation between the retrieved ΔSWE and in-situ observations reaches R=0.48, with a low RMSE of 15.5 mm (n=241) throughout the two whole snow seasons, improving to R=0.47 and RMSE of 15.9 mm for 2019–2020, and R=0.51 and RMSE of 14.8 mm for 2020–2021. These results are achieved without filtering for low coherence or high temperatures. Heavy snowfall leads to decorrelation and phase unwrapping errors, which affect ΔSWE retrieval and are propagated into cumulative SWE. Validation of the cumulative SWE after removing wet snow yields an RMSE of 36.5 mm, which improves to 28.4 mm when high-elevation stations with unwrapping errors due to heavy snowfall are also excluded. InSAR-derived cumulative SWE time series show consistency with ground observations at some stations, though slight overestimations and underestimations are observed due to error accumulation. Various factors combined with validation results show that higher coherence, lower air temperature, and reliable snow density improve the retrieval accuracy. The proposed phase calibration method demonstrates that selecting at least half of the available in-situ ΔSWE values for calibration yields reliable ΔSWE estimates. Calibrating only the integer multiples of 2π provides reasonable accuracy, but is still inferior to the full calibration method, indicating that residual modulo 2π phase has a noticeable contribution to the final inversion accuracy, which highlights that phase calibration plays a key role in the accurate ΔSWE retrieval. This study provides a valuable reference and processing prototype for applying 12-day revisit Sentinel-1 and future NISAR InSAR data to SWE monitoring.