GNSS Observations of the Land Uplift in South Africa: Implications for Water Mass Loss

Continuously operating Global Navigation Satellite System (GNSS) base stations in South Africa show a spatially coherent vertical displacement. While one hypothesis attributes this vertical motion to crustal deformation from mantle flow and dynamic topography (Hammond et al., 2021, https://doi.org/10.1029/2021jb022355), we propose an alternative explanation. Our evidence suggests that land water loss from multiple droughts is a major driver of the observed uplift. In this study, we analyze daily Global Positioning System (GPS) height time series from 2000 to 2021. We use singular spectral analysis (SSA) to separate long-term trends and annual and semi-annual signals from noise. The processed time series were inverted into water mass loading on a uniform grid, with the Earth's crust's rheological properties defined by the Preliminary Reference Earth Model (PREM). Our experimental approach show that a 2$°\times$ 2$°$ grid resolution provides suitable results for most of South Africa. The GPS-derived total water storage change reconcile well with a GRACE-assimilated solution and a hydrological model at the monthly scale across different provinces, showing correlations of up to 90% and 94%, respectively. The long-term trend averaged over the country shows a considerable correlation of 46% and 53%, respectively. These long-term total water storage trends provide strong evidence that the observed land uplift in South Africa is primarily of hydrological origin.