Rainfall decline versus runoff decline

I have been having a look at what happens to runoff, which is the water that ends up in our rivers and dams, when rainfall declines. There are three historical periods in Australia in the Murray-Darling Basin for which we have information on rainfall and runoff.

In the period 1895-1904 - the Federation drought - rainfall was 11 per cent below the long-term average. Runoff during this period was 31 per cent below the long-term average.

In the period 1937 to 1946, rainfall was 14 per cent below the long-term average. Runoff was 22 per cent below the long-term average.

In the period 1997-2006, rainfall was 13 per cent below the long-term average. Runoff was 39 per cent below the long-term average.

While there is a lack of consistency in the periods, what is clear is that a one per cent decrease in rainfall does not equate to a single per cent decline in runoff. Indeed, for the current period, a one per cent decrease in average rainfall has equated to a three per cent decline in runoff. This has potentially disastrous implications for areas such as Canberra, where the projections suggest a decrease in rainfall of 50 per cent by 2050.

I am hoping to get more information from ACTAGL regarding historical dam levels. This should enable me to specifically look at Canberra's situation.