In this paper the authors develop a cross-scale approach by which the water planetary boundary could guide sustainable water management and governance at sub-global contexts defined by physical features (e.g. watershed or aquifer), political borders (e.g. city, nation, or group of nations), or commercial entities (e.g. corporation, trade group, or financial institution).

The application of the planetary boundary for water in these contexts occurs via two approaches: first, calculating fair shares, in which local water cycle modifications are compared to that context’s allocation of the global safe operating space, taking into account biophysical, socioeconomic, and ethical considerations; and second, defining a local safe operating space, in which interactions between water stores and Earth System components are used to define boundaries required for sustaining the local water system in stable conditions. The authors demonstrate how this could be done via a case study of the Cienaga Grande de Santa Marta wetlands in Colombia.

By harmonizing these two approaches, the water planetary boundary can ensure that changes to the water cycle remain within both local and global boundaries and complement existing water management and governance approaches.