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Peer-Review Record

Burning Water, Overview of the Contribution of Arjen Hoekstra to the Water Energy Nexus

Water 2020, 12(10), 2844;
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Water 2020, 12(10), 2844;
Received: 15 September 2020 / Revised: 9 October 2020 / Accepted: 10 October 2020 / Published: 13 October 2020
(This article belongs to the Special Issue In Memory of Prof. Arjen Y. Hoekstra)

Round 1

Reviewer 1 Report

Author answered to all comments, thus I agree to publish it. 

Author Response

We thank reviewer #1 for the review comments given on an earlier version of the paper. There are no additional suggestions for improvement, so we are happy the reviewer finds the paper suitable for publication. 

Reviewer 2 Report

The authors captured my comments and recommendation.

Author Response

We thank reviewer #2 for the review comments given on an earlier version of the paper. There are no additional suggestions for improvement, so we are happy reviewer #2 also finds the paper suitable for publication. 

Reviewer 3 Report

The authors have done a good job in addressing the numerous comments and suggestions of the four reviewers (including my own). Their responses have also clarified that the paper is not considered “…an ordinary review paper, but it was written to honor prof. Hoekstra…” As such, somewhat different evaluation criteria are appropriately to be taken into account.

Given this clarification, I endorse publication of the paper in the journal water, provided that some copy editing (some expressions need improvement) is carried out before publication is finally granted.

Author Response

We thank reviewer #3 for all the suggestions given on an earlier version of the paper and with this help we have been able to improve the paper substantially. This is reflected in the evaluation with all the stars reviewer #3 gives, even more than given by the other two reviewers. 

However, the following comment is rather vague:

I endorse publication of the paper in the journal water, provided that some copy editing (some expressions need improvement) is carried out before publication is finally granted.

In general, we applied definitions related to water footprints that were used before in earlier publications. That has to do with trying to be precise and not changing definitions. Maybe this is what the reviewer means with expressions? Moreover, for us it is difficult to change expressions (or definitions) if the reviewer does not mention which ones. So, we would like to asks the reviewer to provide us a list of expressions we need to change. 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

General comments

This article reviews the knowledge available on water-energy relationships and in particular, sheds light on water requirements needed to produce different energy sources. This is a key information to outline the potential trade-offs that might arise when planning different energy transition pathways. This discussion is also relevant to promote more integrative energy planning. At least to account for the water implications linked to energy policy development.

The paper is very nicely written and I think it is suitable for publication. I have a number of minor comments for the authors to be considered, and some recommendations regarding the structure of section 5. Also, I would suggest the authors to stress more the policy implications of the conducted review. There are some important messages that can contribute to the water-energy nexus debate and energy planning, and these do not emerge very strongly in the current version. 


Specific comments



Line 23. For clarification purposes I suggest to indicate “Arjen Hoekstra´s work on WF of energy generation” not everyone knows Arjen and why he is known for


Line 73. It is convenient to outline already why up to 2009.


Line 103. (4) (Mekonnen et al) typo?


Line 133 who or how?


Line 152 water needed or water consumed. It is confusing of talking about the WF to refer to use


Lines 160-162 where it says “However, nowadays most of the literature has shifted to

describe the freshwater use by energy systems as WFs, highlighting the importance of the WF concept introduced by Hoekstra.” I wonder if this liteature still uses the WF concept correctly or still mixing use with consumption. If the former, in which case I would suggest that the authors explain that despite the progress some clarification is still further required.


Line 235 where is says “In that case the crop WF needs to be reallocated over the crop and

the residue.” This does seem to be an easy job a quite arbitrary unless you know the WF/unit of biomass.


Line 236-237 where it says “An important issue in this respect, however, is whether these crop residues are really available or not.” Provide an example on when can ths occurs. It is provided later but perhaps it makes more sense to place it where



Line 240-241 “where it says “Trees not only use rainwater, but the roots reach

the groundwater causing a blue water consumption” First of all that is not a general rule, and if occurs is tree species-dependent. In fact it depends on the root system and the strategy a tree develops to optimize the capture of water. Iwould suggest that it is clarified that in some cases certain trees can also accounf for a blue WF and why is this.


Line 247-249 where it says “There are large differences among WFs for wood. The study of Schyns et al. [14] calculated only a minor contribution of blue WFs compared to green WFs. However, Das et al. [37] found much larger contributions for wood from India.” Again  different trees have different water demands, such as crops, and different strategies to capture the water. This paragraph needs to outline that differences relay on species, Environmental condictions (e.g. depth of the water table, climate, soils, etc) to avoid passing a simplistic message on WF of trees. Discerning the fraction of blue and green water for trees seems even more complicated than for crops, so lots of further researh are required here. It should be made clear that these are just a few studies but nothing really conclusive



Line 269. This section needs to make clear that WF of electrcity depnds onf the energy mix and timing. Stating that electricity is a source of energy with high WF does not resolve the big challenges that we have ahead. The question is what energy mix is better to offset emissions and have a reduced WF and LF and what type of renewable energies should be promoted. The authors point to this issue in the following section on hydro, where it becomes clear that hydro might offset emissions but is a very water intensive electricity source.



Line 286 section on hydro. Further Research is required, since hydro is in high demand in developing countries as a mean to offset carbon emissions and bridge the energy and electricity gap. Different types of hydro make a big difference and while this is presented, I would suggest the authors to highlight the overall water implications of becoming too hydro dependent and what options exist if this energy source is to be further developed. As Figure 6 outlines the differences between large dams as oppose to run of the river are very large despite the climate particularities.


Section 5 the way forward


The authors have used this section to summarize further Research and uncertainties. Given the different nature of the topics discussed I wonder whether it makes sense to group them into energy groups (using the group typologies). An alternative is to discuss Research gaps and uncertainties within the each of the previous sections. As a reader I had many “but” when Reading the previous sections, and now I see some of my questions addressed all together here. Does not help very much to fix clear messages the current structure.



Lines 339-346 The impact of CC on crop water demands and changes in growing season are definitively a topic of major concern for the entire agricultural sector. There are some studies out there that might be worth citing. From a long term energy planning perspective accounting for these uncertainties is necessary as they might be exacerbated in climate change world.


Lines 347-360 Hydropower. The development of this energy source is going to increase in the developing world. Accounting clearly for the water related implications and how different types of hydro play out it is an important policy message. Allocating the WF among sectors if multi-purpose is relevant, but at the end of the say that total volume of water is going to evaporate to a large extend regardless of whether it is single or multi-purpose. If there are specific recommendations as to how to operate the dams in a way that minimize water losses while delivering water for the different uses, that is probably more relevant than to allocate WF of a large dam to hydro or irrigation. The authors have enough information to substantiate this important debate.


Lines 361-368. As a reader I personally don´t remember anymore where it was said that Grey WF was an important component to produce energy. It will make sense therefore, either to refer to the energy source and research gaps or consider to include this discussion in the corresponding section above. This is a practical observation.




Line 388-389 Statement on “Energy from forestry also has large green WFs, but also a blue WF, even if trees are not irrigated.” Is to conclusive for the evidence gathered Some trees in certain circumstances have capacity to tap into groundwater or water tables from rivers, but that does not mean that is always the case.

Reviewer 2 Report

General Comments

The Water-Energy Nexus is frequently discussed and considered these days. This is due to the realization that important linkages exist between water provision and energy generation, on the one hand, and between the need of energy for the purification and transport of water, on the other. This paper addresses specifically the contribution of water footprint (WF) studies on water for energy relationships. In this context, the authors consider various types of renewables as well as conventional/hydrocarbon energy sources for the provision of water. Since some of these energies utilize agricultural products, their water footprint (WF) is particularly significant and can be quantified by using the WF tool. The paper provides a comprehensive review of important water-energy studies until 2009 and explains the contribution of Hoekstra’s work for these and possible future studies.

While the paper is well conceived and formulated, it goes into details that are, at least in places, not necessary, e.g., in section 4.1, which introduces the water footprint concept at length, while this can be found in many other publications.

While the paper focusses on bioenergy production from various crops, there is hardly any mention about the alternatives, i.e., bioenergy from agricultural waste or from waste originating from food processing, etc. This is issue is briefly addressed in one sentence on p. 6 (l. 220-221 and 234-235).

At some places, the authors state facts and hypotheses, without providing plausible explanations. One example is the discussion on the WF of hydropower generation in Ecuador on p. 9 (l.300-306). The sentence on l. 303-305 :” This probably has to do with the specific climatic and physical circumstances in Ecuador in combination with efficient electricity generation.” does not really explain the spread in WFs as seen in Figure 6. Furthermore, what do the authors mean by “wisely chosen hydropower systems”? Another example is the discussion on p. 10-11 (l. 361-367). It is not clear what the authors intend to say when discussing “nitrogen losses in agriculture”?

A discussion/listing of source categories is given in the abstract (p. 1, l. 25-32), in section 4.1 (p 4, l. 163-169) and in the Conclusions (p. 11, l. 380-387); this is repetitive and should be avoided.

Overall, the paper, in general, and the Conclusions, in particular, remain somewhat weak. Most of the results shown (e.g., Figures 1 – 6) are derived from published papers and do not represent original work. Moreover, as mentioned once by the authors, biofuel/bioenergy production from agricultural sources is highly debated these days and should preferentially be phased out. If this is realized, the relevance of the current paper is even less relevant. The paper might still be published, if the points noted here are adequately addressed and if the role of waste in biofuel/bioenergy production and the related WFs are discussed.

Specific Comments

While in general, well written, there are at places formulations that should be revised. Specifically, a number of unclear formulations and orthographic deficiencies have been observed that are detailed below (this is not a complete compilation of all possible orthographic and/or formulation issues), which should be addressed when revising the paper.







Replace: “…large…” by some quantitative information, e.g., “3 - 4 kWh/m3 for seawater desalination”



The statement “…the use of the renewable energy source biomass has large water demands…” is not universally valid and depends on the kind of source material for biogas generation; using agricultural waste is only indirectly linked to water use.



Reformulate. “Agriculture grows crops that need water, either rainwater, green water, or irrigation water, blue water, and also pollute water creating a grey WF.” Agriculture itself does not “pollute water”, but agricultural practices may lead to water pollution.



The terms “grey WF”, “blue WF”, etc. need to be defined.



It is not clear, what is meant here: “figure also gives the global average WFs”? Is this the global average for all crop types?
Furthermore, it is a bit surprising to see that the average lies well above any of the given individual WFs. This needs some explanation.



Reformulate: “where traditional firewood is used for cooking” to “where firewood is traditionally used for cooking”



Please clarify: “…much larger contributions…” – contributions of what?



In Figure 4, two production technologies are introduced: dry and wet conversion; this needs to be explained.

Reviewer 3 Report

The review on the contribution of water footprint (WF) to the water-energy nexus is a significant issue. This paper gives an overview of the contribution of WF studies on water for energy relationships and discusses the future research trend. Arjen Hoekstra is the esteemed founder of the WF research. The review is a timely meaningful paper in honour of Prof. Arjen Hoekstra.


In my opinion, the approach used is appropriate and the interpretation degree of results is satisfying. This paper provides a comprehensive understanding of contribution of water footprint to water-energy nexus on a global scale for readers. Because of this I feel it can be considered for publication.


I have just two minor comments:

The water-energy nexus is a part of the water-food-energy nexus. Incorporating some food related issue in the section “5. The way forward” may be meaningful. Referring to some WF review literature may be of some help (such as


Why the time scale is pre-2009 needs some clarification.

Reviewer 4 Report

This paper brings a timely and relevant topic about going beyond classical water for energy (ex. water intensity) to a more complete comprehensive WFs energy approach with several energy categories. 

I have some concerns that need to be addressed before publication.

  • The author highlighted “Energy policies need to account for significant trade-offs between carbon, land and water footprints”. What is the global or national trends of energy policies ? Increasing renewable energy or shale gas ? Increasing national energy security ? Decreasing oil trade ? I recommend showing the temporal trends of energy usage by sources and national energy security in terms of energy policy.
  • Economic aspect in water for energy: This paper reviewed a lot of research relating to WFs for energy and focused on quantitative impacts. However, the economic efficiency of WFs was little mentioned, for example, price of energy, recycling water in power plant, water value and so on. Thus, I recommend showing some studies about the WFs energy in economic perspective.
  • Energy that originates in agriculture has a relatively large water footprints. However, in terms of water quality and environmental impacts, we can see different side and trade-offs. Although WFs of energy through agriculture is larger than others, it might bring less negative impacts on environment (ex. water quality). Actually, in shale gas area, water quality becomes important issue such as groundwater pollution. 
  • The title includes the water energy nexus, and the key of nexus is to look holistic impacts based on trade-offs. The author tried to show the main issue of water-energy trade-offs in each energy source categories. Thus, I recommend making one summary table showing WFs centric trade-offs in each energy source categories.
  • What is the main message that you want to deliver ? In coclusion, you showed the summary of WFs energy but little showed the main message from this review paper. For example, what is the biggest difference before and after applying WFs energy in terms of contribution of WFs? How can we use WFs energy in sustainable development era ? What research is needed in WFs energy fields ?
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