Linkages Between Changed Streamflow Response and Weather Types in Victoria, Australia

The "Millennium Drought" was a decade long drought which caused widespread water resource issues in Victoria, Australia. It occurred from roughly 1997-2009 and resulted in the introduction of water usage restrictions throughout the state. Streamflows severely decreased during this time, and 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. This shift in rainfall-runoff relationship experienced during the drought has major implications for the accuracy of streamflow prediction models due to their reliance on stable rainfall-runoff relationships. In addition, despite the end of the drought in 2009, in many Victorian catchments the relationship between rainfall and runoff has not returned to pre-drought levels.

In this study, we examine the changes in rainfall-runoff relationship in terms of changes to the rainfall source at the catchment level. We use a new weather types dataset to associate each rainfall event with their likely origin. The weather types examined include cyclones, fronts, thunderstorm environments, anticyclones and "other" categories, while allowing for multiple weather types to be associated with the same rainfall event. We then investigate the changes in event frequency and rainfall intensity associated with these weather types, to explain how the sources of rainfall have changed between pre, during and post drought eras. These changes in weather types are then related to the observed changes in rainfall-runoff relationships and streamflow.