I have made the prediction on this blog that Canberra inflow - the water that ends up in dams - will effectively drop to zero by around 2017. It should be noted that this is not a solid number - there is a considerable statistical range over which this could happen. But 2017 is the mean.
There are a number of assumptions underpinning this prediction. Any one of those assumptions might be wrong.
The basic argument is that rainfall is declining due to climate change and that declines in rainfall cause declines in runoff. This decline has been observed.
There are three sets of arguments running here.
Premise 1: Climate change will continue.
Premise 2: If climate change continues, daytime temperatures in Canberra will continue to increase.
Conclusion 1: Therefore, daytime temperatures in Canberra will continue to increase.
Premise 3: Increases in daytime temperatures reduce rainfall.
Premise 4: Reductions in rainfall lead to reductions in runoff.
Conclusion 2: Increases in daytime temperatures reduce runoff.
Premise 5: Daytime temperatures in Canberra will continue to increase.
Premise 6: Increases in day time temperatures reduce runoff.
Conclusion 3: Runoff will reduce.
(Note how conclusions 1 and 2 became premises 5 and 6.)
There are other assumptions here - the premises have not been spelt out completely. The main assumptions involve the rate at which day time temperatures will increase and the rate at which those temperature increases will cause rainfall, and hence runoff, to decline. The rates predicted are based on empirical observation.
The problem with this is that the future may well differ from the past in this respect. Just as it is possible that we have undergone a step change in climate here in Canberra, so it is also possible that we will undergo another one soon.
Further, it is possible that there is a limit at which daytime temperature ceases to become the dominant driver of rainfall. Thus, a one degree rise in temperature in the future may not have the same effect as a one degree rise in the past.
So, my prediction could well be wrong. But that is how science advances: by linking empirical observation to a theoretical model and then testing that theoretical model against further empirical observation. At the moment, my theoretical model - based in empirical observation - is predicting an end to Canberra runoff by 2017.