There are several simple ways to minimise the volume of mains water consumed

• Decrease overall water demand
• Use of rainwater in place of mains water
• Use of grey water where it is appropriate

Advice on reducing mains water demand is available from any of the water provider websites.

Grey water re-use can be as simple as using the rinse water from the washing machine to water gardens. More sophisticated systems are available from specialist providers.

How should we use harvested water in place of mains water to maximise savings? For the average household the single biggest use of water (about ¹/₃^rd) is garden irrigation. At first glance, it would seem the best use of rainwater is to meet our irrigation needs. There is a far better use for harvested water than storing it for irrigation later in the year.

How should I use my rainwater tank?

Most of us consider a rainwater tank to be a long term storage vessel. We want to harvest as much water as we can for later use. The amount of water harvested is determined by the size of the tank.

If we re-think our concept of a rainwater tank as being a short term water storage vessel other possibilities appear. Almost ²/₃^rds of Adelaide's rain falls between May and September. Over 33,500 litres falls on a 90m2 roof during this period. To store this volume of water would likely fill our backyard with tanks. By using the water as it falls, we can harvest most of the available rainfall in a relatively small tank.

Consistent use of rainwater allows a relatively small tank to provide as much as 20 or 30 times its storage capacity.

Efficient use of harvested rainwater

Let's compare the water savings possible by using a tank for long term or short term storage.

1. A 10,000 litre tank is installed to store water for garden irrigation. This water is used at the rate of 1,000 litres per week from the beginning of October; or
2. A 2,000 litre tank is installed to provide 200 litres per day for the home's internal water requirements

The large capacity tank used for long term storage would

• be full at the end of May - it cannot harvest any more water until it is partially emptied in October and rain falls again
  
• provide a total volume of 16,000 litres over the year - as water is used on the garden in October storage space is created allowing an additional 6,000 litres of rainwater to be harvested over summer

The small tank used for short term storage would

• be full at the end of July and
  
  • have provided 5,600 litres in May
    
  • 6,000 litres in June
    
  • 6,200 litres in both July and August
  • won't run dry until November
• provide a total volume of 51,000 litres over the year

It is possible to save three times as much water from a tank 1/5^th the size by consistently using the rainwater as it falls.

The family that has saved 51,000 litres of water by using their tank for short term storage could still use 16,000 litres of "guilt free" water on their garden from the mains supply if they wish. They will have reduced their overall mains demand by 35,000 litres.