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Tank Water Savings From Urban Water Tanks

A Letter to the Stormwater Industry Association Bulletin #106

by Greg Cameron

I believe it is important to bring to Members’ attention, the water saving reductions and reduced stormwater run-off provided by a small rainwater tank on the average urban allotment.

Based on rainfall patterns since 1980, a 4,500 litre rainwater tank will yield the following water volumes per year in Australia’s capital cities.

Brisbane101,000 litres/ 65% of indoor use/ 56% less stormwater
Sydney100,000 litres/ 65% of indoor use/ 48% less stormwater
Perth83,000 litres/ 53% of indoor use/ 67% less stormwater
Melbourne80,000 litres/ 51% of indoor use/ 75% less stormwater
Adelaide72 ,000 litres/ 46% of indoor use/ 77% less stormwater
Hobart71,000 litres/ 45% of indoor use/ 75% less stormwater

Estimated maximum rainwater collection efficiency from the roof of an average home is 80% and therefore the maximum possible reduction in stormwater discharge is 80%.

In Sydney and Brisbane, a 10,000 litre capacity rainwater tank will yield about 80% of indoor water use with about a 60% reduction in stormwater. In the other cities a larger rainwater tank would provide a very small additional yield.

Storing rainwater for garden watering is a highly inefficient use of arainwater tank and should never be a primary use if the objective is to maximise water yield.

Australia has 5 million free standing family homes. If 5% of homes install arainwater tank each year, security of urban water supply will be assured by 2014.

BASIS OF ABOVE CALCULATIONS

1. Water Demand Based on average household water consumption for Melbourne.

Total water use for an average household is 240KL/Y comprising:

Garden35%
Bathroom26%
Toilet19%
Laundry15%
Kitchen4%
Drinking1%

65% or 156KL/Y is used indoors.
– Source: Melbourne Water Resources Strategy Committee, June 2001, pages 14, 17.

2. Rainfall

The rainwater tank yield calculations were based on daily rainfall as recorded between 1980 – 2001 inclusive at the following sites of the Bureau of Meteorology:

Sydney Regional Office site 66062
Melbourne Regional Office site 86071
Brisbane Regional Office site 40214 (1980 – 1993)
Brisbane Airport site 40842 (1995 – 2001)
Adelaide Regional Office site 23090
Perth Airport site 9021
Hobart Regional Office site 94029

3. Roof Run-Off

Run-off (litres) = A x (Rainfall – B) x Roof Area, where

A is the efficiency of collection and a value of 0.8 is used.
B is the loss associated with adsorption and wetting of surfaces and a value of 2 mm per month (24 mm per year) is used.
– Source: National Environmental Health Forum Monograph – water series 3: guidance on the use of rainwater tanks, 1998. /http ://enhealth. nphp. gov. au/council/pubs/pdf7rainwtr. pdf

Roof area of average home: 175M2
– Source: Melbourne Water Resources Strategy Committee.Tank draw-down/day: 427 litres (156 000/365).

Amount of rainfall to produce 427 litres: 3MM (175 X 3 X 0.8)
Total stormwater discharge = (Rainfall -24MM) X Roof Area