I feel there has been some valuable discussion on this manuscript. All credit to the reviewers for stimulating this, and to the authors for responding in such detail.

The fundamental points raised by the reviewers relate to the suitability for the SPI when used in combination with bias correction. First amongst these is the fact that both involve normalisation. There are also the inevitable issues around the reference period being used, which to me are inherent challenges in using the SPI (or indeed any indicator in respect to a fixed reference period) in a non-stationary environment, as raised in many previous studies. This is problematic over past timescales and gets even more challenging to interpret in long transient projections.

On the question of normalisation, the authors have taken some steps to address reviewer 1’s comments although have so far just presented correlation. There would be benefits in looking at the other metrics (particularly given the possibility of differences at the extremes, as raised) and the new text needs to concisely capture the key points raised.

The question of reference period is a vexed one, and the decision depends entirely on the objectives of the study. This clarification all needs to be articulated much more clearly, with some discussion on the reasons for the decision and how this affects interpretation. The authors have suggested they will add this discussion.

Most of the other points raised require relatively modest revisions and the authors’ proposed changes seem reasonable. Both referees called for more discussion to be added, and the authors have already provided some new discussion text. This is encouraging and I hope the additional discussion is focused and well integrated with the existing text.

There was also a suggestion to shift emphasis and title away from droughts. I agree with this and I support the suggestion of a title change. I am not sure whether the new title is the best way of capturing the essence of the paper; “seasonally aggregated” could be misleading as the SPI is not necessarily seasonal. Would “monthly aggregated” be better? Or simply “…in projected SPI…”. Worth giving further thought to a title that captures the work succinctly, but with impact.

Personally, given the points raised by the reviewers I wonder whether the title and emphasis should be moved away from bias correction too as that is just one element, and the paper is as much about the trends in the projections and differences between models, so the title could be more generally focused on met drought projections.

Overall the work has potential for publication in HESS and I look forward to seeing a revised manuscript for review.

Best regards
Jamie Hannaford

Dear authors,

Many thanks for your revised version. The referees have considered this new version and appear to be largely satisfied with your responses to their major comments. However, there are a few minor comments which require addressing prior to acceptance for publication - please pay particular attention to referee #1's outstanding concerns.

I would be grateful if you could respond to these modest issues before I appraise the final manuscript.

with best regards
Jamie Hannaford

Dear authors

Thank you for revising the manuscript according to the reviewers' additional comments. I am satisfied that you have addressed the comments appropriately. I consider the new manuscript suitable for publication in HESS. Thank you again for submitting your work to this special issue of HESS - I look forward to seeing the final paper. I spotted a few really minor typos which you can address when uploading production files, see list below.

Best wishes
Jamie Hannaford


P1, L28. Should be plural "different regional climate models"
P2, L5. If referring to SREX surely worth citing the report; it is already in the ref list as IPCC, 2012.
P4, L15 - L20. Quite a long sentence that is quite cumbersome; could break into two with 'Choice of drought indices' being a new sentence.

Figure 5 - unless I am misunderstood, is this in percent (as with Figure 3)? If so should add [%] to the caption.