Changes in annual rainfall-runoff responses in Melbourne's major water supply catchments

Since 1997, the average annual streamflow into Melbourne's four major harvesting reservoirs (Maroondah, O'Shannassy, Upper Yarra and Thomson) has declined by 32% from the pre-1997 long-term average (1913-1996). Hope et al. (2017) attributed streamflow decline in this region to a rainfall decline driven by a shift in climate pattern. Saft et al. (2015) identified how this streamflow reduction has been exacerbated in some catchments following the Millennium Drought (1997-2009). However that study did not include Melbourne's reservoir catchments. This paper presents an assessment of changes in annual rainfall-runoff behaviour for Melbourne's four major harvesting reservoir catchments, as well as preliminary attribution for some of the factors influencing changes in rainfall-runoff response. The assessment uses a 1954-2017 dataset, concurrent and fit for purpose across each catchment, plus longer datasets at individual sites. Using a rainfall-runoff regression modelling technique, it was found that given similar rainfall decline, runoff in the post-1997 period would have been 12% (50 GL/year) higher if the pre-1997 rainfall-runoff relationship had continued after 1997. Rainfall-runoff responses in the Maroondah and O'Shannassy catchments were more resilient to drought, with smaller or no statistically significant changes in rainfall-runoff response before and after 1997. In contrast, larger and statistically significant shifts were detected in the Upper Yarra and Thomson catchments. These changes were found to have persisted after the end of the Millennium Drought, indicating rainfall-runoff behaviour in Melbourne's major water catchments has not yet returned to pre-drought conditions. 30-85% of the changes in annual rainfall-runoff responses post-1997 were attributed to seasonal and multi-year rainfall variance, highlighting the importance of understanding changes in serial correlation of dry years, and changes in seasonal rainfall patterns during and after drought. It was also estimated that changes in rainfall-runoff behaviour post-1997 would have been greater if the 1939 bushfires had not occurred. These changes have important implications for Melbourne's water management. It reinforces Melbourne Water's adoption of a 'post-1997 'step climate change' scenario for drought and operational planning rather than a less conservative 'post-1975' climate scenario, consistent with the Victorian Government's water resource planning guidelines (DELWP, 2016). Ongoing adaptive water management will continue to be supported, by improving our understanding of the drivers and predictability of the observed shift in rainfall-runoff response, through surface-groundwater monitoring, research partnerships, and catchment modelling.