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Questions on dam levels

By Prospector - 26 June 2010 17

We’ve had average to good rainfalls over the past 6 months or so, but the publicised dam levels have been creeping up much more slowly than I’d expected in view of the good rains. It is still barely 55%.

Am I correct in assuming then that the reported dam levels are net of ‘environmental flows’ (i.e. the water which the Government releases from the dam system into the Murray-Darling system)?  So with the better rains of late, is ACTEW taking the plug out of the dam system a little more frequently – increasing environmental releases  –  so dam levels are not rising as quickly as one might expect?

If so, what would dam levels be now if there had been no environmental water releases in 2010?

What’s Your opinion?


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17 Responses to
Questions on dam levels
Prospector 11:57 am 29 Jun 10

Thanks to all those who’ve responded to my query. Yes, I take the points…the water table is still very depleted, and rain doesn’t always fall in the catchments.

Regards Googong, I have vagues recollections that back in the 1970s and 80s, it used to fill quite readily (and the dam wall itself was under threat when it was still being being constructed – the dam began strongly overflowing after a particularly wet period and before the wall had been fully concrete capped).

Is there an issue that extensive farmlet and other housing sub-division construction in the Googong catchment since the Googong was built, with their accompanying private dams, have significantly reduced the run-off that eventually finds its way into the Googong Dam?

Follow up question on environmental releases and whether ACTEW is releasing more water as the rainfall situation has improved – is there any publicly available data on environmental releases from the ACT Dams, say on a month by month basis and what impact they have on dam water storage levels?

georgesgenitals 9:55 am 28 Jun 10

mr reason said :

@#7 ACTEW is not “at least partly privately owned”. ACTEW Corporation is 100% owned by the ACT Government. You are confusing this with the ActewAGL partnerships, of which ACTEW owns half. However ACTEW owns the water and sewerage system and the revnues from them. ActewAGL operates these under contract.

Thanks for that, makes things a bit clearer.

JC 6:08 am 28 Jun 10

True facet, however as the Cotter dam is in the better catchment area one would assume that over time it will fill up more than the Googong and will aid in giving a higher average.

mr reason 4:52 pm 27 Jun 10

@#7 ACTEW is not “at least partly privately owned”. ACTEW Corporation is 100% owned by the ACT Government. You are confusing this with the ActewAGL partnerships, of which ACTEW owns half. However ACTEW owns the water and sewerage system and the revnues from them. ActewAGL operates these under contract.

facet 1:51 pm 27 Jun 10

If we greatly increase the capacity of Cotter Dam, this will mean that the “percentage full” will decrease irrespective of how much rain or run off we get. These figures are a con job.

JC 3:42 am 27 Jun 10

To understand why the levels are so low you need to consider the size of the dams and where the rainfall is actually running off to. Googong is the largest dam at 121GL, Corin, Bendora and the Cotter more or less work as one system (Corin feeds Bendora which can feed the Cotter) with a combined capacity of about 85GL. So in other words Googong has 60% of the total capacity.

When it comes to rain fall most of that has been flowing into the Corin system and bugger all into Googong. Indeed Bendora has at times over the past year been at 100% capacity. It is for this very reason that ACTEW built the pipe line to link Googong to the Bendora system and why the increasing the size of the piddly Cotter dam. It gives them greater storage capacity in the system that gets the best in-flows and gives the ability to pump to Googong when need be.

So to answer the question as Googong is the largest dam but gets the smallest in-flow from a percentage perspective it takes a lot of rain to increase the over all levels any meaningful amount. The rains of the past month for example increased Bendora by 5%, Corin by about 20% but Googong by 0%. In total over all the dams it was about 3%.

As for the figures the ones quote by ACTEW are the amount of water currently in storage in the dams. So clearly takes into account any ‘enviromental’ releases.

bugmenot 9:20 pm 26 Jun 10

I’d still like to see a breakdown of water usage by suburb. Shouldn’t be too hard for them to produce.

It’d also be interesting to see which suburbs are doing their bit (are the O’Malleys, Red Hills, Forrests and Yarralumlas pulling their weight, too?). It would hopefully also show the “benefit” of the rainwater tanks in the newer suburbs.

I see some fairly hefty water usage within the commercial space. I wonder how much better their usage could be? Hell, where I work, we have an exemption to water our *native gardens* twice a week with automatic sprinklers… Hmmm…

Spectra 9:08 pm 26 Jun 10

Am I correct in assuming then that the reported dam levels are net of ‘environmental flows’ (i.e. the water which the Government releases from the dam system into the Murray-Darling system)?

Would you care to explain to me how else they might be reported? Are you suggesting that they should report water that they have released from the dams in “dam level” measurements, even though it’s no longer in the dams?

The percentage measurements are given as a percentage of the total capacity of the dam. If the dam capacity is 100GL and it’s reported as 50% full, that means there’s 50GL in it. If they then release 4GL and there’s no inflow, the reported level (in this scenario) will fall to 46%. I’m not really sure how it could be any simpler.

As other people have already explained, one of the reasons it’s filling so slowly is the prolonged drought we’ve had and its effect on ground moisture. Another is that it takes a hell of a lot of rain to significantly change the levels – your (and my) intuition about what constitutes “a lot of rain” on that scale is a very poor guide indeed.

georgesgenitals 8:48 pm 26 Jun 10

54-11 said :

I cannot understand the mentality of ACTEW when it comes to encouraging less water use.

I don’t think ACTEW seriously encourage less use of resources, as this reduces their revenue. The organisation is at least partly privately owned, so the reduction policies of government have some internal competition, I would think.

nhand42 2:04 pm 26 Jun 10

54-11 said :

However, I think all these water figures are just a fraud anyway. The usage rates include very high commercial and light industrial usage, and the impact of changed household use is both negligible and totally hidden within these overall figures.

As a camapign to reduce/restrict household use, it is ineffective and misleading.

Household and commercial/industrial water usage is public knowledge. There’s no need to guess.

http://www.environmentcommissioner.act.gov.au/publications/soe/2007actreport/indicators/wateruse07

Residential water is 2/3 of usage in the ACT. Reducing your household usage has a measurable and significant impact.

PS: I promise to use preview from now on…

nhand42 2:01 pm 26 Jun 10

54-11 said :

However, I think all these water figures are just a fraud anyway. The usage rates include very high commercial and light industrial usage, and the impact of changed household use is both negligible and totally hidden within these overall figures.
[/quote>

Household and commercial/industrial water usage is public knowledge. There’s no need to guess.

http://www.environmentcommissioner.act.gov.au/publications/soe/2007actreport/indicators/wateruse07

Residential water is 2/3 of usage in the ACT. Reducing your household usage has a measurable and significant impact.

ricketyclik 12:31 pm 26 Jun 10

mr reason said :

While rainfall may have been marginally higher than in recent year, i understand that inflows to the dams are at record lows. this is because the water tables have been depleted over the past 16 years or so.

This is true. Also, on an annual basis, water tables tend to be depleted in late autumn-early winter due to the high evaporation rates of spring, summer and autumn. The rainfall that generally translates directly in to the highest stream flows falls in late winter and spring. Canberra’s dams are typically rising July-December, and falling in January-June. When we’re not in drought that is.

Having said the above, the good rains over autumn will hopefully mean that late winter and spring rains are more productive than they’ve been in recent years, because of what mr reason said – i.e. the water tables will be less depleted.

54-11 12:09 pm 26 Jun 10

I think that’s right, mr reason. The ground is so dry that there is a lot of loss before the water ever reaches the dams.

However, I think all these water figures are just a fraud anyway. The usage rates include very high commercial and light industrial usage, and the impact of changed household use is both negligible and totally hidden within these overall figures.

As a camapign to reduce/restrict household use, it is ineffective and misleading.

Look at your water bill – given the extremely high fixed charges, and no penalty for high usage, there is no incentive to save water at all. For us, over 2/3 of the account is fixed charges, and even though we’ve reduced uasage, our bills are still going up higher that inflation because of these high fixed charges.

I cannot understand the mentality of ACTEW when it comes to encouraging less water use.

buzz819 11:59 am 26 Jun 10

AND rain does not always fall in the catchment area’s cheeky little devil that it is…

mr reason 11:19 am 26 Jun 10

While rainfall may have been marginally higher than in recent year, i understand that inflows to the dams are at record lows. this is because the water tables have been depleted over the past 16 years or so.

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