Does the weighting of climate simulations result  in a better quantification of hydrological impacts?

Wang, Hui-Min; Chen, Jie; Xu, Chong-Yu; Chen, Hua; Guo, Shenglian; Xie, Ping; Li, Xiangquan

doi:https://doi.org/10.5194/hess-23-4033-2019

Articles | Volume 23, issue 10

https://doi.org/10.5194/hess-23-4033-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-23-4033-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 10

Research article

|

30 Sep 2019

Research article |

| 30 Sep 2019

Does the weighting of climate simulations result in a better quantification of hydrological impacts?

Hui-Min Wang, Jie Chen, Chong-Yu Xu, Hua Chen, Shenglian Guo, Ping Xie, and Xiangquan Li

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Final revised paper (published on 30 Sep 2019)
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Preprint (discussion started on 06 Feb 2019)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Reviewer comment', Anonymous Referee #1, 22 Feb 2019
- AC1: 'Replies to comments of Referee 1', Jie Chen, 29 Apr 2019
RC2: 'Review Wang et al.', Anonymous Referee #2, 20 Mar 2019
- AC2: 'Replies to comments of Referee 2', Jie Chen, 29 Apr 2019
RC3: 'Review of Wang et al. (HESSD, 2019)', Anonymous Referee #3, 03 Apr 2019
- AC3: 'Replies to comments of Referee 3', Jie Chen, 29 Apr 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (30 Apr 2019) by Nadav Peleg

AR by Jie Chen on behalf of the Authors (10 Jun 2019) Author's response Manuscript

ED: Publish subject to revisions (further review by editor and referees) (13 Jun 2019) by Nadav Peleg

ED: Referee Nomination & Report Request started (14 Jun 2019) by Nadav Peleg

RR by Anonymous Referee #2 (15 Jul 2019)

RR by Anonymous Referee #3 (02 Aug 2019)

Suggestions for revision or reasons for rejection

I thank the authors for addressing my comments. I must admit that I find the manuscript and their reply a bit difficult to follow, in particular because of grammatical errors - I apologise in advance if some of the points I raise below are the result of a misunderstanding. I am concerned that future readers will also find this study difficult to read and understand, which is unfortunate at this stage of the review process.

In the abstract, the authors now state:

1. “when using raw GCM outputs, streamflow-based weights better represent the mean hydrograph and reduce more biases of annual streamflow than the weights calculated using climate variables”. This is an interesting and potentially impactful result. But where is it shown? I cannot find a Figure or Table supporting the above statement. Table 3 shows the entropy of weights computed using temperature and precipitation, and Figure S1 the weights based on temperature and precipitation, but as I understand it, this does not demonstrate the benefits of computing weights using streamflow instead of temperature/precipitation. In their reply to my comment, the authors refer to P13, L14-18, which is part of the discussion and does not refer to any Figures or Tables in the manuscript. Please clarify. Please also note that the above sentence is not grammatically correct.

2. “when applying bias correction to GCM simulations before driving the hydrological model, the streamflow-based unequal weights do not bring significant differences in the multi-model ensemble mean and uncertainty of hydrological impacts, since bias-corrected climate simulations become rather close to observations.” As also stressed by reviewer 2, this is expected, as by construction, bias-correction forces climate simulations to look like observations, i.e. artificially reduces differences between them (e.g., Hakala et al., 2018), thereby making it difficult to differentiate between good and poor models. Hence, on its own, this result does not justify publication in my view.

The authors conducted additional analysis based on a pseudo-reality experiment to explore the consequence of model weighting under future climate. I thank them for their effort. The results are consistent with those based on current climatic conditions: bias-correction makes it very difficult to distinguish between good and poor models, and there are essentially no benefits to implement a weighting scheme, as the weighting schemes considered do not reduce biases more than an equal weight strategy (see Figures 9d-f and Figures 10d-f). Again, I believe that sequentially applying bias-correction and model weighting based on bias-corrected simulations is a flawed approach.

Furthermore, I am concerned that the results of this study might be misinterpreted. Readers might believe that there are no benefits to use weighting when the climate simulations have been bias-corrected. It is possible, however, that there are benefits of combining bias-correction and model weighting, but I would argue that the model weighting should be done using other criteria than those considered for bias-correction. For instance, the model weighting could reduce the influence of (or exclude) models with erroneous behaviour (for instance climate models creating snow towers, as it is the case for EURO-CORDEX members) or are unable to capture key processes such as atmospheric rivers or atmospheric patterns (e.g., NAO). This could potentially constrain the ensemble (e.g., Padrón et al., 2018) and could be a successful application of model weighting and bias correction. But this would require substantial additional analysis.

Overall, I think that the paper lacks a clear vision and a clear message. In their title, the authors ask “Does the weighting of climate simulations result in a more reasonable quantification of hydrological impacts?” But then they do not clarify what they mean by “reasonable”…

References

Hakala, K., Addor, N. and Seibert, J.: Hydrological modeling to evaluate climate model simulations and their bias correction, J. Hydrometeorol., 19, 1321–1337, doi:10.1175/JHM-D-17-0189.1, 2018.

Padrón, R. S., Gudmundsson, L. and Seneviratne, S. I.: Observational Constraints Reduce Likelihood of Extreme Changes in Multi-Decadal Land Water Availability, Geophys. Res. Lett., doi:10.1029/2018GL080521, 2018.

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ED: Publish subject to minor revisions (review by editor) (12 Aug 2019) by Nadav Peleg

AR by Jie Chen on behalf of the Authors (22 Aug 2019) Author's response Manuscript

ED: Publish as is (29 Aug 2019) by Nadav Peleg

AR by Jie Chen on behalf of the Authors (03 Sep 2019) Manuscript

Short summary

When using large ensembles of global climate models in hydrological impact studies, there are pragmatic questions on whether it is necessary to weight climate models and how to weight them. We use eight methods to weight climate models straightforwardly, based on their performances in hydrological simulations, and investigate the influences of the assigned weights. This study concludes that using bias correction and equal weighting is likely viable and sufficient for hydrological impact studies.