Groundwater storage dynamics in the world's large aquifer systems from GRACE: Uncertainty and role of extreme precipitation

Abstract

Under variable and changing climates groundwater storage sustains vital ecosystems and enables freshwater withdrawals globally for agriculture, drinking water, and industry. Here, we assess recent changes in groundwater storage (1GWS) from 2002 to 2016 in 37 of the world-s large aquifer systems using an ensemble of datasets from the Gravity Recovery and Climate Experiment (GRACE) and land surface models (LSMs). Ensemble GRACE-derived 1GWS is well reconciled to in situ observations (r D 0:62-0.86, p value<0:001) for two tropical basins with regional piezometric networks and contrasting climate regimes. Trends in GRACE-derived 1GWS are overwhelmingly non-linear; indeed, linear declining trends adequately (R2>0:5, p value<0:001) explain variability in only two aquifer systems. Non-linearity in 1GWS derives, in part, from the episodic nature of groundwater replenishment associated with extreme annual (>90th percentile, 1901-2016) precipitation and is inconsistent with prevailing narratives of global-scale groundwater depletion at the scale of the GRACE footprint (200000 km2). Substantial uncertainty remains in estimates of GRACE-derived 1GWS, evident from 20 realisations presented here, but these data provide a regional context to changes in groundwater storage observed more locally through piezometry.