Abstract
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 (ΔGWS) 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 ΔGWS 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 ΔGWS 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 ΔGWS 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 (∼200 000 km2). Substantial uncertainty remains in estimates of GRACE-derived ΔGWS, evident from 20 realisations presented here, but these data provide a regional context to changes in groundwater storage observed more locally through piezometry.
Highlights
Groundwater is estimated to account for between a quarter and a third of the world’s annual freshwater withdrawals to meet agricultural, industrial, and domestic demand (Döll et al, 2012; Wada et al, 2014; Hanasaki et al, 2018)
Gravity Recovery and Climate Experiment (GRACE) TWS in aquifer systems under monsoonal precipitation regimes is strongly correlated with rainfall, with a lag of 2 months (r>0.65, p value
We compute the range of uncertainty in GRACE-derived GWS associated with 20 potential realisations from applied GRACE (CSR, JPL mass concentration blocks (Mascons), GRGS) products and
Summary
Groundwater is estimated to account for between a quarter and a third of the world’s annual freshwater withdrawals to meet agricultural, industrial, and domestic demand (Döll et al, 2012; Wada et al, 2014; Hanasaki et al, 2018). As the world’s largest distributed store of fresh water, groundwater plays a vital role in sustaining ecosystems and enabling adaptation to increased variability in rainfall and river discharge brought about by climate change (Taylor et al, 2013a). Sustained reductions in the volume of groundwater (i.e. groundwater depletion) resulting from human withdrawals or changes in climate have historically been observed as declining groundwater levels recorded in wells (Scanlon et al, 2012a; Castellazzi et al, 2016; MacDonald et al, 2016). The limited distribution and duration of piezometric records hinder, direct observation of changes in groundwater storage globally including many of the world’s large aquifer systems (WHYMAP and Margat, 2008). Since 2002 the Gravity Recovery and Climate Experiment (GRACE) has enabled large-scale (≥ 200 000 km2) satellite monitoring of changes in total terrestrial water storage ( TWS) globally (Tapley et al, 2004).
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