Dear Dr. Switanek,

Let me first of all apologize for the fact that my decision on the further treatment of your very interesting manuscript took a while. This is because the reviews of Uwe Ehret and Andras Bárdossy evaluate your work at two rather different, equally important levels – a) the question whether bias correction is a valid procedure in the context of climate change studies and b) the pros and cons of your without doubt interesting approach.
Although the first issue goes partly beyond the scope of your manuscript, HESS is the right journal to debate it. I very much agree that bias correction is a useful procedure in the context of operational forecasting or merging of rainfall radar and rain gauge data, and in fact application of BM has a long tradition in this context (partly under the name model output statistics MOS). In line with the comments of Uwe Ehret I have great concerns about the use of BM in the context of climate change impact studies. Bias correction is a quick fix, to mask our inability to properly reproduce the recent climatologies of target variables, particularly rainfall. A successful reproduction of recent climatologies is a key benchmark to assess whether our climate models and the underlying theories represent the causal effect relation in our climate systems good enough, to expect reliable extrapolations under non-stationary conditions. Systematic model errors suggest that the representation of these cause effect relations is systematically wrong. The current practice is to bias correct the model results – instead of bias correcting the models. Can we promote climate change impact research without bias correction? Yes we can, it is a matter of communicating the current limits of predictability instead of masking them. So what to conclude from this – certainly not stop the search for improved bias correction schemes. But that those schemes should be introduced and discussed with great care, and that development of bias correction schemes cannot substitute bias correction of the models. Your reply to Uwe Ehrets review reveals that you are very much aware of this problem. I thus kindly ask you to avoid the too much enthusiastic opening statement introducing BM as being "essential" you for instance may use "widely used" instead and to critically reflect on these issues in your revised manuscript.

That said, let me come to the second level the pros and cons of your without doubt very interesting approach. I dare say that I highly appreciate that your study addresses the stationarity assumption, a key weakness of many bias corrections schemes. Your results show evidently that the stationarity assumption is in valid and that your approach preserves the change signal of the climate model. Yet Andras Bardossy came up with several crucial points that should be addressed or at least discussed in the revised manuscript. One is certainly the issue of physical consistency among different fields and the univariate treatment, neglecting spatial correlations. The second important point is to exclude precipitation amounts within the error margin from the analysis and the suitability of the gamma distribution for rainfall extremes. I think you should check how the proposed distributions change when changing the threshold to 1mm. Allow to point out that a definition of a return period as inverse of the exceedance probability requires equal temporal spacing of the selected sample members (for instance annual maxima). If this is not the case here, the term return period is problematic. The suitability of the gamma function for extremes should, as you already stated in your reply, be discussed; the best passage is the context of your Figure 4, panels b and e.

Overall I return this work to you subject to major revisions. Please make sure that the revised manuscript sticks to our mathematical conventions. In line with Andras Bardossy I think that parts of section 3 and 4 should be reformulated as they are not easy to follow.

Best regards,

Erwin Zehe

Dear Dr. Switanek,


Given the postive recommendations of both reviewers I am delighted to accept your manuscript for publication.

Best regards,

Erwin Zehe