water management

rain water storage and
collection

calculating
rainwater runoff from your roof

In order to judge what size water
tanks you should install it is a good idea to work out what
the rainwater runoff from your roof is. Here is a
simple guide as to how to calculate this. All the
calculations shown here are in metric.

BACKGROUND PREPARATION

find the average annual
rainfall for your area

The average annual rainfall for Ballarat is
691 ml. To find out the average annual rainfall for your town in
Australia visit the **Climate Statistics for Australian Sites**webpage on the

**Australian Bureau of Meteorology's website**. People living outside of Australia will probably be able to find their town's annual average rainfall by visiting their own country's weather bureau website or by contacting their local agricultural department.

calculate the area of your roof

You need to calculate your roof area. The formula for this is
:-Length multiplied by width equals
area in (sq/m).

Using my garage's dimensions as a sample the calculation is :-

L 3.4 metres x W 2.6 metres = 8.8 m²

Answer : 8.8 square metres.

work out your
coefficient runoff percentage

Not every drop of water landing on
your roof will runoff down the guttering drainpipes. A thin residue will be
left on the roof after rain and will eventually evaporate in the sun.
For metal and tiled roofs assume a coefficient runoff percentage of 90% or .9.

adjust for first flush devices

First flush devices are designed to siphon of the first rainwater that falls
on your roof. They are used to reduce the amount of dirt and bird droppings
that go into water tanks.
If you have a first flush device fitted to your downpipe
you will have to subtract the amount of water that is diverted
every time it rains by the number of times it is likely to rain.
Different brands of first flush devices siphon off different
amounts of water. Assuming an average of 48 rainfall events each year and your device diverts 20 litres of water each time it rains the calculation for lost water from a first flush device would be :-

20 L x 48 events = 960 L

So if you have a first flush device fitted then you would have to subtract 960 litres from your rainwater calculation.

THE CALCULATION

The rainwater calculation for roof runoff without a first flush
water device is :-Annual rainfall
x roof area x coefficient = annual discharge to your tank in litres.

Using my garage roof dimensions and Ballarat's annual average rainfall as a sample the calculation is :-

691 ml x 8.8 m²
x .9 (90%) = 5,472 L

So I should expect the annual discharge to my garage tank to be 5,472 Litres.

If you have a first flush water device fitted than you have to subtract the amount of diverted water. So the sample calculation is :-

691 ml x 8.8 m² x .9 (90%)
- 960 L (first flush diversion) = 4,512 L