I find the topic of the paper very interesting and potentially very valuable. However, the paper needs a major revision because of the following four comments, whereby my Comment 1 is the most important.

Comment 1
I agree with the two reviewers that on the one hand, the validation of the methodology in the original manuscript lacked robustness, but that on the other hand, without such a validation the manuscript does not have sufficient content to be publishable in HESS.

I therefore encourage the authors to see whether there are any gauged water quality data that can be linked to either one of the three parameters that have been reported by citizens , i.e. either C (water colour), or S (smell) or T (turbidity). If so, then the gauged data could be compared with the citizen contributed data.

I wasn’t convinced by the method adopted namely to compare the three citizen reported parameters (S, T, F) with the combined indicator Q that collapses the scores for E, N and D through averaging.

I would think that turbidity would often be routinely measured; if so that could then be a way to validate, albeit on only one parameter, the VGI method discussed in the paper.

So just throwing out the validation section altogether is not the way forward to get the paper published in HESS.

Comment 2
I do not understand the usefulness of equation 1, which combines all information on different parameters into one aggregate indicator Q through averaging. The question is: does this create additional information or does this in fact result in a loss of information. Also, why average the scores for C, S, and T? One could also aggregate in a different way, such as Min (S,T,F) or Max (S,T,F). And why combine the, arguably more “subjective”, indicator for integrated assessment with that of the three parameters? And if we have to combine them, again why take the average?

I would find it much more interesting to compare the scores for S with Ia, F with Ia and F with Ia, and see whether there are differences in correlations, and whether there can be explanations for these differences (or similarities).

Comment 3
In my opinion Figure 9 adds little information, and can in my view be omitted.

Comment 4
There is need for strict language editing by a native English speaker.

I hope the authors consider my decision in a constructive light. I encourage them to find a way to validate the claim that citizens can indeed contribute data that are valuable in terms of water quality monitoring.

Dear authors,

I noted that you have carefully considered the comments of the referees and the editor. After carefully reading the revised manuscript, I am of the opinion that the paper is of sufficient scientific quality to merit publication.

I have however made some edits in the pdf manuscript (attached, which are marked in the pdf text). I require you to consider my suggested textual changes, and submit a final improved version. Please do not make any other changes.

Success,

Pieter van der Zaag