Thursday, August 18, 2011

August Rain

We had some great rain yesterday: 34.2 mm. This brings the yearly total to around 320 mm, which is still below average for the year. However, we are heading into spring and if ENSO conditions remain neutral or extend to a La Nina we could catch up.

Inflow total thus far is 37,500 megalitres, well below average. There should still be around 3,000 to come from this good rain, but even so we are looking at getting less than 75,000 megalitres of inflow for the year. While this is way better than some recent years, it would be a significant drop on last year. Further, the ratio for inflow to rainfall is 120 megalitres of inflow per mm of rain. This is still a very low ratio, even with last year's very high amount of rain. Remember that the long-term average is 300 megalitres per mm and that inflow declines greater than linearly with decline in rainfall.

This seems to show that the relationship between low rainfall and even lower inflow is still holding. If we get a very bad year or enter drought conditions once more, I would not be surprised to see the inflow to rainfall ratio decline a hell of a lot further in a very short period.

I am still predicting an inevitable collapse of Canberra's water resources that requires urgent action to forestall. However, last year's high rainfall leads me to believe that this will take longer than my initial modelling suggested.

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