20 Apr 2022
20 Apr 2022
Status: a revised version of this preprint is currently under review for the journal HESS.

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 Band3,4, Cuan Petheram5, Sandra Dharmadi6, Kim Seong Tan7, Lu Zhang8, Patrick Lane9, Anthony Kiem10, Lucy Marshall11,12, 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.
  • 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

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.

Keirnan Fowler et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Additional study on rainfall-runoff relationships shift in Europe', Christian Massari, 25 Apr 2022
    • AC2: 'Reply to Christian Massari', Keirnan Fowler, 13 Sep 2022
  • RC1: 'Comment on hess-2022-147', Dengfeng Liu, 16 May 2022
    • AC1: 'Reply to reviewer Dengfeng Liu', Keirnan Fowler, 13 Sep 2022
  • RC2: 'Comment on hess-2022-147', Markus Hrachowitz, 24 May 2022
    • AC3: 'Reply to Markus Hrachowitz', Keirnan Fowler, 13 Sep 2022

Keirnan Fowler et al.

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.