I am very glad to host this opinion paper in HESS and I read the discussion with great fascination. I particularly also thank Dr. Gerten and Dr. Roeckstroem for their valuable and openly stated review and comment. I very much agree with Reviewer 3 (Dr. Gerten) on the importance of the planetary boundary concept as a means of raising awareness that we need to safeguard our globe and that the role of fresh water use should not be downplayed in this context. However, with respect to the specific definition of a planetary boundary for consumptive water use I think the authors raised very thoughtful concerns on a) whether a global annual average of consumptive water use is helpful to judge a safe modus operandi and b) whether regional/ river basin wide regime shifts in environmental system dynamics (and quality) due to an overuse of blue water my pile up to a trigger a regime shift of the earth as a whole.

While consumptive water use implies that the water leaves the local water cycle of a river basin, this flux does not vanish globally. In fact it will be added to the water cycle(s) of one or several other water sheds, either through direct transfer, or ET and precipitation (as outlined by Reviewer 2 Dr. Savenije). While we all agree that consumptive overuse of blue water resources may trigger catastrophic shifts in regional environmental systems, the postulated global dimension is not straightforward, simply because the consumed amount of fresh water is not exported to the universe, it is redistributed and thus remains within the global cycle. One can thus not imagine a planetary scale tipping point that is triggered by this “state variable”, as correctly argued by Dr. Rockstroem.

Can we thus judge the sustainability of global consumptive fresh water use based on a global average annual threshold? Regional/basin scale water resources management cannot rely on averages because one cannot compensate a 100 years drought in summer with a 100 years flood in the following winter. A global mean appears hence a little too simple also because one might draw way too positive conclusions from the fact that mankind operates below this threshold as thoughtfully explained by Maik Haistermann. Yet I agree with Dr. Sivapalan that the hydrological community should not stop with the rejection of those parts of the concepts that appear oversimplified. If one attempts to define global indicators for sustainable fresh water use one might be better off using an average flux in combination with a spatial gini-index. One could imagine that a high and at the same time spatially highly uneven fresh water use could trigger enormous societal and regional regime shifts. The derivation of such a set of indices would require a community effort similar to Pantha Rhei in combination with the best scientific practice. In this respect I highly appreciate Dr. Lorenz comment that one should not use unverified extrapolations with unverified models for such an exercise.
Overall I think that this opinion paper and the entire debate reflects the desperate need for simplification in science to support policy making, and the related cardinal challenge to avoid oversimplifications in this respect. I am happy and grateful to the author that he kicked of such an inspiring debate. This is valuable for science and I believe it will at the end also serve the underlying idea of the planetary boundary concept. As it is cannot be the scope of an opinion paper to improve the PB concept with respect to fresh water use, I think the outlined revision of the manuscript goes into a very appropriate direction.

Erwin Zehe

Dear Dr. Heistermann,

I very much pleased with this revised version of your opinion paper and see no reasons to hold this back from publication!

Best regards,

Erwin Zehe