Uncertainty analysis of groundwater basin declines based on global GLDAS and GRACE data: Is the de-limit state really intended to inform target-setting?
The critique indicates that the ESBs never intended to give a sustainable assessment. Across all domains, the esbs are considered in ref. The Earth system’s de-limit states keep the planetary balance and minimize harm to people who come from it. Although the ESBs can help inform target-setting, the groundwater ESB is not intended to provide a target for sustainable groundwater pumping. We used the GRACE data to determine the broad trends in GWS decline; however, the groundwater extraction that may safely occur within this boundary “should be defined based on local-scale monitoring”1 including assessments of capture across a suitable reference period and groundwater pumping hydraulics as noted by Cuthbert et al.
We understand the uncertainties in the auxiliary global model data products used in GRACE and the challenges of spatial resolution. Global models often underestimate water storage changes compared with GRACE15, but theGLDAS product has been widely used to help quantify groundwater change. A review of GRACE analyses found that there were errors in more than one third of the large basins around the world. Although we discussed these issues in the Supplementary Information of ref. The high priority for future work should be a formal uncertainty analysis.
After analyzing 0.25 GRACE data, trend analyses were used across the globe, following the methodologies previously used for country11 and global12 Recent methodological developments include approaches to resolving GRACE solutions that show very strong correlations between GRACE solutions at different scales, including 0.25° (ref. 13).
Where is the boundary of a subglobal groundwater extraterrestrial stock (ESB)? The role of historical groundwater pumping on environmental justice
If a downward trend is observed within a reference period for reasons other than pumping, or the GWS estimation is incorrect, a subglobal ESB could be transgressed.
Historic pumping may have already reached a new dynamic equilibrium before the reference period for which a subglobal ESB is calculated. If so, previous pumping could have been devastating ecosystems, causing subsidence and inducing saline intrusion, and still be ascertained as ‘safe’ according to the proposed definition.
If the reference period is natural and there is an incorrect estimate of upwards trend by the GSS, a subglobal ESP might not be violated despite some pumping occurring.
Second, under the proposed boundary, many already over-abstracted aquifers, from which wealthier nations have already benefited (for example, via irrigated or industrial productivity before the GRACE reference period), may still be within a subglobal groundwater ESB, for instance, if long-term pumping has led to a new equilibrium with overall lower groundwater levels. The paper3 argues that interspecies justice and future intergenerational justice are not met if local GWS declines over time, but without framing the boundary robustly within an appreciation of the groundwater system dynamics, this is potentially increasing environmental injustice instead.