Assessment and reconstruction of catchment streamflow trends and variability in response to rainfall across Victoria, Australia

Catchments in Victoria, Australia, are essential to the state's water supply, yet have experienced increased stress over the last 30 yr. To gain a better understanding of streamflow variability, 27 catchments across the state were analysed: the majority showed a declining trend in streamflow from 1977-2012, which was largest in the far west and east of the state and smallest close to the Australian Alps. The same spatial pattern was observed for the streamflow reduction during the Millennium Drought. The response of streamflow across the state is strongly related to the mean runoff rather than the pattern of rainfall reduction. In terms of large-scale climate forcings, strong correlations were found during the cool season with the subtropical ridge intensity and to a lesser extent with its position. Tropical modes of variability are important in catchments other than those on the southern flank of the Great Dividing Range; however, they do not explain the streamflow reduction during the Millennium Drought. A simple linear regression was tested based on a temporal range of rainfall parameters with short (from 1 mo) to long (up to 10 yr) memory, combined with temperature, to reconstruct streamflow. High correlations were found between observed and best-reconstructed monthly streamflow. The reconstruction was able to capture the long-term trends and the magnitude of the Millennium Drought well, an important step toward the application of the method to generate streamflow projections using downscaled rainfall from climate models.