Rainfall, evaporation and runoff

Rainfall turns into two things: evaporation and runoff. In Australia, across the entire continent on average 65 per cent of rainfall evaporates and 35 per cent becomes runoff.

As temperature increase, however, the percentage of evaporation tends to increase (there are limiting factors to this - if the atmosphere already contains lots of H2O, evaporation will be slower). Examining the recent evaporation history of the Canberra region, I found that evaporation rates have increased to around four per cent above the long-term average (we have data from 1967 to 2007 inclusive).

Based on the Australian average, this would mean a seven per cent decline in runoff (assuming constant rainfall, of course). Instead of a ratio of evaporation to rainfall of 65:35, we would have a ratio of 67.5:32.5.

I had a look at what this means for the trend in runoff. Over the last 20 years, when changes in evaporation are taken into account, runoff has decreased by around 9 mm per year. The long-term average is 224 mm of runoff per year. At present, we have an average runoff of 166 mm, which is the lowest recorded (note that we only have records for evaporation going back to 1967). This is two standard deviations below the mean.

If the trend continues, effectively we will have no runoff by 2030. None.

The error margins for this estimate are quite large, as we only have a relatively small amount of information. Further, while evaporation rates rise with temperature, they are also affected by things such as the amount of sunlight that reaches the earth. Global dimming, probably caused by aerosols, reduced evaporation between 1970 and 1990, and then as aerosols started to decline somewhat evaporation increased.

However, these trends should be closely examined. And there should be more discussion from government about them and what they are doing to protect Canberra from the effects of climate change. (And I should say here that these effects are almost inevitable at this point, as politically it is unlikely that we are going to significantly slow greenhouse gas emissions between now and 2030.)