The long-term average for the three months of autumn since records began in 1862 is 132.3mm CSIRO report author Dr Wenju Cai said the climate study of the past 58 years showed two significant features.
The number of rain-inducing La Nina events were being increasingly outnumbered by dry El Nino events.
And subtropical Indian Ocean patterns influenced by global warming were diminishing autumn rainfall, particularly in May across northern Victoria.
“When you take these two components together it has a huge impact in the reduction of rainfall and inflows,” Dr Cai said.
“This is not only very important for agricultural but also in wetting the soils and ensuring there are good inflows in the usually wet months of winter and spring to follow.”
The report found that spatially alternating high and low pressure systems, known as pressure wave-trains, are crucial influences for rainfall over Victoria in May.
Such systems have been weakening since 1950.
La Nina transitional periods usually produce wet autumns in northern Victoria and southern NSW are become shorter.
Long-term climate is dominated by more extensive dry El Nino events, another strong indicator of climate change.
Dr Cai said the outlook was serious, and the consensus among scientists was that continuing climate change would reduce inflows by about 15per cent for every degree of warming.
Under the median estimate of a two-degree rise in temperature – which is conservative – there was likely to be a 30 per cent reduction in inflows for the Murray Darling Basin.
The sobering figures are vindication for Coliban Water’s conservative approach to its five-year water plan.
The plan is based on an expected reduction in inflows to the Campaspe system from the long-term average of 38 gigalitres to 15 gigalitres – or more than 60 per cent.
Department of Sustainability and Environment estimates show that surplus water under a ‘medium’ climate change scenario would be about 23 gigalitres by 2030.
The figure would drop to only 8 gigalitres by 2055.
A repetition or continuation of inflows of the past eight years would constitute a more extreme climate change scenario.
Goulburn-Murray Water strategy manager Garry Smith said the data highlighted the importance of the State Government’s Northern Region Sustainable Water Strategy, a draft version of which would be released in coming months.