Hydrological shifts, or changes in the ratio between rainfall and runoff caused by multi-year drought may largely impact urban water supply, agriculture, environment and hydropower generation. A thorough understanding and robust modelling of the hydrological response to multi-year drought are essential for developing water management strategies. Research so far has managed to establish where and when shifts take place, but convincing explanations about underlying processes are still lacking in many cases. There are still many open questions, such as the limited information on historical multi-year drought, insufficient observations of all components relevant for the catchment water balance and modelling ability in highly variable climates.
Now, Keirnan Fowler from The University of Melbourne and colleagues have focused on a case study of the Millennium Drought, an informative multi-year drought causing persistent hydrological shifts — more specifically flow reduction — in many catchments in Australia. First, the authors hypothesized 24 possible process explanations for the hydrological shifts, considering climatic forcing, vegetation, soil moisture dynamics, groundwater, and anthropogenic influence. Then, they systematically tested these hypotheses based on the consistency with diagnostic evidence both temporally and spatially.
The results suggest three plausible process explanations considering climatic drivers and interrelated groundwater processes: long-term or long-memory processes activated by the multi-year drought, reduced subsurface and surface water linkage, and sustained removal of groundwater via multiple fluxes. Other causes include increased evaporation demand, vegetation, agricultural practices and farm dams.
While the paper focuses on individual mechanisms, the authors addressed the need for taking into account and quantifying potential process interactions. In their view, coordinated long-term monitoring of groundwater, vadose zone and other variables may accelerate a holistic understanding of process interactions. The results provide a detailed description of the catchment hydrological response to the Millennium Drought and insights on mechanisms underlying the hydrological shifts. The framework of hypothesized process explanations is transferrable for understanding long-term drying and its hydrological impact in other catchments across the globe, thus contributing to implementing water management strategies under the changing climate.
Original reference: Hydrol. Earth Syst. Sci. https://doi.org/10.5194/hess-26-6073-2022 (2022)
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Chen, Y. How Australia’s Millennium drought induced hydrological shifts. Nat Water 1, 20 (2023). https://doi.org/10.1038/s44221-022-00024-x