Sydney's dam levels are at their lowest since Warragamba Dam began to operate.
Sydney has still got rainfall... what is happening?
Sydney's dams are dominated by one dam, Warragamba, which has 70% of the capacity for the whole system.
Warragamba is located out west, it doesn't necessarily get rain when Sydney does.
Warragamba was last full in 1998. Since then Sydney has used about 2 times the water that Warragamba can hold.
On the other hand, the rain during that time has only filled Warragamba about 1.5 times, so Warragamba dam is about half full.
The water level of Sydney's dams changes slowly. An average month's rainfall may only add 3% to Warragamba Dam's water level.
On the other hand, an average month of Sydney's water usage is about 2% of the water Warragamba Dam can hold.
In drought conditions rainfall is sometimes very low.
Usually in drought conditions the rainfall is only about 30% down on average. But over time, this is still enough for the dam level to drop.
The dams in the Campbelltown area are much more full than Warragamba Dam. So why is the Campbelltown
area on water restrictions? Why doesn't Sydney Water use water from these dams in preference to Warragamba?
The water from the dams in the Campbelltown area can be used by all of Sydney and are owned by all of Sydney.
However, unlike the rest of Sydney, at this stage the Campbelltown area has no access to the water in Warragamba Dam.
If water from the Campbelltown area dams is used for the rest of Sydney, the Campbelltown area's store of
water is reduced. If water from the dams in the Campbelltown area is used in an unrestrained fashion
during drought conditions, this will similarly reduce the Campbelltown area's store of water. This then cannot
be remedied by using water from elsewhere in the system.
If rain falls in Warragamba Dam's catchment area but not in the catchment area for the dams around Campbelltown,
the dam levels around Campbelltown will continue to drop, even if the water level of Warragamba Dam rises.
To reduce the possibility of the Campbelltown dams' water levels from becoming too low, water for use in Sydney is
taken from Warragamba Dam in preference to the dams around Campbelltown,
This can mean that, proportionally, the water levels of dams in the Campbelltown area are higher than those
in Warragamba Dam.
So when can we expect dam levels to recover?
A good way to think about the Sydney dam system is this (it's only approximate but it works pretty well):
Each year toss a coin - if it is heads, dam levels fall by 15% of full capacity - if tails, dam levels
increase by about 45% of full capacity.
So, on average, the dams go up by 15% per year, so there should never be a problem, right? Wrong!
All you need to have is 4 heads in a row to get to our current situation of 40% levels. This is a 1 in 16 chance i.e.
once in every 16 years the dams will get about this low.
Why is a dam water level of 40% such a big problem?
Chances are, after another couple of years (growing at 15% on average) that
the levels will be more like 70%, and 2 years later the dams would be full!
However there is the 1 in 4 chance
of getting 2 more heads and getting down to 10%. Another head after that would send the dam levels down to 0%! So while chances are that the
dams will fix themselves according to this model, there is still a decent chance that, if nothing is done, the dams will drop
below 5% which is a real disaster economically.
Also, this reasoning assumes no climate change effects, just random chance. If some climate change is occurring that has
reduced Warragamba Dam's input of water by 20% on a more systemic basis, then the dam levels will keep getting less!
We only have a bit over 100 years of weather data for the region, that's not much to go on.
Predictions on the question of whether we can expect rainfall to increase, decrease or stay the same vary between
sources. This means that it may be difficult to estimate
how much rainfall will keep the water in Sydney's dams at desirable levels.
Can anything be done about this?
If our water usage over the last 6 years had been 30% less than what it has been, then instead of using
2 times the water that Warragamba Dam can hold, we would have used 1.4 of Warragamba Dam's capacity,
This means that Warragamba Dam would still be basically full! So techniques of reducing
water usage (rainwater tanks, greywater usage) make a big difference to the water levels in Warragamba Dam. Available greywater is
not affected by a drought, rainwater delivered to water tanks only reduces slightly.
You can see graphs and simulations of different water usage and/or desalination plants on our Warragamba Dam Statistics page.
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Comments by visitors:
|by hayley @ 31 May 2005 11:19 pm |
| god we shud sooooo cut down on water |
|by Bill @ 03 Jun 2005 01:30 am |
| Has anyone worked out the economic effect on Sydney/Australia of an empty Warragamba? |
I imagine it could be comparable to the worst terrorist attack. Without water there is no sanitation and food cultivation may be affected.
Imagine the resultant stampede out of the city in the worst case scenario.
|by John @ 13 Jun 2005 03:56 am |
| Dear Sir, |
I believe strongly that harsher water restrictions be put into place, such as put a ban on using hoses. No hoses connected to water taps. Gardeners should use watering cans. Water is too precious to be wasted.
|by Ness @ 19 Jun 2005 11:40 pm |
| Hi....I just want to say something. Yes, water restrictions are good and everything, but they are only a temporary solution. We can expect to be on a GO SLOW ON THE H2O promotion for the next 50years, and then one day we will have so much water. Its not that easy. Australia really has to think about different methods of getting water. for example Nuclear Power. this will save a lot of time, as well as generate a lot of power. It is safe if done correctly. I don't see what the problem is with it. It will stop us from buring up our fossil fuels etc, and will help Australia's water situation. |
|by KK @ 24 Jun 2005 11:33 pm |
| Sydney's water ristrictions aren't harsh enough and thier are still prople who are chooseing not to follow the ristrictions even thought thier are falling dam levels. meaning that people need to wake up and do thier bit and go slow on the H2O |
|by Steve @ 30 Jun 2005 08:30 am |
| Australia is lucky enough to be in a fairly robust western capalist society. Why on earth with the technology available to developed countries haven't we got a desalination system. The government should also invest heavily into subsidising water tanks for homes as i am sure the economic disaster of an empty dam would be far worse than these subsidies. |
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