Global groundwater sustainability puzzles: a coupled human–groundwater systems approach to identify distinct management challenges, strengthen solution networks, bridge global and local scales, and enable pathways to sustainability

Global groundwater sustainability is a grand challenge that requires diverse approaches to account for local contexts. Yet, global groundwater assessments often focus solely on aggregate physical trends in storage, levels, and fluxes, overlooking the diversity of social and ecological functions provided by groundwater and their associated sustainability challenges. Here, we introduce groundwater sustainability puzzles as a concept and approach to identify distinct configurations of human-groundwater system sustainability challenges within heterogeneous landscapes. We apply this new concept and synthesize 17 global datasets on groundwater functions in social-ecological systems (e.g., groundwater-dependent ecosystems, irrigation, climate coupling, etc.) and groundwater system management problems (e.g., depletion, land subsidence, land use change, gender inequality, etc.) through a leading high-dimensional spatial data classification methodology that implements self-organising maps. This data-driven synthesis represents the most comprehensive integration of current data relevant to the global challenge of groundwater sustainability, and generates a refined portrait of the multi-dimensional composition, spatial organization, heterogeneity, and variance of groundwater sustainability challenges worldwide.In total, we identify and map over 200 groundwater sustainability puzzles worldwide. Each puzzle represents a unique configuration of system functions and management problems, corresponding to a specific setting in which sustainability transformations must take root. Notably, half of global land area, population, and crop production situate within fewer than 20 puzzles respectively, indicating a more tractable problem space than the mapping might initially suggest. Groundwater sustainability puzzles help to articulate a coherent system-of-systems problem statement for global groundwater sustainability, bridge the narratives and patterns of global groundwater analyses with local groundwater realities, and strengthen solution networks among researchers and practitioners working in similar contexts. More broadly, the approach offers a new tool for socio-hydrological and hydro-social research to compare systems across regions, facilitate cross-regional learning and network formation, and support context-appropriate pathways to sustainability across freshwater systems.