The authors did work on some changes in the paper. My reading of these changes indicates that authors answered a number of line by line comments of the reviewer. They also added some details on the coupling in the computational implementation section.
Still, authors do not seem to fulfil the reviewers requests for making a major revision by adding a short introduction to the principles of PSO in contrast to PEST; explain the actual coupling is done regarding the fitting process; and clarify what the fitting data are, and the assumptions.

Recent exchanges by mail did clarify a few remaining questions, so that now, after the recent exchanges by mail, everything is ready to be finalised.
I am now requesting a minor revision so that the authors can add the latest manuscript and Author's response on the web site.
After a complete reading of this final version, I think a syntax check is also needed: I found some typos at least in lines 63, 71, 91, 92, 105, 130, 137, 138, 139, 157.
Once this step is completed, from my view, the paper will be ready for publishing.