Contrasts and contradictions comparing hydrological shifts across southern Australia

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, no assessments have yet been undertaken to draw parallels between the processes responsible. We present a comparative analysis of non-stationarity between south-west Australia and south-east Australia, two regions separated by over 2,000km and both experiencing non-stationary streamflow responses. We adopt existing methods of characterising hydrological non-stationarity and apply these to 254 catchments in Eastern and 54 in Western Australia. Of the catchments analysed, 51% of Eastern and 63% of Western catchments displayed a transition from the historical rainfall-runoff relationship to one of reduced flow generation, with the reduced flow state persisting in 31% and 63% of catchments, respectively. Interrogation of characteristics inherent in the transitioned catchments revealed positive correlation in Western Australia between catchment forest coverage and non-stationarity, while an inverse relationship is found in Eastern Australia. Similarly, catchment coverage by land cleared for agricultural purposes was positively correlated to non-stationarity in Eastern Australia and inversely so in Western 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 relative to Eastern Australia. The hydrological non-stationarity in forested western catchments is consistent with previous studies showing the importance of groundwater connectivity in those catchments; in contrast, such links are impossible to explore in eastern forested catchments due to a spatial gap in groundwater monitoring. We recommend further comparative studies be conducted to create a more thorough understanding of this behaviour in order to better inform decisions regarding water planning.