An optimized hydrological drought index integrating GNSS displacement and satellite gravimetry data

Abstract

Space geodetic techniques are rapidly emerging as powerful tools for drought monitoring, including the Global Navigation Satellite System (GNSS) positioning data, and the gravimetric data from Gravity Recovery And Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellite missions. However, while GNSS-based 3D displacements have high temporal resolution, they are impacted by diverse local effects especially for tectonically active regions. Satellite gravimetry derived terrestrial water storage changes (ΔTWS) are continuous in temporal sampling, however, at much coarser spatial resolutions, and there are systematic noise and a large 11-month data gap between GRACE and GRACE-FO observations. Here for the first time, we developed an Optimized GNSS/GRACE/GRACE-FO Hydrological Drought Index (OGHDI), by optimally combining two drought indices derived from GNSS vertical displacements (VDs) and GRACE/GRACE-FO ΔTWS, to better characterize decadal evolution of droughts over Southwest China, 2011–2020. The optimal relative weights of the two drought indices are determined through maximizing the correlations with the Composite Index (CI) commonly used in the study region. Then OGHDI is compared to the self-calibrating Palmer Drought Severity Index (scPDSI). The results show that VDs agree well with ΔTWS at most of the 30 analyzed GNSS sites, with a mean correlation of −0.44. Compared to the drought index developed using only GNSS data, the optimized technique improved the OGHDI-scPDSI correlations at 90% (27 sites) of the sites, with the highest correlation reaches 0.73. Further, more significant improvement is found at the regional scale than at individual sites, and the regional mean OGHDI-CI and OGHDI-scPDSI correlations increase from 0.37 to 0.57 and from 0.42 to 0.68, respectively. All drought indices show that Southwest China experienced two sustained and excessive drought periods from 2011 to mid-2013, and after 2019. We conclude that the novel integration of two distinct contemporary and complemenatry geodetic datasets, GNSS and GRACE/GRACE-FO, improved the ability of drought monitoring in Southwest China.