Dear Mr Tangdamrongsub,

We have received now four reviews of your paper "Data assimilation of GRACE terrestrial water storage estimates into a regional hydrological model of the Rhine River basin". Three of these reviews were invited reviews and one review was spontaneous. Two of the reviewers suggest major revision and two other reviewers suggest minor revision. The reviewers agree that the paper is of high interest and that the work seems sound.

The main points to be addressed in the revision are:
1. Conclusions and recommendations should be more clearly presented and more consistent with findings presented in results (see "spontaneous" review for further details). It should be discussed whether the presented results really imply that GRACE was able to achieve an improvement.
2. Several aspects of the methodology have to be clarified. More details on the implementation of EnKF have to be provided. How were different compartments jointly treated/updated? Details on the following more specific points are also needed:
a. How were GRACE TMS-values assimilated, grid-based or domain averaged?
b. Why was an observation error of 20mm chosen and what is the impact of this decision? How were off-diagonal elements of the covariance matrix of EnKF treated?
c. Why was snow removed prior to from TWS-signal prior to assimilation?
d. How were soil moisture data used?
e. Was water extraction considered and how could it have affected results?
f. What was the role of GLDAS in the assimilation?

Given the amount of points addressed by the reviewers, I think that major revision is need to warrant publication in HESS. The revision should contain a response to each of the comments from the reviewers and explain how the comment was handled. If you disagree with a comment, please explain why. If the text was modified as a response to a comment, please indicate how the text was modified, including the new text and its position in the manuscript.
The new version of the manuscript will be subjected to re-review by the original reviewers.

I am looking forward to the new version of your manuscript.

Best regards,

Harrie-Jan Hendricks Franssen – Editor –

Dear Mr Tangdamrongsub,

We have received now three re-reviews of your paper "Data assimilation of GRACE terrestrial water storage estimates into a regional hydrological model of the Rhine River basin". The reviewers agree that the manuscript improved substantially, and only minor revision is needed at this stage. Please find two reviews with a few minor remaining comments.

The minor revision should contain a response to each of the comments from the reviewers and explain how the comment was handled. If you disagree with a comment, please explain why. If the text was modified as a response to a comment, please indicate how the text was modified, including the new text and its position in the manuscript.

The new version of the manuscript will not be send out for review anymore.
I am looking forward to the new version of your manuscript.

Best regards,

Harrie-Jan Hendricks Franssen – Editor –

Dear Mr Tangdamrongsub,

Your revised version of the manuscript “Data assimilation of GRACE terrestrial water storage estimates into a regional hydrological model of the Rhine River basin” has been reviewed by me and can be published now after making a few technical corrections listed below.

Congratulations and thanks for choosing HESS!

Best regards,

Harrie-Jan Hendricks-Franssen

L100: "attributed to"
L177: "the net removal"
L391: skip "we"
L395: "range provided"

L532, L534, and elsewhere in manuscript: is it appropriate to compare linear correlation coefficients in terms of % increase? I think it is not. For example, an increase of linear correlation coefficient from 0.01 to 0.02 would be a 100% increase then, although the increase is very marginal. I suggest reformulation.

L649: "may not be sensitive".