Mark Olaf Cuthbert , Tom Gleeson, Grant Ferguson, Marc Bierkens, Richard Taylor

Groundwater systems are commonly, but variously, defined as renewable or non-renewable based on natural fluxes of recharge or on estimates of aquifer storage and groundwater residence time. However, we show here that the principle of capture challenges simple definitions so that a groundwater system cannot be renewable or non-renewable in and of itself, but only with reference to how the groundwater is being used. We develop and propose more hydraulically-informed definitions for flux-renewable and storage-renewable groundwater use, and a combined definition that encompasses both the flux-based and storage-based perspectives such that: renewable groundwater use allows for dynamically stable re-equilibrium of groundwater levels and quality on human timescales. Further, we show how a matrix of combinations of (1) the ratio of pumping to possible capture along with (2) the response or recovery timescales implicit in this definition, leads to a useful four-quadrant framework for characterising groundwater use, illustrated using case studies from aquifers around the world. Renewable groundwater use may inform pathways to groundwater sustainability, which encompasses a broader set of dimensions (e.g. socio-political, economic, ecological and cultural) beyond the scope of groundwater science. We propose that separating physically robust definitions of renewable groundwater use from the inherently value-based, normative language of sustainability, can help bring much needed clarity to wider discussions about sustainable groundwater management strategies, and the role of groundwater science and scientists in such endeavours.