Explaining changes in rainfall-runoff relationships during and after Australia's Millennium Drought: a community perspective

Fowler, Keirnan; Peel, Murray; Saft, Margarita; Peterson, Tim; Western, Andrew; Band, Lawrence; Petheram, Cuan; Dharmadi, Sandra; Tan, Kim Seong; Zhang, Lu; Lane, Patrick; Kiem, Anthony; Marshall, Lucy; Griebel, Anne; Medlyn, Belinda; Ryu, Dongryeol; Bonotto, Giancarlo; Wasko, Conrad; Ukkola, Anna; Stephens, Clare; Frost, Andrew; Weligamage, Hansini; Saco, Patricia; Zheng, Hongxing; Chiew, Francis; Daly, Edoardo; Walker, Glen; Vervoort, R. Willem; Hughes, Justin; Trotter, Luca; Neal, Brad; Cartwright, Ian; Nathan, Rory

doi:https://doi.org/10.5194/hess-2022-147

Preprints

https://doi.org/10.5194/hess-2022-147

Preprints

20 Apr 2022

Status: a revised version of this preprint was accepted for the journal HESS and is expected to appear here in due course.

Explaining changes in rainfall-runoff relationships during and after Australia's Millennium Drought: a community perspective

Keirnan Fowler¹, Murray Peel¹, Margarita Saft¹, Tim Peterson², Andrew Western¹, Lawrence Band^3,4, Cuan Petheram⁵, Sandra Dharmadi⁶, Kim Seong Tan⁷, Lu Zhang⁸, Patrick Lane⁹, Anthony Kiem¹⁰, Lucy Marshall^11,12, Anne Griebel¹³, Belinda Medlyn¹³, Dongryeol Ryu¹, Giancarlo Bonotto¹, Conrad Wasko¹, Anna Ukkola¹⁴, Clare Stephens¹³, Andrew Frost¹⁵, Hansini Weligamage¹, Patricia Saco¹⁰, Hongxing Zheng⁸, Francis Chiew⁸, Edoardo Daly², Glen Walker¹⁶, R. Willem Vervoort¹², Justin Hughes⁸, Luca Trotter¹, Brad Neal¹⁷, Ian Cartwright¹⁸, and Rory Nathan¹ Keirnan Fowler et al.

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¹Department of Infrastructure Engineering, The University of Melbourne, Parkville, Victoria, Australia
²Department of Civil Engineering, Monash University, Clayton, Victoria, Australia
³Department of Environmental Science, University of Virginia, Charlottesville, Virginia, USA
⁴Department of Engineering Systems and Environment, University of Virginia, Charlottesville, Virginia, USA
⁵CSIRO Land and Water, Sandy Bay, Tasmania, Australia
⁶Department of Environment, Land, Water and Planning, Melbourne, Victoria, Australia
⁷Melbourne Water, Docklands, Victoria, Australia
⁸CSIRO Land and Water, Black Mountain, Australian Capital Territory, Australia
⁹School of Ecosystem and Forest Sciences, University of Melbourne, Parkville, Victoria, Australia
¹⁰Centre for Water, Climate and Land (CWCL), College of Engineering, Science and Environment (CESE), University of Newcastle, Newcastle, New South Wales, Australia
¹¹School of Civil and Environmental Engineering, University of New South Wales, Kensington, New South Wales, Australia
¹²ARC Training Centre Data Analytics for Resources and Environments, School of Life and Environmental Sciences, The University of Sydney, Camperdown, New South Wales, Australia
¹³Hawkesbury Institute for the Environment, Western Sydney University, Richmond, New South Wales, Australia
¹⁴Climate Change Research Centre, University of New South Wales, Kensington, New South Wales, Australia
¹⁵Bureau of Meteorology, Sydney, New South Wales, Australia
¹⁶Grounded In Water, Adelaide, South Australia, Australia
¹⁷Hydrology and Risk Consulting (HARC), Blackburn, Victoria, Australia
¹⁸School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia

¹Department of Infrastructure Engineering, The University of Melbourne, Parkville, Victoria, Australia
²Department of Civil Engineering, Monash University, Clayton, Victoria, Australia
³Department of Environmental Science, University of Virginia, Charlottesville, Virginia, USA
⁴Department of Engineering Systems and Environment, University of Virginia, Charlottesville, Virginia, USA
⁵CSIRO Land and Water, Sandy Bay, Tasmania, Australia
⁶Department of Environment, Land, Water and Planning, Melbourne, Victoria, Australia
⁷Melbourne Water, Docklands, Victoria, Australia
⁸CSIRO Land and Water, Black Mountain, Australian Capital Territory, Australia
⁹School of Ecosystem and Forest Sciences, University of Melbourne, Parkville, Victoria, Australia
¹⁰Centre for Water, Climate and Land (CWCL), College of Engineering, Science and Environment (CESE), University of Newcastle, Newcastle, New South Wales, Australia
¹¹School of Civil and Environmental Engineering, University of New South Wales, Kensington, New South Wales, Australia
¹²ARC Training Centre Data Analytics for Resources and Environments, School of Life and Environmental Sciences, The University of Sydney, Camperdown, New South Wales, Australia
¹³Hawkesbury Institute for the Environment, Western Sydney University, Richmond, New South Wales, Australia
¹⁴Climate Change Research Centre, University of New South Wales, Kensington, New South Wales, Australia
¹⁵Bureau of Meteorology, Sydney, New South Wales, Australia
¹⁶Grounded In Water, Adelaide, South Australia, Australia
¹⁷Hydrology and Risk Consulting (HARC), Blackburn, Victoria, Australia
¹⁸School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia

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Received: 12 Apr 2022 – Discussion started: 20 Apr 2022

Abstract. The Millennium Drought lasted more than a decade, and is notable for causing persistent shifts in the relationship between rainfall and runoff in many south-east Australian catchments. Research to date has successfully characterised where and when shifts occurred and explored relationships with potential drivers, but a convincing physical explanation for observed changes in catchment behaviour is still lacking. Originating from a large multi-disciplinary workshop, this paper presents a range of possible process explanations of flow response, and then evaluates these hypotheses against available evidence. The hypotheses consider climatic forcing, vegetation, soil moisture dynamics, groundwater, and anthropogenic influence. The hypotheses are assessed against evidence both temporally (eg. why was the Millennium Drought different to previous droughts?) and spatially (eg. why did rainfall-runoff relationships shift in some catchments but not in others?). The results point to the unprecedented length of the drought as the primary climatic driver, paired with interrelated groundwater processes, including: declines in groundwater storage, reduced recharge associated with vadose zone expansion, and reduced connection between subsurface and surface water processes. Other causes include increased evaporative demand and interception of runoff by small private dams. Finally, we discuss the need for long-term field monitoring, particularly targeting internal catchment processes and subsurface dynamics. We recommend continued investment in understanding of hydrological shifts, particularly given their relevance to water planning under climate variability and change.

How to cite. Fowler, K., Peel, M., Saft, M., Peterson, T., Western, A., Band, L., Petheram, C., Dharmadi, S., Tan, K. S., Zhang, L., Lane, P., Kiem, A., Marshall, L., Griebel, A., Medlyn, B., Ryu, D., Bonotto, G., Wasko, C., Ukkola, A., Stephens, C., Frost, A., Weligamage, H., Saco, P., Zheng, H., Chiew, F., Daly, E., Walker, G., Vervoort, R. W., Hughes, J., Trotter, L., Neal, B., Cartwright, I., and Nathan, R.: Explaining changes in rainfall-runoff relationships during and after Australia's Millennium Drought: a community perspective, Hydrol. Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/hess-2022-147, in review, 2022.

Keirnan Fowler et al.

Status: closed

CC1:
'Additional study on rainfall-runoff relationships shift in Europe', Christian Massari, 25 Apr 2022

Dear Authors, congratulations for this excellent work! You might find useful also this recent study on the topic, where the role of evaporation has been investigated as a potential driver to shift the rainfall-runoff relationship in Europe. The study has been published on this journal just some days ago. https://hess.copernicus.org/articles/26/1527/2022/hess-26-1527-2022.pdf

Massari, C., Avanzi, F., Bruno, G., Gabellani, S., Penna, D., Camici, S., 2022. Evaporation enhancement drives the European water-budget deficit during multi-year droughts. Hydrology and Earth System Sciences 26, 1527–1543. https://doi.org/10.5194/hess-26-1527-2022

Citation: https://doi.org/10.5194/hess-2022-147-CC1
- AC2: 'Reply to Christian Massari', Keirnan Fowler, 13 Sep 2022
  
  Thank you very much for bringing this to our attention. The paper is indeed very relevant to this work. We have cited it in the very first paragraph, as part of describing the global context.
  
  Citation: https://doi.org/10.5194/hess-2022-147-AC2
RC1:
'Comment on hess-2022-147', Dengfeng Liu, 16 May 2022

General comments:

The manuscript presents a range of possible process explanations of flow response to the Millennium Drought in Australia, and then evaluates these hypotheses against available evidence. The manuscript is an excellent work to understand the changes in rainfall-runoff relationships after Australia's Millennium Drought. The strength of this work is a large-scale assessment of hydrologic changes and potential drivers. The framework of Hypothesised Process Explanations is also useful to investigate the effects of the drought in other watersheds, and planning more extensive field studies to test predictions of hypotheses.

Specific comments:

In Figure 2, the data of precipitation and runoff from 2011 to 2021 should also be presented to show the hydrological behavior after the Millennium Drought.

Line 234, The manuscript focus on the changes of rainfall-runoff relationships, the annnual runoff coefficient, and those in each season may be necessary to discuss.

The multiple stable states of the watershed may be a potential perspective to explain the change of the behavior of the rainfall-runoff relationship, as mentioned in Line 379, such as that in Peterson et al. (2009). The Millennium Drought is an extreme disturbance that may push the system from one stable state to another. The question is how to quantify the multiple stable states of the watershed. The dry stable state may be seldom presented in the watershed.

Peterson T J, Argent R M, Western A W, Chiew F H S. Multiple stable states in hydrological models: An ecohydrological investigation, Water Resources Research, 2009, 45, W03406, doi:10.1029/2008WR006886.

If the water storage capacity in a watershed is large enough to control all/most of the annual runoff (associated with HPE23), the watershed will be a human-controlled system where the released runoff is regulated by the reservoirs, such as Tarim River basin in China (Liu et al., 2014; Liu et al., 2015). The total water storage capacity of all dams in a watershed may be an important index.

Liu, Y., Tian, F., Hu, H., Sivapalan, M. Socio-hydrologic perspectives of the co-evolution of humans and water in the Tarim River basin, Western China: the Taiji–Tire model[J]. Hydrol. Earth Syst. Sci., 2014, 18, 1289-1303.

Liu D, Tian F, Lin M, Sivapalan M. A conceptual socio-hydrological model of the co-evolution of humans and water: case study of the Tarim River basin, western China[J]. Hydrology and Earth System Sciences, 2015, 19(2): 1035-1054.

Line 570, the spatial distribution of the driving factor should be consistent with the spatial distribution of shifted versus unshifted catchments. Maybe an example will be helpful to understand it.

Line 600, Of the twenty-four HPEs, three are considered plausible, ten are considered inconsistent with evidence, and eleven are in a category in-between. The strength of this work is a large-scale assessment of hydrologic changes and potential drivers. This information should be stated in abstract.

In Figure 10, Higher AET per mm of rainfall, and it equals aridity index=AET/rainfall.

Technical corrections:

L227 and L240, event rainfall->rainfall events

Line 714, check the citation of Figure 4c. Maybe it is Figure 5d.

Citation: https://doi.org/10.5194/hess-2022-147-RC1
- AC1: 'Reply to reviewer Dengfeng Liu', Keirnan Fowler, 13 Sep 2022
  
  Thank you for the supportive comments and constructive feedback. Please see the attached table of responses.
  
  Citation: https://doi.org/10.5194/hess-2022-147-AC1
RC2:
'Comment on hess-2022-147', Markus Hrachowitz, 24 May 2022

The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2022-147/hess-2022-147-RC2-supplement.pdf

Citation: https://doi.org/10.5194/hess-2022-147-RC2
- AC3: 'Reply to Markus Hrachowitz', Keirnan Fowler, 13 Sep 2022
  
  Thank you for the very detailed feedback and encouragement! Please see the attached table of responses.
  
  Citation: https://doi.org/10.5194/hess-2022-147-AC3

Status: closed

CC1:
'Additional study on rainfall-runoff relationships shift in Europe', Christian Massari, 25 Apr 2022

Dear Authors, congratulations for this excellent work! You might find useful also this recent study on the topic, where the role of evaporation has been investigated as a potential driver to shift the rainfall-runoff relationship in Europe. The study has been published on this journal just some days ago. https://hess.copernicus.org/articles/26/1527/2022/hess-26-1527-2022.pdf

Massari, C., Avanzi, F., Bruno, G., Gabellani, S., Penna, D., Camici, S., 2022. Evaporation enhancement drives the European water-budget deficit during multi-year droughts. Hydrology and Earth System Sciences 26, 1527–1543. https://doi.org/10.5194/hess-26-1527-2022

Citation: https://doi.org/10.5194/hess-2022-147-CC1
- AC2: 'Reply to Christian Massari', Keirnan Fowler, 13 Sep 2022
  
  Thank you very much for bringing this to our attention. The paper is indeed very relevant to this work. We have cited it in the very first paragraph, as part of describing the global context.
  
  Citation: https://doi.org/10.5194/hess-2022-147-AC2
RC1:
'Comment on hess-2022-147', Dengfeng Liu, 16 May 2022

General comments:

The manuscript presents a range of possible process explanations of flow response to the Millennium Drought in Australia, and then evaluates these hypotheses against available evidence. The manuscript is an excellent work to understand the changes in rainfall-runoff relationships after Australia's Millennium Drought. The strength of this work is a large-scale assessment of hydrologic changes and potential drivers. The framework of Hypothesised Process Explanations is also useful to investigate the effects of the drought in other watersheds, and planning more extensive field studies to test predictions of hypotheses.

Specific comments:

In Figure 2, the data of precipitation and runoff from 2011 to 2021 should also be presented to show the hydrological behavior after the Millennium Drought.

Line 234, The manuscript focus on the changes of rainfall-runoff relationships, the annnual runoff coefficient, and those in each season may be necessary to discuss.

The multiple stable states of the watershed may be a potential perspective to explain the change of the behavior of the rainfall-runoff relationship, as mentioned in Line 379, such as that in Peterson et al. (2009). The Millennium Drought is an extreme disturbance that may push the system from one stable state to another. The question is how to quantify the multiple stable states of the watershed. The dry stable state may be seldom presented in the watershed.

Peterson T J, Argent R M, Western A W, Chiew F H S. Multiple stable states in hydrological models: An ecohydrological investigation, Water Resources Research, 2009, 45, W03406, doi:10.1029/2008WR006886.

If the water storage capacity in a watershed is large enough to control all/most of the annual runoff (associated with HPE23), the watershed will be a human-controlled system where the released runoff is regulated by the reservoirs, such as Tarim River basin in China (Liu et al., 2014; Liu et al., 2015). The total water storage capacity of all dams in a watershed may be an important index.

Liu, Y., Tian, F., Hu, H., Sivapalan, M. Socio-hydrologic perspectives of the co-evolution of humans and water in the Tarim River basin, Western China: the Taiji–Tire model[J]. Hydrol. Earth Syst. Sci., 2014, 18, 1289-1303.

Liu D, Tian F, Lin M, Sivapalan M. A conceptual socio-hydrological model of the co-evolution of humans and water: case study of the Tarim River basin, western China[J]. Hydrology and Earth System Sciences, 2015, 19(2): 1035-1054.

Line 570, the spatial distribution of the driving factor should be consistent with the spatial distribution of shifted versus unshifted catchments. Maybe an example will be helpful to understand it.

Line 600, Of the twenty-four HPEs, three are considered plausible, ten are considered inconsistent with evidence, and eleven are in a category in-between. The strength of this work is a large-scale assessment of hydrologic changes and potential drivers. This information should be stated in abstract.

In Figure 10, Higher AET per mm of rainfall, and it equals aridity index=AET/rainfall.

Technical corrections:

L227 and L240, event rainfall->rainfall events

Line 714, check the citation of Figure 4c. Maybe it is Figure 5d.

Citation: https://doi.org/10.5194/hess-2022-147-RC1
- AC1: 'Reply to reviewer Dengfeng Liu', Keirnan Fowler, 13 Sep 2022
  
  Thank you for the supportive comments and constructive feedback. Please see the attached table of responses.
  
  Citation: https://doi.org/10.5194/hess-2022-147-AC1
RC2:
'Comment on hess-2022-147', Markus Hrachowitz, 24 May 2022

The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2022-147/hess-2022-147-RC2-supplement.pdf

Citation: https://doi.org/10.5194/hess-2022-147-RC2
- AC3: 'Reply to Markus Hrachowitz', Keirnan Fowler, 13 Sep 2022
  
  Thank you for the very detailed feedback and encouragement! Please see the attached table of responses.
  
  Citation: https://doi.org/10.5194/hess-2022-147-AC3

Keirnan Fowler et al.

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Short summary

Recently, we have seen a tendency for multi-year droughts to cause shifts in the relationship between rainfall and streamflow. In shifted catchments that have not recovered, an average rainfall year produces less streamflow today than it did pre-drought. We take a multi-disciplinary approach to understand why these shifts occur, focusing on Australia's 10+ year "Millennium" drought. We evaluate multiple hypotheses against evidence, with particular focus on the key role of groundwater processes.


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