Globally widespread and increasing violations of environmental flow envelopes

Virkki, Vili; Alanärä, Elina; Porkka, Miina; Ahopelto, Lauri; Gleeson, Tom; Mohan, Chinchu; Wang-Erlandsson, Lan; Flörke, Martina; Gerten, Dieter; Gosling, Simon N.; Hanasaki, Naota; Müller Schmied, Hannes; Wanders, Niko; Kummu, Matti

doi:https://doi.org/10.5194/hess-26-3315-2022

Articles | Volume 26, issue 12

https://doi.org/10.5194/hess-26-3315-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-26-3315-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 26, issue 12

Research article

|

01 Jul 2022

Research article |

| 01 Jul 2022

Globally widespread and increasing violations of environmental flow envelopes

Vili Virkki, Elina Alanärä, Miina Porkka, Lauri Ahopelto, Tom Gleeson, Chinchu Mohan, Lan Wang-Erlandsson, Martina Flörke, Dieter Gerten, Simon N. Gosling, Naota Hanasaki, Hannes Müller Schmied, Niko Wanders, and Matti Kummu

Download

Final revised paper (published on 01 Jul 2022)
Supplement to the final revised paper
Preprint (discussion started on 11 May 2021)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on hess-2021-260', Anonymous Referee #1, 20 May 2021

Environmental flow envelopes: quantifying global, ecosystem-threatening streamflow alterations

General comments

The authors quantify the environmental flow violation for most worldwide sub-basins using environmental flow envelopes (EFE). EFE are based on previously established minimum environmental flow requirements and a newly established maximum environmental flow requirement. EFE quantification was based on pre-industrial simulations of several global hydrological models (GHM), under various global circulation models (GCM), and applied to historical simulations for each GHM. Results inform about the frequency and severity of EFE violation, and could thus indicate the state of riverine ecosystem security.

Environmental flow is a concept often used in hydrological modelling in order to simulate sustainable water use scenarios that do not interfere with riverine ecosystem security. Moreover, they can be a useful tool to determine sustainable dam and water management policies. The authors aim to reduce the uncertainty in the current (historical) violation of these requirements, and to expand the violation assessment to the worlds sub-basins, by using a GHM-GCM model ensemble. However, I have some concerns regarding the EFE quantification methods, the attribution of EFE violation, and some general comments

EFE quantification methods

The authors aim to advance environmental flow assessments in four ways: (1) improve the environmental flow concept to include an upper bound (line 95, line 114, line 186, line 458), (2) reduce the uncertainty in the actual quantification of environmental flows, in particular the large GHM uncertainties (line 87, line 92, line 112, line 409), (3) use the pre-industrial river streamflow as the environmental flow basis (line 116, line 160, line 460), and (4) present the quantification of environmental flows in terms of frequency, severity and trends (line 98, line 115, line 458).

1. While I understand the adverse effects of upper EFE violation, the selection of the upper bound seems arbitrary. What is the reason for the 95th percentile?

2. I feel this study does not properly address the GHM uncertainty. The ensemble strategy is only applied to the EFR methods and GCMs, not the GHMs. EFE violations are still determined for each model separately, without averaging, which can lead to additional (compounding) uncertainties. Moreover, if the aim of the authors is to provide accurate quantifications of EFE for each river, why did the study not use observational data or select the “best” performing GHM model for each basin?

3. What is the rationale behind using pre-industrial river streamflow? Should the simulated historical pristine (no human influence) river streamflow not be better to estimate EFE (as is done by other modelling studies)? It is briefly discussed that historical pristine river streamflow may already be violating EFE (line 399). Should this be the case?

4. I am assuming monthly pre-industrial EFE quantifications are combined into a multi-year monthly averages. However, the EFE are applied to historical monthly streamflow values. This could result in EFE violation during dry years (even if EFE were applied during the pre-industrial period). Is this correct?

Attribution of EFE violation

The authors describe the EFE violation in terms of frequency, severity and trends. To that end they use the gross total of all 1440 historical GHM-year-month combinations (line 232). Subsequently the key drivers are shortly explained in the discussion (line 353).

5. I assume the authors use multi-year monthly averaged pre-industrial simulations to quantify the EFE and subsequently apply these values to the historical period (see my comments above). Can the authors show the impact of this choice on your results? It would be useful to separate the effects of climatic changes (human induced or otherwise) from direct human alterations (e.g. abstractions and dam construction). Moreover, it would be useful to see the temporal variability in EFE violation, especially during dry years.

6. Results are given a as the gross total of all GHM-year-month combination. However, the results do not show the variability between the GHMs. Could it be that a majority of the EFE violations originate from a single model?

7. The authors state that their findings show that EFE violations are widespread around the world (line 239). However, to me the results shown seem quite modest (about half the basins violate EFE for 5% of the time, not even a month per year). Even more so as dry years could contribute to this number. How do these results change for 10% or 25% of all months?

Specific comments

1. Line 110 and 658: “The significantly advanced global environmental flow assessments with the novel methodology of environmental flow envelopes” is overstated. The concept of EFE is heavily based on previous lower limit environmental flow requirements, while the newly introduced upper limit is unsubstantiated (in this study). Rather the application of the EFE methodology is new.

2. Line 177 and 178: Why are high and low flows omitted? Should they not be part of the natural flow regime (wet and dry periods)?

3. Line 241: Do these results still take into account the three consecutive month exceedance threshold (line 210)?

4. Line 284: Is it implied this trend is due to changes in climate? If so, a thirty year period to detect climate trends is rather short.

5. Line 296: It is interesting to see that the trend in upper bound EFE violation mostly increases in the northern regions, while the lower bound EFE violation mostly decreases in these regions. Do the authors know if this is a climatic or direct human alteration effect?

6. Figure 5: This figure is a little confusing to me. Firstly there is no y axis for the trend slope graph. Secondly there are clusters that have prevalent violations during all seasons, but subsequently do not display violations during for example the high flow months? Lastly, more information could be given about the clustering and why some areas could not be assigned to a cluster.

7. line 329: The first section of the discussion could be incorporated in the conclusions.

8. Line 430: As the aggregated monthly sub-basin assessment restricts the conclusions that can be made, why did the authors not do a daily gridded assessment? The data is available.

9. Line 329 and 461: Where is it shown that the flow alterations reported here are recurrent or long-standing?

Technical corrections

1. Line 51: Should “because of their importance” be removed?

Citation: https://doi.org/10.5194/hess-2021-260-RC1
- AC1: 'Reply on RC1', Vili Virkki, 03 Sep 2021
  
  Dear Anonymous Referee,
  on behalf of all authors, I would like to thank you for your time and effort on providing thoughtful and constructive Referee comments related to our manuscript on environmental flow envelopes.
  Please find our point-by-point responses to Referee comments by all three Referees in the supplementary pdf file.
  With kind regards,
  
  Vili Virkki
  
  Citation: https://doi.org/10.5194/hess-2021-260-AC1
RC2:
'Comment on hess-2021-260', Anonymous Referee #2, 23 Jun 2021

Please see attached document. Note that these are merged comments from two reviewers.

Citation: https://doi.org/10.5194/hess-2021-260-RC2
- AC2: 'Reply on RC2', Vili Virkki, 03 Sep 2021
  
  Dear Anonymous Referees,
  on behalf of all authors, I would like to thank you for your time and effort on providing thoughtful and constructive Referee comments related to our manuscript on environmental flow envelopes.
  Please find our point-by-point responses to Referee comments by all three Referees in the supplementary pdf file.
  With kind regards,
  
  Vili Virkki
  
  Citation: https://doi.org/10.5194/hess-2021-260-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (27 Sep 2021) by Graham Jewitt

AR by Vili Virkki on behalf of the Authors (05 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (10 Jan 2022) by Graham Jewitt

RR by Anonymous Referee #1 (26 Jan 2022)

Suggestions for revision or reasons for rejection

Widespread and increasing violations of environmental flow envelopes

General comments
Since the last revision, the authors have answered and addressed the most important comments of the reviewers. Especially the inclusion of pre-industrial violation results and the omission of overly strong language has markedly improved the manuscript.

However, there is still need for further clarification and specification in some aspects of the abstract and discussion. By improving these sections this study could be presented more robust. Moreover, there is also an opportunity to improve the writing. Therefore, I recommend minor revisions.

Specific comments
Note that some of the comments to do with the writing are based on my personal writing experience, and are not “wrong”. However, they may help the authors to improve their writing throughout the document.

Line 28: What uncertainty? Please clarify
Line 29: “The sub-basin specific EFE is” to “Sub-basin specific EFEs are”
Line 30: “and its derivation considers the methodological uncertainties”, again please clarify
Line 32: Maybe mention Q95 here?
Line 32: “This” to “This upper bound”
Line 33: “Long-term” is used throughout the manuscript, for different meanings. This is sometimes confusing. See also some comments below. “Long-term” to “annual”?
Line 37: Best to move “global hydrological model outputs from the ISIMIP2b ensemble” to its own sentence (perhaps at the start of the paragraph “Discharge was derived from ...”).
Line 40: “widespread, occurring” to “widespread and occurring”, as one refers to the spatial extent and the other the temporal extent.
Line 44: “spatially distributed” to “dispersed” as in the discussion.
Line 46: Ending on a positive note would be stronger, what is the broader application of your study?
Line 54: “population growth”, not only growth but also agricultural and population development (irrigation expansion and diet changes).
Line 63: Probably remove “in addition” as it specifies, and not adds to, the previous sentence?
Line 124: What are “mechanistic equations”? Probably use “process-based”?
Figure 1: Could the three month threshold be included here?
Line 165: What do these levels mean? Maybe omit?
Line 200: Normal distribution cannot be assumed for discharge. I did a quick test for 100 of the world’s largest rivers, and normality could only be assumed for half of them (using the Shapiro-Wilk test on multi-year average measured discharge). Moreover, why is it assumed there are errorous outliers in model simulations that should be removed, how do simulations make errors? This also affects the Q95 upper bound in the study. If so, this should be discussed.
Line 328: “figures” to “values” or “results”.
Line 423: “both long-term and recent”. “Long-term” here is referring to “more than three consecutive months”. However, as long-term is used in combination with “recent”, which refers to the last 30 years, which is confusing. As an alternative to using “long-term” I would propose to use “extended” or “persisted” throughout the manuscript?
Line 425: If possible, could the absolute values be included in the supplementary?
Line 434: What is a possible reason for this wider spread? Is it related to the model ensemble?
Line 437: Can you expand upon these limitation (perhaps with citations)? Or maybe move this to section 4.3, where these limitations are discussed.
Line 441: “(i.e. not taking potential climate change impacts into account as we do)” would be better at the end of the sentence, were you discuss what your study does. “(...) which is different from our seasonal analysis based on the pre-industrial period and includes potential climate-change impacts”
Line 451: “cautious interpretation of our results” to “cautious interpretation of our results in these regions”
Line 465: Would omit this sentence as it is discussed before and is addressed in the results.
Line 491: Although the limitations of EFs are described, could you indicate how they are useful? Currently this section seems to undermine the value of your study.
Line 509: Important for what? Please elaborate.
Line 513: Why this specific number? Maybe just omit?
Line 507: “On the one hand (...) but on the other hand”. These sentences do not contradict, just use “and”.
Line 508: Is there also an advantage to the spatial aggregation (as there is for the temporal aggregation)? If so it would be good to mention here.
Line 521: Could you elaborate on why the separation of natural and anthropogenic flow alterations would give additional information on the “seriousness” of major violations?
Line 530: “On one hand (...) On the other hand”. These sentences do not seem to contradict. Maybe use “nevertheless” instead of “on the other hand” (and omit “on the one hand”).

Hide

RR by Chris Dickens (03 Feb 2022)

Suggestions for revision or reasons for rejection

General:
This reviewer was party to a submission of considerable detail in the first round of reviews. To their credit the authors have clearly attended to most if not all of the comments previously given, which has considerably upgraded the paper. It is thus recommended for publication with minor edits as detailed below.
A major finding of the first review was that the absence of ecological consideration in this hydrological approach was not acknowledged. This has been partially rectified in a clumsy way and deficiencies remain, some of which are indicated below.

Specific comment:
Title –"Widespread and increasing violations of environmental flow envelopes" – this heading does not convey the depth and substance of the paper particularly as the concept of flow envelopes is not immediately intuitive. At least the word "global" should be included.
175 – quasi natural – while dams may not have been prevalent, canals were common enough in a few areas e.g. in England the construction of canals began in the mid-1700s. This would have impacted streamflow but only in selected areas.
250 – it would be good to include at least a sentence of summary of the supplementary material.
480 - these purely hydrological methods aim to establish the hydrological conditions that would be acceptable to ecosystems, but do not include any metric whereby this may be tested. Suggest this perspective be included.
484 – "This is because the ecosystem response to altered flow regimes varies across spatial and temporal scales, as well as between different species" – this sentence does not cover the issue so suggest mentioning that altered flows affect a range of ecological characteristics from sediments, to stream morphology including riparian banks, to biodiversity and community dynamics of most fauna and flora. Any of the reviews of EF will spell this out.
487 – "Though fish make up only a part of a riverine ecosystem, these studies support incorporating water quality–related factors in a comprehensive EF definition." It is not clear why water quality is given prominence here – when in most EF studies it is the response indicators (biota) that are most important as they are the best indicators of the success of the EF. It is also complex to include WQ in any comprehensive way in determination of EF because the presence of WQ issues may be and generally is completely non flow-related.
494 – floodplains - this seems a strange example, because from an ecosystem point of view you would hardly call the necessary flooding of a floodplain a violation of the environmental flows.
500 – " detrimental to riverine ecosystems outside monsoon regions" – it is not clear why there is the limitation of monsoon areas. Does this imply that they are detrimental to ecosystems inside monsoon areas?
516 – "In the future, the EFEs should be developed by complementing our global analysis with more advanced EF methods and more detailed hydrological data that better correlate with riverine ecosystem status" – this is a presumptuous way of stating this, presumptuous in that it is your model that should form the basis of future EFEs

Hide

ED: Publish subject to revisions (further review by editor and referees) (14 Feb 2022) by Graham Jewitt

AR by Vili Virkki on behalf of the Authors (28 Mar 2022) Author's response Author's tracked changes Manuscript

EF by Polina Shvedko (29 Mar 2022) Supplement

ED: Referee Nomination & Report Request started (10 Apr 2022) by Graham Jewitt

RR by Anonymous Referee #1 (10 May 2022)

ED: Publish as is (11 May 2022) by Graham Jewitt

AR by Vili Virkki on behalf of the Authors (19 May 2022) Manuscript

Short summary

Direct and indirect human actions have altered streamflow across the world since pre-industrial times. Here, we apply a method of environmental flow envelopes (EFEs) that develops the existing global environmental flow assessments by methodological advances and better consideration of uncertainty. By assessing the violations of the EFE, we comprehensively quantify the frequency, severity, and trends of flow alteration during the past decades, illustrating anthropogenic effects on streamflow.