Lessons from a decade-long drought and non-recovery: hydrological processes understanding and modelling

Prolonged changes in climatic conditions can induce unexpected shifts in catchment hydrologic functioning due to the indirect impact of catchment adaptation on streamflow. In a multiyear drought, streamflow can be reduced significantly more than expected from the typical response to the same annual precipitation. For example, the same annual rainfall in the first and the tenth years of a dry period is likely to result in significantly lower streamflow during the tenth year than in the first year. Such hydrological shifts were first detected in Australia during the Millennium Drought (1997 – 2009). Since then, similar shifts were also reported in studies from other continents. Subsequently, it was also discovered that catchments, once they shift their hydrologic behaviour, may not necessarily recover back to the pre-drought behaviour even after record-breaking floods and years of annual rainfall similar to the pre-drought conditions. Observed shifts not only challenge some common assumptions of long-term hydrologic functioning but also present an interesting practical problem as hydrological models tend to reproduce historic behaviour and systematically overestimate the streamflow when the hydrologic shifts occur. Here we present the results from a large collaborative project in Australia devoted to two questions (1) which hydrological processes are responsible for the observed shifts and (2) how to improve our hydrological models to provide robust predictions under non-stationary climate? We hope that the lessons from the Millennium drought and post-drought period will be helpful in the other parts of the world where similar hydrological shifts were or will be observed.