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 28, 2011
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.
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.
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.
Monday, September 19, 2011
Rainfall and inflow update after a lengthy absence
I have been on my second teaching prac for a month and thus have not had time to keep this updated.
I overestimated the expected inflow from the good rain that we received in August. As of 19 September, we are only up to about 38,000 megalitres of inflow from yearly rain totalling close to 340 mm. Below average rainfall so far for this year and below average inflow. It does not appear that last year's huge amount of rain changed the dynamics significantly: megalitres per mm has dropped to 113.
La Nina has rolled in officially, so it is probable that we will get some good rain over the remaining four months of the year. We need it to push as to the average or above. I admit that I am concerned that we could be heading into another period of drought. At least our dams are full, though.
I overestimated the expected inflow from the good rain that we received in August. As of 19 September, we are only up to about 38,000 megalitres of inflow from yearly rain totalling close to 340 mm. Below average rainfall so far for this year and below average inflow. It does not appear that last year's huge amount of rain changed the dynamics significantly: megalitres per mm has dropped to 113.
La Nina has rolled in officially, so it is probable that we will get some good rain over the remaining four months of the year. We need it to push as to the average or above. I admit that I am concerned that we could be heading into another period of drought. At least our dams are full, though.
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.
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.
Monday, July 11, 2011
New figures
I am probably going to have to change the way I look at rainfall and runoff to a financial year model, as it seems that that is the main way that ACTEWAGL are reporting data.
However, as of this date we have had 270.6 mm of rainfall for the calendar year and - by a revised calculation that removes the 130 megalitres per day of overflow that ACTEWAGL reported for March and April from my May, June and July figures - 33,000 megalitres of inflow. We have had 23,000 megalitres of water usage over that period also, which is sitting at slightly above last year, but - give the uncertainties - is basically the same.
However, as of this date we have had 270.6 mm of rainfall for the calendar year and - by a revised calculation that removes the 130 megalitres per day of overflow that ACTEWAGL reported for March and April from my May, June and July figures - 33,000 megalitres of inflow. We have had 23,000 megalitres of water usage over that period also, which is sitting at slightly above last year, but - give the uncertainties - is basically the same.
Friday, July 8, 2011
June/July update
I haven't posted for over a month, as I have been elsewhere.
Unfortunately, so has ACTEWAGL's daily usage data while they switched to a new website design ...
This means that I am having to estimate the amount of runoff that we have received. That estimate is currently at around 41,000 megalitres from rainfall of not much more than 280 mm. This estimate is at the high end--we may have received closer to 30,000 megalitres. Rainfall thus far this year has been low, with very low April, May and June. We have already had some rain in July, but it is not yet enough to counter the previous low months. Spring will be all important in determining annual rainfall.
Unfortunately, so has ACTEWAGL's daily usage data while they switched to a new website design ...
This means that I am having to estimate the amount of runoff that we have received. That estimate is currently at around 41,000 megalitres from rainfall of not much more than 280 mm. This estimate is at the high end--we may have received closer to 30,000 megalitres. Rainfall thus far this year has been low, with very low April, May and June. We have already had some rain in July, but it is not yet enough to counter the previous low months. Spring will be all important in determining annual rainfall.
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