Dear Erwin and coauthors;

I am happy that you already wrote quite detailed responses to the reviewers' comments. The main concerns of both referees do not affect the theoretical concept presented in the manuscript itself, but rather the first section. In particular, one of the reviewers would probably have expected a bit more critical discussion of the applicability of the ideas of optimality in subsurface hydrology, and both reviewers suggested to include some more literature about previous work in this field. In turn both reviewers pointed out the potential of the concept presented in the second section. I would therefore see the need for a moderate revision where the aspects that you addressed in your responses are already a good basis.

All the best,
Stefan

Dear Erwin,

both reviewers appreciated the improvement by the revised version, although stating that some points are still a matter of discussion. However, I think this is within the scope of the special issue. In the report of the second reviewer (Gerrit) you will find several minor points and suggestions for further improvement. So I think the paper will be ready to publish after taking into account his suggestions.

All the best for 2019,
Stefan