the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Feb 2026
Comparing drivers of hydrological shifts across regions: the case of southern Australia
Abstract. Several regions globally have recently experienced persistent shifts in the relationship between rainfall and runoff, triggered by multi-annual drought. These regions are climatically diverse; however, few assessments have yet been undertaken to draw parallels (if any) between the processes responsible. We present a comparative analysis of these hydrological shifts between south-west Australia and south-east Australia, two regions separated by over 2,700 km (~1,700 miles). We apply existing methods based on Hidden Markov modelling to characterise shifts in rainfall-runoff relationships in 254 catchments in Eastern and 54 in Western Australia. Of the catchments analysed, 51 % of Eastern and 63 % of Western catchments displayed a movement away from the historical rainfall-runoff relationship to one of reduced flow generation following a multi-year period of drier climate. The reduced flow state persisted in 31 % of catchments in Eastern Australia despite a return to near-normal climatic conditions after multi-year drought, whereas in Western Australia neither the climate nor the flow states have returned to earlier norms (i.e. nearly all shifted catchments have stayed shifted). Interestingly, some catchment characteristics that were correlated with shifts in one region were anticorrelated in the other, possibly indicative of different causative processes. For example, in Western Australia the shifted catchments are typically those that have not been cleared for agriculture and thus retain forest coverage; the opposite is true in Eastern Australia. We suggest a possible link to pre-existing trends in groundwater for cleared catchments, where those in Western Australia may have been experiencing rising groundwater levels due to clearing occurring recently (mid-1900s) relative to Eastern Australia (late-1800s). These findings suggest the importance of land use history when considering changes in rainfall-runoff relationship. We recommend further comparative studies be conducted to synthesise understanding across geographies and better inform water planning decisions under climate change.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Hydrology and Earth System Sciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
(2325 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-378', Anonymous Referee #1, 03 Mar 2026
-
RC2: 'Comment on egusphere-2026-378', Anonymous Referee #2, 15 May 2026
Reviewing Comparing drivers of Hydrological shifts across regions: the case of southern Australia. HESS
The authors explore the dynamic of the rainfall/runoff relationships by comparing a number of catchments in two diverse southern Australian areas. In both cases, more than half of the catchments have exhibited a flow reduction compared to previous conditions, with some catchments in the east and none in the west returning to previous conditions. To explain this behaviour, the authors explore the role of groundwater and the influence of LULC change on groundwater. The manuscript is grammatically well written, but it may benefit from more proofreading. Logically, the cohesion and coherence of the paper spark some concerns, as groundwater sounds a bit like a posterior addition to try explaining the model results better. Although it is treated properly in the introduction, it does not give the same feeling throughout the paper. The model setup and other methodological choices need to be clarified better. The paper would benefit from an improved discussion section, especially 3.2. Figures 1, 4, 5, and 6 need improvements before being acceptable for publication. Only after thoroughly answering all comments can the paper be considered for publication.
A list of major comments and concerns:- The main concern is related to the comparability of the two areas in South Australia that are not climatically linked. This becomes especially relevant for the temporal analysis within the first research question, where different climatic drivers are acting. I don't fully get the rationale for the selection of these two areas, especially when you look deeper, and in the supplementary information, you say "...the intent with the selection of eastern catchments was to ensure they were not too different from western...", which I don't see as a very solid choice. Can you justify your selection better? At the moment, the conclusions drawn from the analysis might be hindered by the different nature of these areas. Would it not be better to restructure the analysis, grouping the catchment by some climatic indices?
- Can you discuss how the Hidden Markov model deals with different population sizes, and, more generally, the uncertainty of the model? You have a ratio of 5 to 1 catchment east to west. I think the method is still fine as long as you acknowledge a wider uncertainty in the west. An assessment of the model uncertainty would also be positively seen.
- I could not see anything about soil types in your discussion, while in the CAMELS-AUS v2 dataset, soils are present (as in your supplementary material). Please elaborate on the effects of soil type on your analysis, and probably add a sentence about this in the limitations.
- You suggest groundwater to partially explain the cause of the different behaviour from east to west. To check the importance of this, you should differentiate catchments with a deep groundwater level, where groundwater is disconnected from the surface water, from others with high surface water/ groundwater interaction. In this way, you can test your assumptions. Have you tried anything like this?
- In the introduction section, references need to be better distributed. At present, there are several claims not appropriately backed up by the associated reference.
- Figure 1 is not particularly clear. The background colours confound the main message of where the red-marked catchments are. Figure 3 does this in a much better way. It is not clear what the endpoints of the arrow are (Perth to Melbourne?). Is the change in precipitation expressed in %?
- I recall the model was introduced by Peterson in 2021. However, can you improve the model description in this paper to avoid flipping too much between papers? How was the model run (i.e. each catchment individually)? Did you produce different annual models per catchment, as in Petersen et al. 2021, accounting for effects such as temporal autocorrelation or lags with climatic inputs?
- How will the choice of the annual data affect the variability due to the seasonality of precipitation? This might be particularly relevant for catchments exhibiting low synchronicity between climatic drivers (such as the high PET in summer, high rainfall in winter or the Mediterranean-like climate in Western and South Australia). Is it what you are showing in Figure A1? Can you elaborate on this?
- Line 221-222 and Figure 4. You mention a change in the rainfall-runoff relationship, but Figure 4 only shows flows. Better to try quantifying rather than saying "clear peak". Are these “two part” related, or is something missing? How did you select the groups in Figure 4?
- The conclusion section is pretty thin and needs improvement to convey the message better.
Minor comments
- Line 136, 150, 290, etc…. Consider not starting the sentence with these circumlocutions.
- Line 165. Why did you Box-Cox transform and then use a gamma distribution? The gamma distribution could have fitted the raw data already, but once you transformed wasn't it better to go for a normal distribution?
- Line 182-184, can you elaborate on how you chose 1997? Was it by your method or another? In the second case, you might need a citation.
- Line 210. Avoid using non-quantifiable adverbs.
- Line 210b. "To the east" of what?
- Line 233. A bit confusing.
- Figure 5. I'm having some trouble understanding the x-axis. Isn't it just catchments, rather than proportion? The lines 229-235 do not do a good job either. To help me understand it better, can you, for example, explain what drives these drops in the proportion of areas forested for the shifted catchments of WA, like the one around x=0.58?
- Fig 6. The text is disproportionated compared to the rest of the figure.
- How did you choose the results presented in Figure 7? Why are you only talking about 3 metrics out of 118? Wouldn't it better to present a table rather than a figure?
- Can you elaborate/speculate about the sentence at lines 284-285?
- Section 3.2 is cumbersome and does not feel like making the point.
- Line 303. Instead of "proposing" trends in groundwater levels, it would be better to present a couple of case studies to back up your suggestion.
- Section 3.2.1 Can you say land clearing is a cause of what you want to explore if there is such a large time lag between the cause and the state of equilibrium? Can't something else also playing a hidden effect here?
- Line 371. What is "our" east region?
- Line 382. Again "our". Now the paper sounds like a study of a group of the Eastern part of Australia to which some catchments have to be added in the west to try explaining something.
Citation: https://doi.org/10.5194/egusphere-2026-378-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 844 | 546 | 123 | 1,513 | 81 | 86 |
- HTML: 844
- PDF: 546
- XML: 123
- Total: 1,513
- BibTeX: 81
- EndNote: 86
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Review of “Comparing drivers of hydrological shifts across regions: the case of southern Australia” by Campion and co-authors
General comment
Campion and co-authors present a comparative analysis of characteristics and causes of hydrological shifts following drying in southern Australia, by means of Hidden Markov modelling and a correlation analysis. They further put their findings in the context of groundwater and land-use changes, for which they provide some evidence from available – sparse - data. In so doing, they illustrate widespread shifts in rainfall-runoff (R-R) relationships in both south-west and -east Australia during the Millennium drought around the beginning of the century, but different possible causes for such shifts (evapotranspiration sustainment by high groundwater tables in forested catchments in the south-west, and low groundwater tables in cleared catchments in the south-east). The topic is timely and of interest to HESS readership, given the increasing evidence of worldwide changes in catchment behaviour under drying, our still poor understanding of their causes, and the socio-ecological implications of such changes. While the authors rely on well-established methods to investigate the problem at hand and standard statistical analyses, they present some interesting new insights on the topic, and I commend the manuscript for its comparative nature and the amount of data – from different sources – brought together. I believe that this can be considered suitable for publication in HESS after a round of revisions to address my comments below, mostly related to clarification on data consistency and the proposed interpretation of results.
Major comments
1. Streamflow dataset consistency
I think more information on this point is needed. The authors thoroughly describe how they selected catchments according to their hydroclimate, but they fall short in the rationale for selecting catchments according to data availability, if any. They disclaim that they analyse a varying number of catchments over the years (caption of Fig. 4) and some infilling for missing data (L138–141). But have thresholds on maximum missing data allowed and minimum number of years with available streamflow observations been set? I ask the authors to consider these points and clarify them in the text. Also, adding the number of analysed catchments over the years in Fig. 4 would be appreciated.
2. Correlation analysis
The authors explore correlation between 118 (!!) catchment properties and the magnitude of shifts in R-R relationships, but they only report results in Fig. 7 and in the text for a bunch of them. I see that reporting all correlation values, even in an appendix or supplementary, may be overwhelming, but I wonder whether the authors could find a way to summarize them, for instance by reporting the maximum correlation for property type (e.g., climate/land-cover).
3. Groundwater data and proposed explanation for hydrological shifts
In the discussion, the authors introduce data from 9 groundwater wells to support their hypothesis of diverging causes for hydrological shifts in southern Australia, together with information on land-use changes. While I believe introducing the groundwater data in the discussion only is a legitimate choice, I recommend adding more information on this dataset (e.g., in Sect. 2 or in an appendix as done for the land-use datasets). How were the groundwater wells selected? I would assume for data availability, but this is not stated. Where are the wells located? Showing their location in a map with respect to the catchments under study would be interesting to me. Are the groundwater wells potentially influenced by human activities? Are data before 1990 available? I believe that data even from just a few wells before the 1990s would be highly beneficial to support the hypothesis of still increasing groundwater tables at the onset of the Millennium drought because of land clearing in the 1940s. If no data is available, could the authors better discuss how this hypothesis fits in the broader groundwater literature and previous works on shifts in south-western Australia during the Millennium drought because of decreases in groundwater contribution (e.g., Kinal & Stoneman, 2012 and others already cited in the introduction)? An alternative (or add-on) I see to the use of data before 1990 would be showing also groundwater data for catchments without (or with low) clearing in south-western Australia, if available. Also, is the reported rainfall decline since the 70s similar in western and eastern Australia? Adding information on this should be rather straightforward for the authors from the dataset they currently use and it would support more strongly the point of differences between the two regions because of differences in land-use history. In summary, I believe the authors could better work out their proposed mechanistic explanation of causes in shifts in south-western Australia showing some additional data, if available, and adding some more discussion.
4. Language
The paper is well written, even though I found it at times slightly difficult to follow, in two directions in particular. First, the authors refer quite often to previous works for their datasets and methods (e.g., Sect. 2.2). I believe providing some more details on the main characteristics of the datasets and methods used, rather than just referring to previous papers, would help the readers. Second, in the discussion Sect. 3.2 the main points are not so easy to grasp (also because of the many interactions in place). I suggest the authors streamlining this part. Maybe a summary conceptual figure could help? Not required, just as an idea that I leave the authors to decide on.
Minor comments
5. L11–12 and L87, as a reader not particularly familiar with this geographical area, I would appreciate here some information on catchment differences between the two regions, other than the distance one to another only, to better appreciate the variety of the case study.
6. Abstract, I would suggest clarifying the study period and when the reported shifts occurred.
7. L118, could the authors please quantify this? Also, could focusing on arid catchments (aridity as defined by the authors less than 1) be maybe more informative?
8. Fig. 1, the text in panels b-d is rather small to read, I wonder whether a different arrangement of this figure could help in making these panels bigger and thus allow to increase font size. Also, I would find useful information on land use here as well, given the central role in the manuscript.
9. L162–163, could the authors please provide more details on how the magnitude and timing of onset of hydrological shifts are estimated? This would ease the readers in following the work without having to overly rely on previous works in my view (see comment #3).
10. L186, why was the year 1991 chosen?
11. L198–200, I assume these numbers relate to Fig. 3, which I would suggest placing before current Fig. 2.
12. L220–222, isn’t this because the normal state is defined as the flow in 1975 (L178)?
13. L320–321, what exactly the authors did here is not totally clear to me. I believe that with this sentence they mean that they consider wells with data over the common period 1990–2024 and they use different y axes, however, I would suggest clarifying the data pre-processing, if any.
14. L363–369, could the authors actually show these differences in e.g. maps? It seems to me that they have all the required information from their dataset anyway.
15. Fig. 9, what do the abbreviations on the y axis stand for?
Technical corrections
16. Sect. 3 is repeated twice.
References
Kinal, J., & Stoneman, G. L. (2012). Disconnection of groundwater from surface water causes a fundamental change in hydrology in a forested catchment in south-western Australia. Journal of Hydrology, 472–473, 14–24. https://doi.org/10.1016/j.jhydrol.2012.09.013