Peer review of: Drought and climate change impacts on cooling water shortages and electricity prices in Great Bri (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Review 
 
The manuscript presents a novel integrated approach that includes a physically-based model – 
statistical model and a power system model to evaluate the evolving economic cost of droughts on the 
UK electricity sector. 
The subject is of high interest to the Nature Communications audience. The approach includes a 
robust probabilistic analytics, careful attention has been given to evaluating models, and the no-go-
hands-off approach to derate the thermo-electric power plants generating capacity based on water 
availability is an improvement with respect to existing literature. Overall this is a very well performed 
analytics that requires clarification in some of the assumptions and reporting of the results, and a 
sensitivity analysis of the results with respect to fuel prices which is important since the system scale 
cost is at the center of the analytics. 
 
Couple comments in no particular order: 
 
1) Based on the supplementary material, it seems that there is minimal reservoir regulation on the 
observed flow at the monthly time scale. Since the derating is however performed at the daily time 
scale, is there an assessment on how reservoir operations could alleviate some of the derating in the 
results? While this is not necessary to add in the manuscript, it seems worth mentioning as a 
limitation. 
 
2) Are droughts over the UK a year-long process? Some of the results are at the annual time scale 
while the seasonal and monthly analysis is taken into account to describe the inter-actions with the 
renewables. While the analysis allows for the author to comment on how renewables can compensate 
for droughts in terms of cost, it might not be a “drought” process anymore? Could you clarify? 
 
3) Use of $US currency in the introduction and UK pounds in the remaining of the text – while I 
understand, it makes the paper a bit confusing. 
 
4) Page 2: Voisin et al. 2016 and 2018 used distribution of capacity derating as a way to evaluate 
potential vulnerabilities of the grid and associated the risk with system-scale economic cost, with an 
explicit “risk-based approach”. While those applications were not done under climate change and 
neither with as many runs and robust statistics as done in this analytics, those papers should however 
be mentioned to nuance the claim of the paper that this is the first attempt to use this risk-based 
approach. 
 
[ presentation of the distribution and risk-based approach] 
Voisin, N., M. Kintner-Meyer, J. Dirks, R. Skaggs, D. Wu, T. Nguyen, Y. Xie, M. Hejazi, 2016. « 
Vulnerability of the US Western Electric Grid to Hydro-Climatological Conditions: how bad can it get? » 
Energy (115) pp. 1-12. doi: 10.1016/j.energy.2016.08.059. 
 
[ supplemental material in particular for an application of the risk based approach for the 55-year 
historical benchmark] 
Voisin, N., M. Kintner-Meyer, D. Wu, R. Skaggs, T. Fu, T. Zhou, T. Nguyen, and I. Kraucunas, 2018: 
Opportunities for joint water-energy management: sensitivity of the 2010 Western U.S. electricity grid 
operations to climate oscillations. Bull. Am. Meteorol. Soc., BAMS-D-16-0253.1, doi:10.1175/BAMS-D-
16-0253.1  
5) The authors made careful sensitivity analysis. Beside clarifications throughout the paper, I would 
recommend the authors to perform a sensitivity analysis on the results with volatility in prices. While 
the focus is on climate change, it would be more transparent for the electricity sector to also address/ 
comment on how fuel price volatility affects the perceived cost of drought. Cost of fuel is a major 
source of uncertainty for long term planning. It may also affect the statement on how renewables 
counter-balance the cost of droughts. Or there might be UK price fluctuation regulation in place that 
already address this point. Please consider. 
This comment is in relation to a paper published this year on the sensitivity of the system-scale and 
regional scale cost to fuel prices. O’Connell et al. (2019) demonstrated a significant sensitivity with the 
cost of drought being as significant as extreme natural gas price volatility. It might be more significant 
over the UK , or not, but it brings more perspective on the findings of the analytics for long term 
planning purposes. 
 
O'Connell M., N. Voisin, J. Macknick, and T. Fu. 2019. "Sensitivity of Western U.S. power system 
dynamics to droughts compounded with fuel price variability." Applied Energy 247. 
doi:10.1016/j.apenergy.2019.01.156 
 
6) Drought definition is missing. Authors seem to use a plant-scale drought definition ( flow 
percentile), yet the drought definition at the system scale is based on the overall derating or weighted 
flow at the different stations? Please clarify. 
 
7) In the discussion, please clarify that the demand is changing only in response to temperature and 
not to changes in population or technology innovation. Another assumption important to clarify with 
respect to expectation for long term planning in the electricity sector is that the generation portfolio is 
not changing – no new plants are added and old / aging plants are not retrofitted. While not change in 
the analytics is needed, this is worth mentioning. 
 
8) Clarification of the numerical experiment in the introduction: the baseline is presented as “ no 
derating” but there is inter-annual derating – please clarify. 
 
Well done . Best, 
Nathalie Voisin 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
Dear Authors 
Congratulations for your research. 
I have short comments and suggestions as opportunities for improvement. 
 
1. During the 2001/2002 crisis, there was no increase in dispatch of thermal plants in Brazil, especially 
because that country did not have many thermal plants at that time (this picture has changed 
considerably since then). There was, however, a great rationing. The figures presented by the authors 
demonstrate that the direct economic impact of a higher use of thermals was not relevant in 2001, 
just compare $ 41 million (2001-2002) against 19.1 Billion (2013-2016). The indirect cost to the 
economy of rationing, however, was brutal. 
2. 2. The paragraph beginning with “Under NF and FF ...” on page 5 deserves to be rewritten clarifying 
its understanding. 
3. 3. In item 3.4 the authors should inform which plants are eligible as renewable. A table of installed 

capacity statistics for each generation source considered renewable would be of good use to the 
reader. It is not clear whether Hydros are considered renewable. Reservoir capacity influences this 
analysis. In the UK are the Hydros considered renewable? I ask this because in Brazil in reason of the 
impacts of large dams, many Hydros are no more acceptable as renewable (in the sense of being 
environmentally friendly). For this classification, we consider only the run of a river power projects or 
the small ones . 
4. 3. The scales and explanations related to figure 7 deserve to be improved. 
5. 4. I recommend a broader explanation for the 1st. Paragraph of item 5. 3 
 
Finally I recommend the publication of the article considering the small revisions suggested.