In case of emergency, empty dam
Russell Degnan

We've had a fair amount of rain recently, so rightly, the fountains are being turned back on. Despite this, Melbourne Water have said that we still need to save water, so the depleted catchments can refill. Again, rightly. But beyond that some of the assumptions about water conservation don't match the reality of our water usage.

That reality is of ever-increasing per capita usage, despite claims to the contrary. What Melbourne Water do claim is a per household reduction from 340 kL in the early 1980s to 220 kL in 1993. This looks fantastic except for two points. One, average household size has reduced substantially in that period, so much of that gain is illusory. Two, the points chosen for comparison are right before the start of water restrictions in the early 1980s and during restrictions now. Examining water usage as a trend tells a different story.

source: Melbourne Water

What this shows (and a per capita graph shows it better, but I had none at hand), is that water usage is inversely related to rain, and that we continue to use more and more as every year passes on.

One thing at a time. Average summer water usage in Melbourne is 1600 ML a day. The average generally is 1200 ML. The average this week was 1000 ML. The reason: when it rains, people don't water their garden, and gardens are both the biggest and most fluctuating use of water. During a drought, they can use up to 60% of total water, as compared to 35% normally.

There is a constant pattern over the course of the drought cycle (a complicated and unpredicatable thing, but which generally runs in patterns of 4, 7 and 11 years). Water usage when restrictions are eased rises quickly, before smoothing out until the drought starts. At this point, the lack of rain means people start watering their dying gardens, water storage levels drop, and restrictions are introduced until the drought eases. Then the pattern begins again.
Except that each successive cycle has seen higher usage than the previous one. I am not sure why. It is not dissimilar to the Induced Demand Hypothesis as it applies to roads. But road use that can be explained as a willingness to travel further in the same time frame. Increased water use because it is available has no similar rhime nor reason. Except that people water gardens because they can, and every year we get more and more greenery.

It is for this reason that water savings measures cannot be the be-all and end-all of Melbourne's water policy, as proposed. Droughts cause more water to be used, droughts occur often in Melbourne, and Melbourne has a finite supply of water that is nearing (in fact it is past) the limit of what can be taken. And rationing water more heavily in the wet part of the drought cycle doesn't help when uses that normally make no, or small demands on water - such as most gardens - start making heavy demands on water for lack of a natural supply.

For that we will need water restrictions. Or harsh water pricing that achieves the same ends. Till next time though, we have our fountains again. And a fine sight they are.

Sterner Matters 29th July, 2004 00:21:11   [#] 


Russ, did you see that desalination is being used to provide water in London?

I know the green paper didn't consider this for the short term, but I wonder whether in 20 years if there will be desalination plants in the lower reaches of the Yarra.
Rob Merkel  30th July, 2004 15:09:59  

Almost certainly
Given that I think there are great environmental benefits to desalination (for rivers anyway) - unless someone knows otherwise. I'd be surprised if it doesn't become much more common. The impediment is obviously cost, both in processing, and in pumping uphill. But as a drought independent source of water I'd say you are right.

In my opinion, the projections of the green paper are very optimistic for water saving (for reasons noted above). Finding additional sources of water will definitely be on the agenda in the future. The only reason it isn't now is that the Thompson is so big we haven't needed to for 20 years.
Russ  31st July, 2004 19:02:55  

Is it a domestic or commercial issue?
Russ, can we break up the figures of the graph to distinguish from household water trends and the commercial/industry trends to actually get the true picture of water use?
LisaB.UrbnPl  31st July, 2004 19:48:10  

City-wide, commerical use is quite low (less than 20%). Indutry smaller again. This page gives some statistics, although I'd be a little dubious of the exact proportions. Parks and Gardens seems a bit low, and commerical use is well above Sydney, which implies that the former may be being counted in the latter. The other problem with that page is that it is aggregating statistics that have no business being aggregated.

I would like to show an up to date per capita usage graph, because it shows just how variable usage is; but the only one I have is 12 years old, and Melbourne Water haven't printed one. Nevertheless, as I explained in the original post, gardens are the biggest issue, because they are the thing that creates the biggest drain on supply in a drought. This is why water restrictions focus almost exclusively on garden related activities.
Russ  1st August, 2004 01:56:02