Rainwater Harvesting

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Rainwater harvesting for irrigation purposes

Rainwater harvesting has been practised in rural areas of Australia for many years. On sites with ample space and regular rainfall, large water storage tanks can provide sufficient water to sustain a house and small garden.

But as Perth and much of WA is subject to dry summers and a very short wet season, there is limited value in collecting rainwater for residential garden irrigation unless a large tank is installed. Standard domestic tanks (1,500L – 3,000L) do not have the capacity to provide sufficient storage to last through the dry season.

If space and budget permit large water storage systems can be installed for external use with a pump and drip system
for garden irrigation.

Underground water storage is becoming more popular as block sizes become smaller.

Types of rainwater storage

Rainwater tanks come in a range of shapes, sizes and materials, including small, medium and large circular or oblong tanks, under eave and slimline styles, bladders and single or modular underground tanks.

The best way to…

utilize rainwater in our Mediterranean (dry summer/wet winter) climate is to connect a tank with automatic mains
water backup to the toilet, basins and washing machine. This leads to significant water savings because rainwater is used through the wet months of the year when indoor use remains constant and regular rainfall refills the tank. It is important to emphasis this fact to clients who are planning to install a new tank/tanks or retrofitting them to an existing home or building.

Planning for these tanks is sometimes overlooked at the building design stage or
clients may wish to retrofit to an existing property.

The slimline tanks are useful for the often narrow or ‘dead’ side of the house where there are fewer windows and the area is not used for other purposes.

The size of a tank is determined by available space, size of the residence and the household needs.

Generally, rainwater tanks can be classed as:

  • Small – less than 2,000L
  • Medium – between 2,000L and 10,000L
  • Large – greater than 10,000L.

From a landscape design point of view, the following styles of tank are very useful in terms of their space saving benefits:
Bladder tanks are a flexible rainwater storage system that hides away under decks or floors making them much more discrete than traditional water tanks. Once the bladder or bladders are full, any excess water should be diverted to the existing storm water system. Multiple bladders can be installed either side-by-side or end-to-end for maximum water storage.

Underground tanks are ideal for large storage requirements, particularly modular style products as these can be designed to fit available spaces and customised to the client’s requirements. With the modular systems there is the option of infiltration or re-use applications. The size of these systems is limited only by available underground space and constraints on excavation.

Rainwater harvesting yield

For every 1mm of rain that falls on a square meter surface, such as a roof, one litre of water can be collected. The formula to calculate rainwater harvesting yield is:

Amount of rainfall (in mm) x Area of the roof (in square metres) = litres of rainfall

So for example:

Area of roof = length x width

20m x 15m = 300m2 (square metres)

Amount of rain = 1 mm
Litres of rainfall = area of roof x amount of rainfall

300m2 x 1mm = 300 litres

For more information

The Rainwater Design and Installation Handbook HB 230-006 is a useful reference and is available from: www.arid.asn.au

Produced by the Australian Rainwater Development Association (ARID), MPMSAA and Standards Australia, the handbook provides guidelines, references and information and covers design objectives, calculations, sizes, water quality and treatment.