|I have reviewed the revised manuscript and the author’s response. Indeed, I had accidentally reviewed the original version of the manuscript and the revised version is certainly improved. As I stated before, I consider the methodological framework presented in this work novel and as such, I wish to see the paper published. I have included some further comments below:|
1. Regarding my previous comment on the connection between the modeled 1% AEP and the 100yr flood map generated by FEMA. I would like to add few more comments as a response to author’s comments. The authors stated that
“We disagree with the assessment of the Referee , for two reasons.. First,, we note that the core of our methodology is to examine changes in the frequency of modeled 1% AEP events.. These changes are based on model to model comparisons through the 21 st century. While we recognize that the time period of our “baseline”” does not coincide with the time period during which the 100 - year floodplains were mapped, this is likely to make our results conservative if one assumes that extreme hydrologic events have already been increasing through the late 20th century.”
I am fine with your methodology for assessing changes in modeled 1% AEP events, as you mention. However, your assessment of changes in frequency is based on a threshold (the one you identified at the 1%AEP from the 2001-2020 period), which means that your results are also threshold-dependent. Considering a longer period could (certainly would) lead to a different threshold that would lead to potential differences in the results. In fact, this relates to your discussion on the sampling uncertainty which you have identified to be in the order of 5-20% that you chose not to propagate in your assessment. However, presenting some evidence on the sensitivity of the results on the threshold (even just for the min/max possible values) would be beneficial for the readers. Again, due to the complexity of the problem you are addressing it is understandable that all the different sources of uncertainty cannot be realistically quantified. However, since you have already estimated a range of uncertainty for the 1% AEP I would strongly encourage you to show how this affects the results.
The authors' second argument stated
“Second, because we are using a delta method, we neither expect nor require our model to perfectly simulate 100 - year events in any one location. The mapped 100 - year floodplains are used only to calculate the damages associated with 1% AEP events in each stream reach, and our method does not require that we demonstrate the perfect correspondence between modeled and observed hydrologic extremes. We believe that the discussion of the model performance in Supplemental Information File #11 and in Mizukami et al., (2017), both of which were added to the revised manuscript, provide sufficient documentation of the model’s performance to justify its use under the delta method framework we applied. Furthermore and as described above, Figure 7 demonstrates that despite the uncertainties described above and in Section 2.4, the simulated damages in the baseline model run very closely mimic observed flood damages over the late 20th century.”
I understand the point of the authors and my point was not to require that the simulated flows matched perfectly the observed. My point connects to the previous point on the impact of uncertainty in the estimation of 1% AEP. Let’s assume that FEMA calculated the 100yr flood based on N observations available from the simulations. Take the N simulated events and calculate the 100yr flood (Q100N). If we take a sub-sample n<N of those observations and recalculate the 100yr flood we will end up with a different estimate Q100n (uncertainty due to sample size). The number of times the future (simulated) flows exceed Q100N and Q100n will be different and thus the associated estimates of future damages would be different.
Again, in my opinion, this point could be addressed (at least partially) by providing some indications on the sensitivity of the results to the estimation of the baseline 1% AEP.
2. My second comment relates to page 5, L5 where the authors define as “flood” the annual maximum flow value that exceeds the baseline 1% AEP. Why are you considering only the annual max? There can be also other events within the year that exceed the baseline 1% AEP. Accounting for these as well will have a significant impact in the future change of “flood” frequency and associated damages. Please clarify/justify your choice.