The role of glacier changes and threshold definition in  the characterisation of future streamflow droughts in glacierised catchments

Van Tiel, Marit; Teuling, Adriaan J.; Wanders, Niko; Vis, Marc J. P.; Stahl, Kerstin; Van Loon, Anne F.

doi:https://doi.org/10.5194/hess-22-463-2018

Articles | Volume 22, issue 1

https://doi.org/10.5194/hess-22-463-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-22-463-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 22, issue 1

Research article

|

22 Jan 2018

Research article |

| 22 Jan 2018

The role of glacier changes and threshold definition in the characterisation of future streamflow droughts in glacierised catchments

Marit Van Tiel, Adriaan J. Teuling, Niko Wanders, Marc J. P. Vis, Kerstin Stahl, and Anne F. Van Loon

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Final revised paper (published on 22 Jan 2018)
Preprint (discussion started on 04 Apr 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review', Anonymous Referee #1, 11 May 2017
- AC1: 'Response to referee comment Anonymous Referee #1', Marit Van Tiel, 24 May 2017
RC2: 'An interesting mix of state-of-the-art and obvious "don'ts"', Massimiliano Zappa, 23 May 2017
- AC2: 'Response to referee comment Massimiliano Zappa', Marit Van Tiel, 04 Jul 2017
SC1: 'Remark on drought terminology', Lena M. Tallaksen, 02 Jun 2017
- AC3: 'Response to short comment Lena Tallaksen', Marit Van Tiel, 04 Jul 2017

Peer-review completion

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

ED: Publish subject to revisions (further review by Editor and Referees) (06 Jun 2017) by Matthias Bernhardt

AR by Marit Van Tiel on behalf of the Authors (01 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (29 Aug 2017) by Matthias Bernhardt

RR by Anonymous Referee #3 (13 Sep 2017)

Suggestions for revision or reasons for rejection

Comments to the Author
Summary of the manuscript
This manuscript (ms) presents climate change projections of future streamflow droughts in two glacierized catchments. For this purpose the HBV model was used to project future runoff in two case studies (Wolverine in Alaska and Nigardsbreen in Norway) using two future climate change scenarios. Furthermore, four types of projections are presented in order to address the effects of glacier retreat and drought thresholds: i) simulations with constant glacier area, ii) simulations with dynamic glacier area (using the delta-h parametrization), iii) simulations with a drought thresholds (consecutive numbers of days with defined low discharge) defined in the reference period and iv) simulations with drought transient variable thresholds (TVT) defined for future periods. The study concludes that glacier dynamics and the threshold approach can significantly affect the assessment of future streamflow droughts.

Evaluation
In summary I think that an assessment of future stream flow droughts is extremely important for water managers and accordingly, I do think that the topic of the study is relevant. However, in my opinion there are some major concerns that should be addressed to make the study valuable:

- I am not sure why it is necessary to analyze the effects of constant glacier areas. To me this seems redundant, as it is evident that glacier areas will get smaller. I would rather like to see how much the glacier retreat affect streamflow drought. I recommend removing the simulations regarding constant glacier areas, and providing ice-melt contributions to runoff. In my opinion the simulations with constant glacier area could lead to misleading interpretations by some readers and diverts from the actually interesting topic: future stream flow droughts.

- I am also not convinced why a TVT approach is helpful. For water managers a detailed assessment of future droughts based on present flow observations would be helpful. The TVT-approach might be misleading, as it may suggest that droughts will not be relevant in the future. I recommend focusing the results on streamflow drought based on the historic reference period, rather than investigating the effect of hypothetical numerical assumptions.

- The authors use discharge (Q) and glacier mass balances (MB) to calibrate their model; however they fail to present a validation of MB. For Q simulated and observed average daily values for the entire historic period are presented, making it impossible for the reader to judge if extreme events, i.e. droughts, are well reproduced by the model. Since an analysis of droughts is the objective of this ms, I believe a thorough discussion regarding the efficiency of extreme events should be discussed and visually presented.
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- Recent research has been focusing on estimating future rainfall, snow and ice runoff in glacierized catchments. In mountain areas snow melt has been identified as a dominant source of runoff generation, even in catchments with over 40% glacierization (how much glacierization is in the two case studies?). Accordingly, I would strongly recommend validating the snow cover (satellite data are available worldwide) before investigating streamflow droughts in mountain catchments. This is especially important for the HBV model, which accumulates snow height in higher altitudes to unrealistic heights if not calibrated adequately. The accumulation of snow height is of particular concern if simulations are run over several decades, as unrealistic snow heights can falsify the contribution of snow melt after few decades.
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- Multi-objective function (pg2): the attribution of 40% to MB, 40% to Q in April-September and 20% to peak flow is arbitrary; what is the scientific rationale behind these weighing factors? I recommend giving all datasets equal weight. I also recommend providing the reader with the individual efficiencies for each part of the function (KGE might provide an overall efficiency, but is not suitable to discuss the efficiency regarding drought modelling, or glacier contribution, which are both essential to address climate change projections of stream flow droughts).
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- Climate scenarios (RCP4.5 and 8.5): since this study focuses on extreme events, i.e. droughts, it would be helpful to address how well the climate scenarios reproduce such extreme events. I recommend providing a table showing mean and standard deviation during QM correction.
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- Selection of case studies: why were two study sites chosen that are geographically so far apart? What is the scientific rationale behind the selection of the study sites?
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- Conclusions: in my opinion the conclusions are flaw and do not really reveal new insights into drought dynamics. I recommend that the conclusions focus on future droughts based on present threshold (see comment above). By addressing all the concerns above, valuable conclusions could be generated, addressing the risk of future water shortages in the two case studies.
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- Figures: please provide short titles (as done in fig 6) for all panels in all figures (next to the letters), this would help providing the reader an overview of the numerous sub-panels
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- Finally, I recommend to add a reflection why stream flow droughts are an important issue in the two selected case studies.

I leave it up to the editor and the readers of HESS to decide if the comments above should be implemented in the frame of revisions or in the frame of a new ms. All of the concerns above have been addressed in recent papers and I am convinced that this study could become a valuable contribution the water research if the concerns are addressed adequately.

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RR by Massimiliano Zappa (18 Sep 2017)

ED: Publish subject to revisions (further review by Editor and Referees) (09 Oct 2017) by Matthias Bernhardt

AR by Marit Van Tiel on behalf of the Authors (31 Oct 2017) Author's response Manuscript

ED: Publish as is (16 Nov 2017) by Matthias Bernhardt

AR by Marit Van Tiel on behalf of the Authors (22 Nov 2017) Manuscript

Short summary

Glaciers are important hydrological reservoirs. Short-term variability in glacier melt and also glacier retreat can cause droughts in streamflow. In this study, we analyse the effect of glacier changes and different drought threshold approaches on future projections of streamflow droughts in glacierised catchments. We show that these different methodological options result in different drought projections and that these options can be used to study different aspects of streamflow droughts.