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

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 (Delta GWS) 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 Delta GWS is well reconciled to in situ observations (r = 0.62-0.86, p value <0.001) for two tropical basins with regional piezometric networks and contrasting climate regimes. Trends in GRACE-derived Delta GWS are overwhelmingly non-linear; indeed, linear declining trends adequately (R-2 >0.5, p value <0.001) explain variability in only two aquifer systems. Non-linearity in Delta GWS 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 (similar to 200 000 km(2)). Substantial uncertainty remains in estimates of GRACE-derived Delta GWS, evident from 20 realisations presented here, but these data provide a regional context to changes in groundwater storage observed more locally through piezometry.