Questions on dam levels

All you wanted to know about environmental flow (hey it even rhymes):

http://www.environment.act.gov.au/water/act_water_resources/environmental_flows

This site is hysterical.

“It’s all industrial usage! The impact of changed household use is negligible and totally hidden!”
“No, it’s isn’t; no, it isn’t; and no, it isn’t.”
“I bet it’s all those rich people using all the water, then!”

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?

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.

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.

@#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.

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.

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.

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…

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.