Characterizing hydrological droughts within three watersheds in Yunnan, China from GNSS-inferred terrestrial water storage changes constrained by GRACE data

SUMMARY The spatiotemporal evolution of drought is often modulated by climate and watershed characteristics. While numerous drought studies using space geodesy have been conducted in Yunnan, the scarcity and limited sensitivity of observation instruments have hindered the development of watershed-scale drought analyses. This study aims to accurately characterize hydrological droughts within three watersheds in Yunnan from 2011 January to 2021 May by a Global Navigation Satellite System (GNSS) inversion constrained by Gravity Recovery and Climate Experiment (GRACE) data. Initially, we employed Variational Bayesian Independent Component Analysis to reconstruct the 3-D crustal deformations at 43 GNSS stations resulting from hydrological loadings. We then computed the time-series of vertical displacements caused by GRACE Mascon water products. Subsequently, utilizing the method of least squares, we derived the scaling factors between the vertical crustal displacements (VCD) obtained from GNSS observations and the synthetic displacements derived from GRACE data. By combing scaling-factor-adjusted VCDs derived from GRACE with the GNSS data, we have obtained accurate estimates of water storage for three Yunnan watersheds. Finally, we identified drought events characterized by abnormal decreases in water storage and used climatological methods to quantitatively describe the severity, extent and recovery of these drought extremes. Additionally, we evaluated the influence of various earth elastic structures on the scaling factors, and demonstrated their advantageous contribution to aligning GNSS and GRACE observations. In conclusion, our study introduces a novel approach to integrate GNSS and GRACE retrievals, allowing for accurate characterization of droughts in data-scarce regions, which cannot be achieved by GNSS or GRACE individually. Moreover, our results underscore the potential benefits of watershed-scale drought monitoring and analysis for effective water resource management.