FEATURE ARTICLE 1: EXPERIMENTAL ESTIMATES OF HOUSEHOLD WATER CONSUMPTION FROM RAINWATER TANKS IN AUSTRALIA


1.0 INTRODUCTION

1. In recent years the installation of rainwater tanks has grown in Australian households due to a number of factors, including the increasing cost of distributed (mains supply) water in Australia, the availability of subsidies for households installing rainwater tanks, and the implementation of mandatory building regulations requiring water efficiency savings for new dwellings.

2. Currently, water use estimates for households in the Water Account Australia (WAA) do not include water usage from rainwater tanks for households connected to mains supply. This article explores how the ABS could further develop the WAA via a method for estimating household storage and use of rainwater and include the estimates in the physical and monetary supply and use tables. The ABS welcomes comments on the methodology as well as information on other methods used to measure own-account production of water or other natural resources.

3. There is no comprehensive data on rainwater tank usage across Australia. As such this article has drawn on regional studies of urban water issues which focussed on water security and recycling through the use of rainwater tanks in South East Queensland. The studies were conducted by the Urban Water Security Research Alliance, a partnership between the Queensland government, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Griffith University and University of Queensland.

4. The WAA presents information on the physical and monetary supply and use of water in the Australian economy compiled mostly in accordance with the System of Environmental-Economic Accounts (SEEA). The SEEA water flow accounts describe water flows, in physical units, from the initial abstraction of water resources from the environment into the economy, to the water flows within the economy in the form of supply and use by industries and households, and finally flows back to the environment.

5. Water may be abstracted from artificial reservoirs, rivers, lakes, groundwater and soil water. The capture of precipitation, for example, via the capture of water from the roofs of houses in water tanks, is recorded as abstraction via precipitation. In the SEEA, abstraction is defined as the amount of water that is removed by any source, either permanently or temporarily, in a given period of time.

6. The WAA is conceptually aligned with the SEEA although there are differences in terminology and presentation as well as the aggregate known as “water consumption”, a type of net water use. For household water use in the WAA water use can include self-extracted water, distributed water, or reuse water and sometimes a combination of all three sources are used. Distributed and reuse water use is the amount supplied to households by water providers (these data are sourced mainly from the ABS Water Supply and Sewerage Services Survey).

7. In the WAA, self-extracted water use by households is calculated by applying average state "volume (kL) per connection" coefficients and applying these to the households known not to be served by water providers (estimated by subtracting the connections served by water providers from the total number of households in each State and Territory).

8. The location of a dwelling is a major factor that influences the sources of water used by Australian households.

9. Mains or town water was the most common source of water for Australian households in 2010, with 93% of households being connected to either mains or town water (see the ABS publication Environmental Issues: Water Use and Conservation, March 2010 (cat. no. 4602.0.55.003) - Table 2).

10. The prevalence of rainwater tanks as a source of water for Australian households continues to increase. Twenty six per cent of households used a rainwater tank as a source of water in 2010 compared with 19% of households in 2007 and 17% in 2004.

11. In 2010 over 90% of households in capital cities reported mains or town water as their main source of water for drinking compared with 69% of households outside capital cities. For households outside of capital cities, water from rainwater tanks was the second most popular main source of water for drinking (22%) (Environmental Issues: Water Use and Conservation, March 2010 - Table 3).

12. From March 2007 to March 2010, households with a suitable dwelling which had a rainwater tank installed increased from 24% to 32%. During this period, households in capital cities experienced the greatest increase in the proportion of rainwater tanks installed at their dwelling (from 15% in 2007 to 26% in 2010) (Environmental Issues: Water Use and Conservation, March 2010 - Table 5).

13. Households in newly built dwellings were more likely to have a rainwater tank installed. Rainwater tanks are mandatory in New South Wales, Australian Capital Territory, Victoria and South Australia for all new dwellings. Rainwater tanks were also mandatory for new dwellings in Queensland from 2006 to 2013.

14. The size of the rainwater tank installed will depend on a range of environmental, economic and legal factors including rainfall in the household area, catchment area (size of the roof), garden type and size, the number of people living in the dwelling, water usage by households with rainwater tanks, income and government mandates.

2.0 DATA SOURCES AND METHODOLOGY

15. The data required for modelling the rainwater tank production of water by households includes rainfall, number and sizes of tanks used by households, volumes of water collected by rainwater tanks, overflow amounts and water usage within households. Data would be required for different geographical areas, e.g. capital cities and the balance of Australia. To collect all of these data would be extremely costly and impose significant respondent burden and consequently existing data sources were used.

16. The method applied utilises regional studies, ABS household surveys and modelling. It involved the use of estimates for the number of private dwellings in capital cities and the balance of Australia which have rainwater tanks sourced from the ABS publication Environmental Issues: Water Use and Conservation, March 2010 (cat. no. 4602.0.55.003).

17. The method assumed the type of dwelling (number of bedrooms) was an indication of the size of rainwater tank the household would be able to use and accommodate. Larger dwellings have a greater catchment area (roof size) enabling the household to install larger tanks and capture more water and hence to improve the tank’s capacity to provide a reliable water supply. Smaller dwellings are also limited to the size of tank that can be installed due to the lack of space available to site a rainwater tank. Estimates for the number and size of dwellings are based on ABS census data and household estimates.

18. Estimates of mains water savings through the use of rainwater tanks were based on studies of four local government areas of South East Queensland by the Urban Water Alliance, a report by the Master Builders Queensland on domestic rainwater tanks in Queensland and their cost effectiveness and impact on housing costs, the ACT Planning and Land Authority rainwater tank guide and a report by Ballarat council on the benefits of rainwater tanks in Ballarat.

19. Water savings arise when rainwater tanks are plumbed into washing machine cold water taps and toilets. Greater savings can be achieved if they are plumbed into hot water washing machine taps and bathroom taps. Internal hot water taps use 30 to 35 per cent of mains water. External use of rainwater tanks can yield savings from 12% to 43%. The average external use of water in South East Queensland was estimated to be 12%, a study in Ballarat showed evidence of 17%, and Canberra 43%. The average savings for a household in a capital city was assumed to be around 30%.

20. In 2010, 832,100 households in Australia used a rainwater tank as their main source of drinking water - 84% (702,800) of these households were located in areas other than capital cities. For this reason, for the balance of Australia savings on mains water use from rainwater tank usage were estimated at 60%.

21. In summary, the total water use from rainwater tanks is dependent on the water practices of each household. For capital cities it was estimated that mains supply water savings of 30% would result if a household was using a rainwater tank; for the balance of Australia estimated savings were 60%.

22. These estimates of water savings (30% in capital cities and 60% for the balance of state) are fairly general in that they separate households into only two categories. Accuracy in the estimates could be increased by splitting households into finer categories based on size of household, uses of rainwater tanks (e.g. whether water is used outside, for toilets, for other internal cold water uses, for hot water, for drinking water, etc.) and then estimate savings on each category of use.


3.0 RESULTS

23. Table 1 shows estimates for the total storage capacity of rainwater tanks in Australia from 2006 to 2011. The total storage capacity in Australia has increased by 30% (6,345 ML), largely fuelled by increased storage capacity in capital cities (4,227 ML or 67% of total storage increase).

Table 1 - Total storage capacity of rainwater tanks, Australia, 2006 to 2011 (ML)
	2006	2007	2008	2009	2010	2011
Capital city	5 024	5 574	6 220	6 335	8 142	9 251
Balance of Australia	16 346	16 638	17 077	17 188	18 061	18 463
Total	21 369	22 212	23 296	23 521	26 203	27 714

Sources: ABS Water Account, Australia, 2010-11 (cat. no. 4610.0); ABS Water Account, Australia, 2009-10 (cat. no. 4610.0); ABS Water Account, Australia, 2008-09 (cat. no. 4610.0); ABS Household and Family Projections, 2004 (cat. no. 3236.0) and ABS Household and Family Projections, 2010 (cat. no. 3236.0)
24. The water use from tanks in capital cities and the balance of Australia is shown in Table 2. The average household water use from all sources decreased from 0.261 ML/hh/year in 2006 to 0.184 ML/hh/year in 2011 and water use from rainwater tanks has increased during this period in capital cities. Water use from tanks has increased by 66% in capital cities and the number of households with tanks has increased by 129% during the period 2006 to 2011. However, for the balance of Australia, water use from rainwater tanks has decreased by 8% from 2006 to 2011. The number of households with rainwater tanks has increased by 31% but, coupled with the fall in average household use, the water use from tanks has decreased by 12,005 ML.

25. Water use from rainwater tanks from 2006 to 2011 has remained relatively constant at approximately 200,000 ML per year. In 2006 water use from rainwater tanks in capital cities was 41,969 ML (21% of total household water use from rainwater tanks) and has increased to 69,916 ML (33% of total household water use from rainwater tanks) in 2011. Total water use from rainwater tanks for Australia has increased from 197 GL in 2006 to 213 GL in 2011.

Table 2 - Water use from rainwater tanks, Australia, 2006 to 2011
		No. of households with tank	Ave. household use (ML/hh/yr)	Household water use (ML)	Water use from tank (ML)	Total water use from tank (ML)
2006
	Capital city	536 600	0.261	139 896	41 969
	Balance of Australia	989 500	0.261	258 260	154 956	196 925
2007
	Capital city	625 500	0.247	154 499	46 350
	Balance of Australia	1 019 800	0.247	251 891	151 135	197 485
2008
	Capital city	738 100	0.234	172 715	51 815
	Balance of Australia	1 081 500	0.234	253 071	151 843	203 658
2009
	Capital city	758 400	0.198	150 163	45 049
	Balance of Australia	1 085 200	0.198	214 870	128 922	173 971
2010
	Capital city	1 073 400	0.200	214 680	64 404
	Balance of Australia	1 222 800	0.200	244 560	146 736	211 140
2011
	Capital city	1 266 600	0.184	233 054	69 916
	Balance of Australia	1 296 800	0.184	238 611	143 167	213 083
Sources: ABS Water Account, Australia, 2010-11 (cat. no. 4610.0); ABS Water Account, Australia, 2009-10 (cat. no. 4610.0); ABS Water Account, Australia, 2008-09 (cat. no. 4610.0); ABS Household and Family Projections, 2004 (cat. no. 3236.0) and ABS Household and Family Projections, 2010 (cat. no. 3236.0)

26. Whereas total water use from rainwater tanks for 2006 to 2011 has increased by 8%, Table 3 shows own account equivalent monetary expenditure on tank water from 2006 to 2011 has increased by 45% ($358 million to $519 million) due to the average price of household water increasing from $1.82 to $2.44 (figures sourced from WAA).

Table 3 - Own account equivalent monetary expenditure on tank water, Australia, 2006 to 2011 ($ million)
	2006	2007	2008	2009	2010	2011
Capital cities	76	85	97	87	134	170
Balance of Australia	281	278	283	248	306	349
Total	358	363	380	335	441	519

4.0 CONCLUSION

27. Water use by households in 2010-11 was 1,699 gigalitres and the estimated water use from rainwater tanks by households in 2011 is 213 gigalitres (or 12% of total household use).

28. A total of 71,796 GL of water was supplied by the environment and used within the Australian economy in 2010-11. Water use by all industries and households was 13,337 GL in 2010-11 and the remainder was mainly in-stream use for hydro-electricity. When compared to the total water supplied by the environment rainwater tank extraction is 0.3% and in comparison to water use by industries and households 1.6%.

29. Overall the amount of water extracted by rainwater tanks is small but the estimated amount used by households from rainwater tanks is significant and inclusion of this data in the annual ABS WAA would provide improved reporting on the physical and monetary supply and use of water in the Australian economy. In previous WAA publications information on the supply and use of water in the Australian economy has not included the water from rainwater tanks (for households connected to mains supply). The SEEA Central Framework which is used by the ABS records as completely as possible the stocks and flows relevant to the analysis of environmental accounting and the inclusion of water use from rainwater tanks will complete the picture provided by the WAA.


5.0 THE WAY FORWARD

30. The ABS is seeking feedback on the methodology proposed to estimate rainwater tank use with a view to incorporating it into the main supply and use tables of the ABS Water Account Australia.

31. Specific questions are:

• Is the method proposed in this article suitable for estimating household production of water from rainwater tanks? How can the method be improved?
• Are there alternative methods currently in use or proposed that could estimate rainwater tank use by households?
• Is there other significant work that investigates the use of water from rainwater tanks by either households or industry?

Comments on the methods and sources of data used in this article can be submitted at <environment@abs.gov.au>.


BIBLIOGRAPHY

32. Bibliography available on request from <environment@abs.gov.au>.