This report from ACTEW (Australian Capital Territory Electricity and Water) is essential reading for anyone interested in the future of Canberra's water supply.
Of particular note is this:
"Whilst global warming progresses proportionally to the build up of greenhouse gases in the atmosphere, it can result in rapid ‘step’ climate changes in a particular region. It is possible that the recent drought represents a shift in climate for Canberra. The past 5 to 10 years are clearly the most severe long-term dry period in the 1871 to present extended historic record inflow sequence."
I have looked at the data on inflows in this report at page 23 (PDF page 25) and tried to estimate the figures from the graph. While I may be out somewhat, it will not be by all that much (although I wish that they had included the data or at least a reference to it). When graphing the values for the last little while (14 years of 10-year averages, which includes 23 years of data) against the rolling 10-year rainfall figures for Canberra, I found that every 1 mm decline in the rainfall average means around a 70 megalitre decline in inflows. This means that when the 10-year average rainfall drops to around 450 mm, inflows drop to zero. Zero.
If the current trend in declining rainfall continues, this point will be reached by 2017, eight years from now. You heard it here first.
Again, the error margins here are reasonably large because of the autocorrelation in the 10-year averages. However, this is significantly sooner than my previous estimate of 2030 for zero inflow into Canberra dams.