Rainfall and streamflows in the Greater Melbourne catchment area: variability and recent anomalies

Observed rainfall and water availability is investigated across several catchments northeast of Melbourne using gridded rainfall data over the last 113 yr and reconstructed streamflow observations for the last 100 yr, focusing on the 1997-2009 record-breaking rainfall deficits, associated record-low streamflows and subsequent recovery from 2010 to 2012. These catchments provide drinking water for about 90% of the state of Victoria's population and hence are critical. The influence of large-scale tropical modes of climate variability affecting rainfall, and subsequently reservoir streamflows, are shown to be modulated by the orographic features marking this region. These remote large-scale tropical climate forcings have contributed strongly to recovery since 2010. However, across these catchments, the large-scale modes of natural variability do not explain the long-term deficit in streamflows in the last 15 yr. Annual streamflow in these wet catchments can skilfully be reconstructed month by month using catchment-wide observed rainfall. The year-to-year variability, decline during the last 30 yr and magnitude of the deficiency during the Millennium Drought are reasonably well captured but not fully accounted for by the linear combination of rainfall in the current month, the previous month and the previous 12 mo. Maximum temperature does not have a sizeable additional impact when added to the reconstruction, while the previous 12 mo of rainfall contribute to about 25% of the reconstruction's ability to capture many of these statistics.