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

Abstract. The Millennium Drought lasted more than a decade and is notable for causing persistent shifts in the relationship between rainfall runoff many southeastern Australian catchments. Research to date has successfully characterised where when occurred explored relationships with potential drivers, but convincing physical explanation observed changes catchment behaviour still lacking. Originating from large multi-disciplinary workshop, this paper presents evaluates range of hypothesised process explanations flow response Drought. hypotheses consider climatic forcing, vegetation, soil moisture dynamics, groundwater, anthropogenic influence. are assessed against evidence both temporally (e.g. why was different previous droughts?) spatially did rainfall–runoff shift some catchments not others?). Thus, strength work large-scale assessment hydrologic drivers. Of 24 hypotheses, 3 considered plausible, 10 inconsistent evidence, 11 category between, whereby they plausible yet reservations applicable others). results point unprecedented length drought as primary driver, paired interrelated groundwater processes, including declines storage, altered recharge associated vadose zone expansion, reduced connection subsurface surface water processes. Other causes include increased evaporative demand harvesting by small private dams. Finally, we discuss need long-term field monitoring, particularly targeting internal processes dynamics. We recommend continued investment understanding hydrological shifts, given their relevance planning under climate variability change.