Meteorological drought projections for Australia from downscaled high-resolution CMIP6 climate simulations

Eccles, Rohan; Trancoso, Ralph; Syktus, Jozef; Chapman, Sarah; Toombs, Nathan; Zhang, Hong; Ma, Shaoxiu; McGloin, Ryan

doi:https://doi.org/10.5194/egusphere-2024-2341

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https://doi.org/10.5194/egusphere-2024-2341

Preprints

29 Jul 2024

| 29 Jul 2024

Meteorological drought projections for Australia from downscaled high-resolution CMIP6 climate simulations

Rohan Eccles, Ralph Trancoso, Jozef Syktus, Sarah Chapman, Nathan Toombs, Hong Zhang, Shaoxiu Ma, and Ryan McGloin

Abstract. Climate change is projected to lead to changes in rainfall patterns, which, when coupled with increasing evapotranspiration, has the potential to exacerbate future droughts. This study investigates the impacts of climate change on meteorological droughts in Australia using downscaled high-resolution CMIP6 climate models under three Shared Socioeconomic Pathway (SSP) scenarios. The Standardised Precipitation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI) were used to assess changes to the frequency, duration, percent time, and spatial extent of droughts. There were consistent increases in droughts projected for south-west Western Australia, southern Victoria, southern South Australia, and western Tasmania using SPI and SPEI. There were significantly larger increases for SPEI derived droughts, with consistent increases projected for most of the country. The largest increases occurred at the end of the century and under the high emissions scenario (SSP370), demonstrating the influence of emissions on extreme droughts. For instance, if emissions reached high levels by the end of the century, the area subject to extreme drought in drought prone Southern Australia would be 2.8 greater than if they were kept to low levels using SPI, and 4 times greater if assessed using SPEI. The insights generated from these results and supplementary tailored datasets for Australian Local Government Areas and River Basins are essential to better inform decision making and future adaptation strategies at national, regional, and local scales.

Received: 24 Jul 2024 – Discussion started: 29 Jul 2024

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Journal article(s) based on this preprint

29 Sep 2025

High-resolution downscaled CMIP6 drought projections for Australia

Rohan Eccles, Ralph Trancoso, Jozef Syktus, Sarah Chapman, Nathan Toombs, Hong Zhang, Shaoxiu Ma, and Ryan McGloin

Hydrol. Earth Syst. Sci., 29, 4689–4710, https://doi.org/10.5194/hess-29-4689-2025,https://doi.org/10.5194/hess-29-4689-2025, 2025

Short summary

Rohan Eccles, Ralph Trancoso, Jozef Syktus, Sarah Chapman, Nathan Toombs, Hong Zhang, Shaoxiu Ma, and Ryan McGloin

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2341', Anonymous Referee #1, 12 Nov 2024
The paper describes a set of drought projections for Australia developed by dynamically downscaling CMIP6 global climate models. A possible range of future drought conditions is considered that span multiple emissions pathways and model configurations. Future drought conditions are described through event frequency, duration, spatial extent and time spent in drought, in terms of changes in two commonly used drought metrics. The paper is well-structured, the information is clearly presented and the key results appropriately discussed. I recommend publication after minor revisions, which I describe below.
One key issue is the use of SPEI. This metric, being the difference between precipitation and potential evapotranspiration, is intended to reflect the atmospheric water balance and thereby give a complementary view to SPI-based drought. However, the use of potential evapotranspiration in the calculation of SPEI makes SPEI unrealistic in many water-limited parts of Australia, where actual evapotranspiration does not approach the potential upper limit. So, any projected worsening of PET-related conditions is merely an indication of an increase in atmospheric demand for moisture, rather than a conclusive reduction in water stores. I suggest this issue is more adequately discussed in the paper, including the implications in the interpretation of SPEI-based projections of drought.
The second key issue is the lack of attention given to the uncertainty of the projections. While using a multi-model ensemble and multiple emissions pathways goes some way to addressing uncertainty, the drought projections should be presented along with quantified uncertainty estimates. Moreover, the issue of uncertainty propagation from GCM through to downscaling technique to RCM was not addressed.
The final key issue is that one of the most crucial findings of the study needs to be made more prominent. The results show that more time is projected to be spent under extreme conditions, both wet and dry, and less time under ‘normal’ conditions, for some parts of Australia (Table 3). This result should be made more prominent, for example by featuring in the abstract. This result is important because it suggests that the combination of projected changes in the climate system is shifting the dial towards more extreme climatic conditions and motivates future research in understanding the physical processes responsible for the shift.
Minor comments:
L84: this is a bit of a throw away line. I suggest turning this around by stating that since RCMs have been shown to estimate regional rainfall features with higher precision than GCMs, RCMs are more appropriate to study drought on the regional scale.

Inter-model variability (Figure 11) is shown to be higher for SPEI and some drought characteristics. Can an explanation be offered for why this is? What are the implications of this variability on the interpretation of future drought changes?
Citation: https://doi.org/10.5194/egusphere-2024-2341-RC1
- AC1: 'Reply on RC1', Rohan Eccles, 13 Feb 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2341/egusphere-2024-2341-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2341-AC1
CC1: 'Comment on egusphere-2024-2341', Sivarajah Mylevaganam, 22 Nov 2024

HESS is an open-access journal. For your review, I have included an excerpt from the journal office's official website(https://www.hydrology-and-earth-system-sciences.net/) below.
“Hydrology and Earth System Sciences (HESS) is a not-for-profit international two-stage open-access journal for the publication of original research in hydrology. HESS encourages and supports fundamental and applied research that advances the understanding of hydrological systems, their role in providing water for ecosystems and society, and the role of the water cycle in the functioning of the Earth system. A multi-disciplinary approach is encouraged that broadens the hydrological perspective and the advancement of hydrological science through integration with other cognate sciences and cross-fertilization across disciplinary boundaries.”
Why would the journal office entertain anonymous referees? If the journal is an open-access journal, how could the anonymous referees’ reviews fit into the journal’s statement of openness?
Do the authors have rights to request the journal office to disclose the names of the referees who are scared to disclose their names?
Do the readers of the journal have rights to request the journal office to disclose the names of the referees who are scared to disclose their names?
Could it be possible for the journal office to have clerical staff to post reviews as invited referees? Is this what we expect from EU and its official statements?
What is the relationship between the handling editor and the invited referees? Are the referees paying the handling editors to get them invited?
Why would an invited referee be scared to disclose his/her name in an open-access journal? Is it because he/she is unqualified and lacks potential and expertise in his/her field of specialization to demonstrate in an open-access journal?
I am requesting the handling editor to disclose the reason for promoting anonymity in an open-access journal. I am also requesting the journal office to disclose the rationale behind appointing the handling editors and referees. I request the journal office to disclose academic transcripts of the handling editors and referees. What did they do in their PhD work? Who were their supervisors? What were their GER scores in their postgraduate studies? How many billions have been invested in their PhD philosophies? These details are pertinent to showcase that the EU and the journal office don’t favor appointing handling editors and referees in an open-access journal.

Citation: https://doi.org/10.5194/egusphere-2024-2341-CC1
RC2:
'Comment on egusphere-2024-2341', Anonymous Referee #2, 08 Feb 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2341/egusphere-2024-2341-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-2341-RC2
- AC2: 'Reply on RC2', Rohan Eccles, 13 Feb 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2341/egusphere-2024-2341-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2341-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2341', Anonymous Referee #1, 12 Nov 2024
The paper describes a set of drought projections for Australia developed by dynamically downscaling CMIP6 global climate models. A possible range of future drought conditions is considered that span multiple emissions pathways and model configurations. Future drought conditions are described through event frequency, duration, spatial extent and time spent in drought, in terms of changes in two commonly used drought metrics. The paper is well-structured, the information is clearly presented and the key results appropriately discussed. I recommend publication after minor revisions, which I describe below.
One key issue is the use of SPEI. This metric, being the difference between precipitation and potential evapotranspiration, is intended to reflect the atmospheric water balance and thereby give a complementary view to SPI-based drought. However, the use of potential evapotranspiration in the calculation of SPEI makes SPEI unrealistic in many water-limited parts of Australia, where actual evapotranspiration does not approach the potential upper limit. So, any projected worsening of PET-related conditions is merely an indication of an increase in atmospheric demand for moisture, rather than a conclusive reduction in water stores. I suggest this issue is more adequately discussed in the paper, including the implications in the interpretation of SPEI-based projections of drought.
The second key issue is the lack of attention given to the uncertainty of the projections. While using a multi-model ensemble and multiple emissions pathways goes some way to addressing uncertainty, the drought projections should be presented along with quantified uncertainty estimates. Moreover, the issue of uncertainty propagation from GCM through to downscaling technique to RCM was not addressed.
The final key issue is that one of the most crucial findings of the study needs to be made more prominent. The results show that more time is projected to be spent under extreme conditions, both wet and dry, and less time under ‘normal’ conditions, for some parts of Australia (Table 3). This result should be made more prominent, for example by featuring in the abstract. This result is important because it suggests that the combination of projected changes in the climate system is shifting the dial towards more extreme climatic conditions and motivates future research in understanding the physical processes responsible for the shift.
Minor comments:
L84: this is a bit of a throw away line. I suggest turning this around by stating that since RCMs have been shown to estimate regional rainfall features with higher precision than GCMs, RCMs are more appropriate to study drought on the regional scale.

Inter-model variability (Figure 11) is shown to be higher for SPEI and some drought characteristics. Can an explanation be offered for why this is? What are the implications of this variability on the interpretation of future drought changes?
Citation: https://doi.org/10.5194/egusphere-2024-2341-RC1
- AC1: 'Reply on RC1', Rohan Eccles, 13 Feb 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2341/egusphere-2024-2341-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2341-AC1
CC1: 'Comment on egusphere-2024-2341', Sivarajah Mylevaganam, 22 Nov 2024

HESS is an open-access journal. For your review, I have included an excerpt from the journal office's official website(https://www.hydrology-and-earth-system-sciences.net/) below.
“Hydrology and Earth System Sciences (HESS) is a not-for-profit international two-stage open-access journal for the publication of original research in hydrology. HESS encourages and supports fundamental and applied research that advances the understanding of hydrological systems, their role in providing water for ecosystems and society, and the role of the water cycle in the functioning of the Earth system. A multi-disciplinary approach is encouraged that broadens the hydrological perspective and the advancement of hydrological science through integration with other cognate sciences and cross-fertilization across disciplinary boundaries.”
Why would the journal office entertain anonymous referees? If the journal is an open-access journal, how could the anonymous referees’ reviews fit into the journal’s statement of openness?
Do the authors have rights to request the journal office to disclose the names of the referees who are scared to disclose their names?
Do the readers of the journal have rights to request the journal office to disclose the names of the referees who are scared to disclose their names?
Could it be possible for the journal office to have clerical staff to post reviews as invited referees? Is this what we expect from EU and its official statements?
What is the relationship between the handling editor and the invited referees? Are the referees paying the handling editors to get them invited?
Why would an invited referee be scared to disclose his/her name in an open-access journal? Is it because he/she is unqualified and lacks potential and expertise in his/her field of specialization to demonstrate in an open-access journal?
I am requesting the handling editor to disclose the reason for promoting anonymity in an open-access journal. I am also requesting the journal office to disclose the rationale behind appointing the handling editors and referees. I request the journal office to disclose academic transcripts of the handling editors and referees. What did they do in their PhD work? Who were their supervisors? What were their GER scores in their postgraduate studies? How many billions have been invested in their PhD philosophies? These details are pertinent to showcase that the EU and the journal office don’t favor appointing handling editors and referees in an open-access journal.

Citation: https://doi.org/10.5194/egusphere-2024-2341-CC1
RC2:
'Comment on egusphere-2024-2341', Anonymous Referee #2, 08 Feb 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2341/egusphere-2024-2341-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-2341-RC2
- AC2: 'Reply on RC2', Rohan Eccles, 13 Feb 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2341/egusphere-2024-2341-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-2341-AC2

Peer review completion

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

ED: Publish subject to revisions (further review by editor and referees) (28 Feb 2025) by Shreedhar Maskey

AR by Rohan Eccles on behalf of the Authors (18 Mar 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (02 Apr 2025) by Shreedhar Maskey

RR by Anonymous Referee #3 (23 May 2025)

Suggestions for revision or reasons for rejection

Minor comments
My main issue in the paper is related to the section “2.2 Data”, which I feel could be made clearer. It would be helpful to better present the project for which the downscaling was done, to explain some of the choices made. Why “some model variants were downscaled multiple times” (l.114) ? This sentence seems to come too early, and corresponds to the explanation given l. 124 and Table 1?
How did the authors select their model ensemble? Why are some CMIP6 models downscaled with different set-ups and not others?
This needs to be better explained, especially since in the later sections, we need to understand when it is pertinent to analyse the 15-model ensemble, or rather consider an 11-model average.
It would made the section 2.4 clearer (l. 196 to 204).
Also, the authors mention that the downscaling approach significantly improve the performance on temperature and precipitation than using the coarse scale GCMs. Against which observation data? Since we are in the data section and that observation data are directly mentioned in the following paragraph, I feel this information could be added.
Still in the methodological part, section 2.3, the authors mention a calibration on the historical period to fit the SPI and SPEI distribution in the future period.
I do not understand fully how this was conducted, and also how it fits with the analysis later on of the changes in the SPI and SPEI distributions and shifts in percentiles, if these distributions have been calibrated…
The paragraph from l. 174 to l. 180 needs to be made clearer, with more details given on the transfer function used for the calibration, if there was a calibration. The first section of the results (3.1) seems to imply that there was none, since you look at the biases between observation and simulation. Otherwise, how does the calibration changes the performances? I am confused.

Specific comments for the authors:
Introduction
- There is a redundant information in the introduction, with l.65 to 69 similar to l.84 to 86. Since the structure of your introduction shows: the use of GCM (l.54 to 64), their limits and the use of RCMs (l.65 to 70), but last studies used for CMIP5 (l.71 to 81), I feel you could reorganise slightly l81 to 86. It would avoid the redundancy and better highlight that you want to work with CMIP6, and it is the main novelty of your study.
- To help the method section of the article, I think you could mention the RCM you used in the end of your introduction and detail a bit more the Queensland Future Climate Science Program (QFCSP).
Method
- P.7: you use a notation SPI/SPEI. If I understood correctly, it means “SPI or SPEI”. Be careful, it looks like a ratio. I feel this need to be changed.
- Sentence l. 212-213 is not clear. A percentile can not be a shift. The sentence is not correct. Ta define a shift, you need a reference. So I think I understand what you mean. If the 50th percentile in the projected distribution matches the 40th percentile in the historical period, therefore there is a 10% shift towards dryness.
Results
I feel the results are very well explained with pertinent illustration. Still I have a few comments.
- Section 3.2.2: how do you define the “area affected by droughts”? Is it all pixels for which there is at least one drought event (depending on the severity of the drought event defined)?
- L. 352-354: unclear : “differences between the 10th and 90th percentiles”?
Discussion
- L. 383: “time in” ?
- L. 424: What does “this” refer to? The elevated PET?
- L. 454: “may better”  “would rather” ?

Referee Report: PDF

Hide

ED: Publish subject to revisions (further review by editor and referees) (01 Jun 2025) by Shreedhar Maskey

AR by Rohan Eccles on behalf of the Authors (09 Jun 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (18 Jun 2025) by Shreedhar Maskey

RR by Anonymous Referee #3 (26 Jun 2025)

ED: Publish subject to technical corrections (04 Jul 2025) by Shreedhar Maskey

ED: Publish subject to technical corrections (09 Jul 2025) by Giuliano Di Baldassarre (Executive editor)

AR by Rohan Eccles on behalf of the Authors (15 Jul 2025) Author's response Manuscript

Journal article(s) based on this preprint

29 Sep 2025

High-resolution downscaled CMIP6 drought projections for Australia

Rohan Eccles, Ralph Trancoso, Jozef Syktus, Sarah Chapman, Nathan Toombs, Hong Zhang, Shaoxiu Ma, and Ryan McGloin

Hydrol. Earth Syst. Sci., 29, 4689–4710, https://doi.org/10.5194/hess-29-4689-2025,https://doi.org/10.5194/hess-29-4689-2025, 2025

Short summary

Rohan Eccles, Ralph Trancoso, Jozef Syktus, Sarah Chapman, Nathan Toombs, Hong Zhang, Shaoxiu Ma, and Ryan McGloin

Supplement

https://doi.org/10.5194/egusphere-2024-2341-supplement

Rohan Eccles, Ralph Trancoso, Jozef Syktus, Sarah Chapman, Nathan Toombs, Hong Zhang, Shaoxiu Ma, and Ryan McGloin

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Month	HTML	PDF	XML	Total
Jul 2024	50	17	2	69
Aug 2024	96	43	107	246
Sep 2024	46	15	2	63
Oct 2024	23	14	0	37
Nov 2024	100	32	4	136
Dec 2024	42	10	0	52
Jan 2025	30	13	1	44
Feb 2025	63	10	3	76
Mar 2025	13	17	0	30
Apr 2025	55	22	1	78
May 2025	46	19	1	66
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Jul 2025	47	8	2	57
Aug 2025	136	11	2	149
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Rainfall and evaporation are two key variables influencing when droughts occur and their severity. We use the latest climate simulations for Australia to see how changes to rainfall and evaporation influence future droughts for Australia and show increases are likely over most of Australia, especially in the south. Increases to evaporation are shown to be larger than changes to rainfall over most of the continent. We show that keeping emissions to lower levels can work to mitigate these impacts.


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Meteorological drought projections for Australia from downscaled high-resolution CMIP6 climate simulations

Journal article(s) based on this preprint

Interactive discussion

Interactive discussion

Peer review completion

Suggestions for revision or reasons for rejection

Suggestions for revision or reasons for rejection

Journal article(s) based on this preprint

Supplement

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