Monday, November 28, 2011

Weekend downpour and inflow update

On the weekend, 26-27 November, we received a very nice downpour of 53 mm. This has pushed November well above average in terms of rainfall. However, we have still had less than 500 mm of rainfall this year, which means that we need a wet Christmas to reach Canberra's average.

The inflow since the start of this year has been, by my calculations, 47,175 megalitres, with an error margin of 2,000 megalitres. The megalitre per millimetre ratio is around 98, but that should improve some as we are due more inflow from the downpour - probably 2,000 to 3,000 more.

If we have a few years without drought, it might be possible for us to return to something closer to the average megalitre per millimetre ratio of 300. But I do not think that we will ever get back to that level.

Oh, and a word on consumption: ACTEW releases annual figures, but for financial years. This financial year, we have used 17,142 megalitres thus far, an average of close to 3,500 per month. This points to another year of 40,000 megalitre consumption. This is good: lowest ever consumption was last year's 40,923 megalitres. While we get more than that on average, we will be fine.

Monday, November 14, 2011

Good rainfall to start November; la nina confirmed

We have had a good start to November, with 27 mm thus far, putting us on track for the average of around 60 mm. However, ACTEWAGL have been slow updating their dam level numbers and so I will have to estimate the inflow for the year thus far. My calculation put it around 44,000 megalitres, with probably a little more to come from last week's rain. However, we are having hot weather for November, which increases evaporation and transpiration, so there might not be much more to come. We will see.

La Nina has been confirmed as present in the Pacific, so above average rainfall is projected for the summer. If above average rainfall eventuates, it will be interesting to see the inflow figures. The heavy rainfall that we had from February 2010 and February 2011 did not bring us back to average inflow levels, but perhaps a few further months of good rain will go some way towards doing so.

Wednesday, November 2, 2011

Poor start to spring

Spring is when the rain should fall in Canberra, although in recent years the rain has been coming more in the summer months. Thus far this year, we have had below average rainfall for the first two months of the season. Our yearly total has not yet crested 400 mm, which means that in the last eight weeks or so  of the year we need more than 210 mm to reach the annual average.

The 13 months from February 2010 to February 2011 were months of extremely good rain. Since then, however, only around 210 mm have fallen, leaving the ground parched even though the dams are still full.  The megalitres per mm rate has dropped from a high of 120 early in the year (still low) to a low of 101. And in the past three months we have had less than 7,000 megalitres of runoff.

While I do not think that permanent drought will come upon Canberra in the next decade, which is what I was predicting previously, I think that we are heading in that direction. Large rainfall every few years may make us relatively secure, especially with the new dam. But I am still very concerned.