Towards a revised planetary boundary for consumptive freshwater use: role of environmental flow requirements

We review the conceptual and quantitative foundation of the recently suggested 'planetary boundary' for freshwater (PBWater; 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 similar to 2800 km(3) yr(-)1 (the average of an uncertainty range of 1100-4500 km(3) yr(-)1). This is notably lower than the earlier suggestion based on a simpler top-down analysis (4000 km(3) yr(-)1, the lower value of a range of 4000-6000 km(3) 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 km(3) yr(-)1, PBWater 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.