Do rainwater tanks save water?

Research published in the latest issue of the journal Geographical Research indicates that residents with a rainwater tank don’t reduce their consumption of mains water by any more than those without a tank.

The study finds households in NSW’s Illawarra region who installed a rainwater tank reduced their consumption of mains water on average by 10.3%. Households without a water tank reduced their consumption on average by 10.8%.

On the face of it, this is a worrying finding given at least one State Government is now promoting the virtues of rainwater tanks as an alternative to the astronomical capital and operating expense of desalination solutions to water security.

Researcher Candice Moy from the University of Wollongong examined water consumption by all households in NSW’s Illawarra region (Wollongong and Shellharbour municipalities) from 2002 to 2009 (you can read the full paper here). Level 1 water restrictions were made law in 2003, Level 2 in 2004 and Level 3 in June 2005. The latter meant gardens could only be watered by hand-held hose on two days per week, between 4pm and 10am.

7,125 households (about 8% of all households in the region) installed a rainwater tank in the three years between 2005 and 2007. Their consumption was documented for two years prior to and after installation and compared against all households in the region. Ms Moy also conducted interviews with a small number of these households and analysed a separate survey of tank-owning households in the Illawarra.

She reckons there’s a straightforward explanation for why rainwater tanks make no difference to mains consumption. The great bulk of households who install a tank don’t do it in order to reduce overall water consumption or to protect the environment. Rather, the tank is a means of maintaining “luxurious water behaviours” in the face of mandatory restrictions.

Most tank-owning households aren’t concerned, she says, with saving water but with using it. Since they collected it off their own roofs, they see rainwater as a private good. It’s a commodity they’re free to use in any way they wish, whether for gardens, pools, or hosing the driveway.

This interpretation is supported by the small proportion of tank-owning households – just 10% – who use rainwater for internal uses, notwithstanding the availability of a connection subsidy. About 5% connected their tank to either the washing machine or the toilet , and another 5% connected it to both. The latter group was the only one to show a substantially larger reduction in mains use (16.1%) than households without rainwater tanks (see exhibit).

The writer says her study challenges the finding by the ABS that 41% of households who install rainwater tanks do so to “save water”. On the contrary, the research shows “that water for outdoor usage was the clear motivation for the large majority of participants”. She also argues her findings suggest the proportion of household water used for outdoor purposes is much lower than the 28% assumed by water authorities.

Her conclusion is tanks have the potential to generate significant water savings, but water-intensive behaviours have to be modified. If tanks are not plumbed indoors:

the potential of this alternative water source will not be met. Currently, rainwater tanks facilitate water users as much as water savers.

I have some misgivings about this paper, though. It’s ambiguous in parts, relevant information isn’t always shown, and some comments seem contradictory. Nor is there much effort to canvass other possible explanations.

It’s not discussed in the paper, but perhaps households with tanks didn’t have the scope to make bigger savings simply because there was insufficient rainfall to fill them often enough. Or perhaps, as cynics often suggest, some households filled their tanks from the mains system.

Another point is households who installed a water tank between 2005 and 2007 were already on Level 2 restrictions from June 2004, limiting them to hand-held hosing on three days per week. Their scope to reduce mains water consumption further would accordingly be limited. There’s also the “rebound effect” – households who install a rainwater tank might now feel it’s OK to (say) take longer showers with mains water.

The immediate threat of drought has receded across most of Australia, but it would be unwise to ignore the possibility it could come back sooner rather than later. Governments see rainwater tanks as having a major role in water security, so it’s important their limitations are properly understood. Notwithstanding some reservations on my part, Ms Moy’s findings certainly warrant close examination by water authorities.