| 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.
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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