We review the conceptual and quantitative foundation of the recently suggested ‘planetary boundary’ for freshwater (PB-Water; i.e. tolerable human ‘blue’ water consumption), and propose ways forward to refine and reassess it. As a key element of such a revision we suggest a bottom-up quantification of local water availabilities taking account of environmental flow requirements. An analysis that respects these requirements in a spatially explicit manner suggests a PB-Water of ~2800 km3 yr-1 (the average of an uncertainty range of 1100–4500 km3 yr-1). This is notably lower than the earlier suggestion based on a simpler top-down analysis (4000 km3 yr-1, the lower value of a range of 4000–6000 km3 yr-1). The new estimate remains provisional, pending further refinement by in-depth analyses of local water accessibility and constraints up-scaled to the global domain, including study of cascading impacts on Earth system properties. With a current blue water consumption of >1700 km3 yr-1, PB-Water is being approached rapidly. Thus, design opportunities to remain within PB-Water are imperative. We argue that their quantification requires analysis of tradeoffs with other planetary boundaries such as those for land use and climate change.
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