Rainwater Tank Calculator

Discover how much water you could save by installing a rainwater tank!

Download DKA COOLmob's Rainwater Tank Calculator and input your roof size, daily water use and tank size.

The calculator is especially for Alice Springs residents and uses data from the Bureau of Meterology.

How to use the Alice Springs Rainwater Tank Calculator

Go to www.dkacoolmob.com and click on the Calculator link on the left. Download the appropriate Excel spreadsheet and then open the spreadsheet on your computer. Check the macro security settings are on low and enable macros [Download excel troubleshooting PDF]

Enter DRAINAGE: Firstly you need to measure the roof area.  You do this by measuring the length and width of your roof or use an estimate of 150-250m2

Enter RAIN LOST: The average amount of rain lost before capture is generally 1mm. This goes down the first flush diverter.

Enter DAILY WATER USE: Input how much water from your tank you plan to use per day. E.g. A tank connected to a dual flush toilet in a four person house will use about 80L per day.

Enter TANK SIZE: Input your tank size. If you don’t have a tank yet and are not sure what size, try inputting 5,000L or 10,000L.

Check RESULTS: Considering what you want to use the water for and your average daily usage, the calculator then estimates

• How many days a year the tank will have water
• How many days a year the tank will be empty
• How many litres you will save a year from going to waste.

HAVE A GO: Experiment with different tank sizes and daily water use volumes to see what will work for your house.

Some things to think about before getting a tank

Rainfall in arid climates can vary significantly, some years a tank may be full to capacity and other years there may be little rainfall. The calculator bases the number of days a year the tank will have water on historic data for Alice Springs from the Australian Bureau of Meteorology. The calculated output is an estimate based on these averages. The calculator also does not take into account changes in rainfall patterns which might occur due to climate change. The calculator uses a model created by Dr Mark Stafford-Smith.

Are gardening and drinking good uses for rainwater?

The average household in Alice Springs uses 1500 Litres a day, and about 990 Litres (65%) of that goes on gardens.  You can see that even with a 5000L tank filled to the brim, you’d only have enough for 5 days of garden watering! 

On the other hand, the volume of water used in a house for drinking is quite small. You might not reduce the level of water in the tank enough in between rains.

How about these other options?

• Washing machine 80-250 L per wash, depending on your machine.
• Toilets: about 20L per person per day with a dual flush
• Solar hot water system- 9.5L shower* 3 people * 4 minutes+ washing dishes + washing machine (60L)=  about 200L per day

Other harvesting options: sumps and swales

You don’t need a rainwater tank or even gutters to make good use of rainwater. Look where the water on your property goes when it rains. Can you contour the ground so that it is channelled towards your plants instead of the road or the drain? Some people make swales and depressions to keep the water where it is needed most, or create sump areas with wetland plants.

Water efficiency is the cheapest way of reducing mains water use

We use a massive amount of water in Alice Springs. Simple, relatively cheap changes to hardware and usage patterns can make a huge difference to your water bills. Before making the investment of installing a tank, it is recommended that you take as many other measures as possible to increase your water efficiency. See links below.

Have a home water audit with DKA COOLmob and get a break-down of where your water goes- how much is on the garden, how much does the washing machine use? Ring 8952 0299 or email dka.coolmob@coolmob.org. Join up and receive our monthly newsletter for updates on latest rebates and options.

Benefits of using rainwater 

• Fresh rainwater is soft water. Aesthetically it is great for washing clothes or hair and watering sensitive plants
• You don’t use as much soap in soft water, suds are formed more readily
• Using less soap is better if the greywater is used in the garden later
• Supplementing mains groundwater supply will conserve it for future use
• Alice Springs currently “mines” groundwater i.e. Uses it at a greater rate than it is replaced. Based on 2004 figures Alice Springs extracts 11 000ML a year from groundwater basins.
• When people use rainwater tanks they tend to become more conscious of how much water is used around the home.
• With a roof area of 250m2 for Alice Springs a 10,000 litre tank will provide approximately 40,000 litres a year.

Will I save money by installing a rainwater tank?

Realistically, you are unlikely to recover your costs with the current price of water- 75cents per 1000L.  For example, if you use 100% of 40,000 litres from a 10,000L tank, you will save approximately $30 dollars a year on your water bill.  It’s not much money. On the other hand:

• The cost of water is likely to increase soon to be in line with the rest of the country.
• If every house in Alice Springs house had a 10,000 litre tank and utilised 100% of the

40,000L of water in an average year the town could, given constant use patterns, save 366 920 000L, or 366 920kL or 367ML from groundwater aquifers.  (Based on Australian Bureau Statistics 2006 census 9173 occupied private dwellings).

Buying a tank

Tanks come in all shapes and sizes. Some are round, some are long and rectangular walls. There are bladders which fit under your house and others which are installed underground.  Local suppliers of tanks include:

Alice Tanks; Dave Douglas Tyre City; Reece Plumbing; Stratco

Building approvals

In Alice Springs, no building approval is required for tanks which sit on the ground. If they are elevated on a tank stand, then any tank stand greater than 400mm in height on which the tank is sitting on will require a building permit.  When installing a tank the plumbing connections are required to be installed by a licensed plumber. If a structure requires a building permit then the Planning Scheme setback clause applies.  If you have any queries please contact Building Advisory Services, Ph: 08 8951 9235.

A list of Green plumbers for the Alice Springs region can be found on the greenplumbers website http://www.greenplumbers.com.au/directory/index.php?pageID=143

Water Tank Maintenance

Water Tanks must be maintained to minimise risks of pathogens.  Luckily, the amount of maintenance needed to do this is low.

• Check screens are in good condition to prevent animals, including mozzies, from getting inside
• Minimise organic matter build up by cleaning gutters.
• At the end of a dry period and between big rain events clean gutters and screens and flush out first flush tanks.
• If tank is not maintained, chlorinate the supply
• Clean out sludge in bottom of tank every few years
• Shade tank from sun to extend life

If your tank is contaminated, under no circumstances use kerosene! Kerosene is corrosive to plastic tanks and the interaction creates all sorts of nasty by-products. Use chlorine if you need to kill pathogens. Likewise, avoid other oil based products in plastic rainwater tanks.

The Enhealth report, Guidance on Use of Rainwater Tanks is endorsed nationally and is your first port of call for detail on tank maintenance. Another very handy document is Report 39 Guidance Manual for the Design and Installation of Urban Roof water harvesting Systems in Australia (Edition 1). Go to http://www.waterquality.crc.org.au/ and navigate to publications.

The enHealth document : Guidance on the use of Rainwater Tanks (2004) [PDF file, 979k]

 www.icat.org.au has good simple information on rainwater tank maintenance, and there is plenty of useful literature out there.

Health and Water Quality

Microbiological

Environmental Health recommends that you filter or boil rainwater before drinking.

The biggest health risk from drinking rainwater is contamination from microorganisms, specifically pathogens that make people sick. Pathogens can enter your tank through animal droppings on the roof or animals getting into the tank. Proper tank maintenance (see below) will minimise contamination risks within rainwater tanks.

Special care must be taken to prevent Legionella bacteria. Hot water should be stored at a minimum of 60ºC. Untreated rainwater should not be used in evaporative air conditioners. The real danger of Legionella lies in inhaling the bacteria into the lungs, rather than from ingesting them in the stomach.  Avoid uses of rainwater which create fine mists (aerosols).

Microbiological Water Quality can be tested in Alice Springs at the Water Microbiological Laboratory at AZRI, however, what the tests can tell you will be limited, Legionella bacterial tests are not available and only very limited indicator organism tests are carried out. You are better off chlorinating your tank if you have doubts about your maintenance program.

The Victorian rainwater tank information booklet is a good guide with chlorination instructions and other FAQs and can be found here: http://www.health.vic.gov.au/environment/downloads/your_private_drinking_water_supply.pdf

Chemical

Roofs accumulate matter from the surrounding environment, including dust and soot.  That’s why some people call rainwater “roofwater.”

A first flush system will eliminate most dust and particulate matter. In terms of roofwater quality, roof materials are important. Most tin and tile roofs are okay.

Never drink water harvested from a roof painted with lead paint. Also avoid: cracked, flaking paint, tar roofs and tiles that might contain asbestos.

Some people worry about chemicals leaching from the tank itself. This is not considered to be a health risk (unless you use kerosene in a plastic tank).  All rainwater tanks are tested to Australian standards for storage of drinking water.

Rebates for rainwater harvesting and more links

NT Government rebates

NT Waterwise Central Australia Rebate Scheme, offers a plumbing rebate of up to $500 to households in Alice Springs and Tennant Creek for eligible services associated connecting a rainwater tank to a household hot water system, toilet, washing machine, air conditioner or to supplement household drinking water supplies. Refer to the waterwise website www.nt.go.au/waterwise  for more information.  Or book a DKA COOLmob water audit. Your auditor will bring a rebate pack with all the information.

There has been talk of a Northern Territory wide rebate on rainwater tanks in the 2009-2010 Financial Year. Stay tuned.

Federal Government Rebates

The national Greywater and Rainwater Initiative will provide grants of up to $500 for households to install greywater and rainwater systems. This rebate is meant to be available sometime in the 2008-2009 Financial Year. For more information, see  http://www.environment.gov.au/water/programs/nrgi/index.html

More links to technical information

For information about Alice Springs water quality go to the water quality report published by the Power and Water Corporation

http://www.powerwater.com.au/powerwater/docs/water_quality/DW_quality_report_2007.pdf

The Water Quality Co-operative research has information on the benefits of rainwater tanks and useful links to installation information. Have a look at Report 39 Guidance Manual for the Design and Installation of Urban Roof Water harvesting Systems in Australia.

http://www.waterquality.crc.org.au/ and navigate to publications.

http://www.habitathardware.com.au/rebates.html

The Waterwise website for NRETAS also has a good technical factsheets.

http://www.nt.gov.au/nreta/publications/natres/water/rainwatertanks.pdf

The following sites have comprehensive rainwater tank information.

http://enhealth.nphp.gov.au/council/pubs/documents/rainwater_tanks.pdf

http://www.ircsa.org/factsheets/Cunliffe.pdf

www.arid.asn.au

www.icat.org.au

www.watertankco.com.au/benefits.html

www.sustainable –water.com

www.rainharvesting .com 

www.leafbeater.com

http://www.savewater.com.au/

http://www.nt.gov.au/nreta/water/wise/index.html

http://www.powerwater.com.au/powerwater/factsheets/how_much_water_do_we_use_july_2008.pdf

http://www.powerwater.com.au/powerwater/factsheets/saving/simple_tips_to_save_water_in_the_garden.htm

http://www.powerwater.com.au/powerwater/factsheets/saving/simple_tips_to_save_water_at_home.htm

Rainwater resources compiled by Tracey May and Kat taylor December 2008. Thanks also to Robyn Grey-Gardener, Nerida Beard and Glenn Marshall for your help.

The rainwater tank calculator was made possible with the support of the Northern Territory Government