Tuesday, May 25, 2010

Working out our water storage needs...

Earlier in the year we did a water usage audit for our home - if you want to do your own water audit look here.  As a follow-up I have been meaning to do a water storage needs analysis - so here is the start of the analysis.

I based my workings on the information given in Rosemary Morrow's book "Earth User's Guide to Permaculture"


I wanted to calculate how much rainwater tank volume I would need to have to flush the toilet and use the washing machine for the whole year.  I will explore the amount of water I use on the garden and in the glasshouse at a later date.

At the moment, we have a single tank (10,000 litres) which we use for the garden, including the glasshouse as well as the main toilet and washing machine.  In summer I switch off the rainwater to the toilet and washing machine to conserve water for garden and glasshouse.

Here is my water storage needs analysis for the toilet and washing machine.

1. Household consumption of water for the washing machine and the main toilet was 4,800 litres per month.
2. Rainfall in Orange is spread fairly evenly throughout the year - although it is driest from March - May.  Mean monthly rainfall ranges from 52 mm in April to 97 mm in August (from Climate Data Online by the Bureau Of Meteorology) - I should aim to have a month supply (about 5,000 litres or half a tank) in reserve to use as a buffer.
3. Our area of roof that is hooked up into our 10,000 litre tank is 100m2.
4. Based on this roof area and the average monthly rainfall I worked out the volume of rainwater we can harvest each month for this main water tank (= roof area draining to tank x average monthly rainfall).  Depending on the month we can harvest between 5,200 or 9,700 litres of water per month. So that's at least half a tank full depending on the month - and sometimes nearly a full tank.  Even in the drier months of the year (March - May) we can harvest at least half a tank of water which is our current estimated usage for the toilet and washing machine.

So my conclusion from these calculations is that our current tank is adequate to run the toilet and washing machine all the time.  However, if I want water for the garden and glasshouse I should investigate where else we can put another rainwater tank - in a small yard space is limited.  We currently have a 1,000 l tank under the back deck which we rarely use.  Is this enough for our glasshouse and garden? - that's for another day's calculations.

If I can save 4,800 litres per month of town water, or 57,600 litres per year we could save nearly $90 a year - which would be about a quarter of our water costs.

I write this as is lightly rains outside - its a good feeling to know that we are harvesting rainwater as I sit at the computer.

Do you have the resources to be less dependant on the town water supply.

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