During the "Millennium Drought" from roughly 1997-2009, most rivers and creeks throughout Victoria, Australia experienced a significant decline in runoff and streamflow. In many catchments, the reduction in runoff was larger than expected from the reduction in rainfall, reflecting a decrease in the rate of conversion from rainfall to runoff. Unusually, after the drought ended in 2009, the rainfall-runoff relationship in many Victorian catchments did not return to pre-drought levels. The shift in the rainfall-runoff relationship has major implications for the accuracy of streamflow prediction models due to their reliance on stable rainfall-runoff relationships.

In this study we examine the changes in the rainfall-runoff relationships in terms of the changes in rainfall experienced from the start of the Millennium Drought. A new weather types dataset is used to identify the cause of each rain event from either cyclones, fronts, thunderstorms, their combinations, or other weather systems such as anticyclones. Changes in rainfall events are identified and their contributions to the shift in rainfall-runoff relationships are assessed.