water
tanks
the truth about
water tanks
Water tanks are popular, but many of them are far too small to be of much use in the garden.
Geoffrey Burnie shares his experiences with his tanks and has plenty of practical tips for you.
44 Burke’s Backyard
F
irst up, know this. To be
of any use in the garden,
you need to be able to
store a lot of water, and
that means a big tank or several
smaller tanks. How much is a lot?
That depends on the size of the
planted area you want to water
and how often you want to
water it. It’s better to apply a lot
of water in one go than to try
to conserve what you have by
sprinkling it on. Why? Because
light sprinklings barely wet the
surface of the soil and that only
encourages roots to grow closer
to the surface where they
are vulnerable to drying out.
Do your sums
Aim to apply 25mm (1 inch) of
water every time you water.
That amount will wet sandy soil
to a depth of 150mm (6 inches)
or heavy clay soil to a depth
of 75mm (3 inches). To apply
25mm takes 25 litres of water
for each square metre of soil. So,
if you have 100 square metres of
garden beds you’ll need to apply
2500 litres each time you water
it. So, a 5000-litre tank will give
you just two waterings. But
remember, one application of
25mm is probably enough for
two more weeks without water.
How often?
How long 25mm of water (or
rain) will last depends on the
depth and water-retentiveness of
your soil, the plants grown and
how hot and/or windy it is. But
say it does last two weeks. That
means if you get a good fall of
rain 25mm on, say, the first of
the month, you don’t need to
water again until the 14th. You
apply your water and that lasts
until the end of the month, by
which time there’s likely to be
more rain (on the coast, at least).
Rooftop collectors
3m
5 x 3m
= 15m2
6m
7m
11 x 7m
= 77m2
2.5 x 2.5m
= 6.25m2
2.5m
2.5m
Illustration by Pamela Horsnell, photos by Brent Wilson, BlueScope Water, Aussie Bladda, Waterwall
5m
77 + 15 + 6.25 = 98.25
mUltiply by 1.06 for roof pitch
Total roof area = 104 square metres
This illustration gives an approximation of the
volume of water a roof sheds. Measure the
outside of the house, adding the width of the
eaves to your measurements. Multiply the
length by the width to get the area of the
roof (in square metres). Multiply this by 1.06
if the roof is pitched. In our example, the
little house has a roof area of just over 104
square metres*. This means that for every
millimetre of rain that falls, the roof sheds
104 litres of water. If all the downpipes are
connected to the tank, you’ll collect all that
water. If only some downpipes are connected
you will collect that proportion of the water.
* While house sizes vary, most houses would
have a bigger roof area than this example,
probably in the 150-200 square metre range.
Burke’s Backyard 45
water
tanks
measuring
the flow
You can calculate the
volume of water you
are applying through
the pump or with
gravity feed from your
water tank. Turn the
tap on full then stick it
into a watering can with
litres marked on the
side. Time how long it
takes to fill five litres.
Say it takes 16 seconds.
There are 3600 seconds
in an hour so divide
3600 by 16 to get 225.
Multiply that figure by
5(litres).Yourtankemits
1125 litres per hour.
How big a tank?
The amount of water you will
want is linked to the size of the
garden and how often you want
to water it. But it’s no good
installing a huge tank if you don’t
have the rain to fill it. In some
parts of Australia, rain is seasonal.
In Brisbane, the tropics and subtropics, most of the rain falls in
summer and there can be
months with little or no rain. In
Perth and Adelaide, autumn and
winter are rainy with summers
very long, hot and dry. In Sydney
and Melbourne, rain can fall anytime but in recent years it has
been erratic and there have been
long dry periods in both winter
and summer. It’s those dry periods when you’ll want tank water
so the size of the tank you need
depends on how long your dry
spells are. The longer the dry the
more water you’ll have to save
up. But if you install, say, 30,000
litres of storage, how much rain
will it take to fill it?
In our example on page 45,
the house would need 288mm
falling on it to fill a 30,000 litre
tank. If the house were bigger,
say 150sq m of roof, 200mm of
rain is needed. Unless you have
a big house or unless there are
times of the year when you
receive very high rainfall, storage
of 20,000 litres is about the maximum to aim for. That is a lot of
water and gardeners will be very
pleased to have it. Most people
will probably find they have room
for much less storage than that.
what do tanks cost?
Small tanks start from a few hundred dollars and go up to
around $12,000 for a big concrete tank in the ground. My
two bladder tanks (see right for more) cost $7600 for 8500
litres but that includes installation, all the plumbing and
piping, two new garden taps and the plumbing to them
and the pump to distribute the water. Bladders are more
expensive than conventional tanks but have the advantage
of being able to go under the house. To the cost of the
tank you may have to add a concrete pad for it to sit on, a
stand to raise it so the water can drain out, plus plumbing.
If a pump is needed, it has to be wired in. There’s a lot of
information, including costings, on the internet but your
particular site will probably have a bearing on costs. Both
NSW and Victoria offer a cash rebate and your state may,
too. The amount varies between states and with the size of
the tanks you put in and whether or not you connect them
to the toilet or washing machine. I received $500 back.
With a tank you can water any day and you can use the
sprinkler or your irrigation system to apply that water.
What’s your local rainfall?
This information is easy to
find. On the net go to www.
australianweathernews.com
Choose ‘recent & climate’
from the options at the top
of the page. Then select
‘daily weather by station’
and find your town or somewhere close and you can see
how much rain fell on which
day for the whole year. You
can also see the averages
46 Burke’s Backyard
listed so you know whether
you are getting more or less
than usual and you can go
back a few years too. You
can use this useful website
to see how long between
rain and how much you
could expect when it does
fall. The last few years have
been dry so pay attention
to the actual figures rather
than historical averages.
Case history: Geoffrey Burnie
I
have 8500 litres of
water storage in two
bladder-type tanks
(similar to the one pictured above). One sits
under the deck, the other
under the house. I have
about 280sq m of garden
to water and live on the
coast in Sydney. My roof
area is 150sq m but only
two of three downpipes
feed into the tanks, so
it takes 85mm of rain to
completely fill the tanks.
If I did what I am
telling you to do in this
story and applied 25mm
of water to my garden
in one go, I’d use up
7000 litres, leaving just
1500 litres in the tanks.
That’s enough to put
only 5mm of water on
later on, and I think
that’s a waste. What I
do is apply 6000 litres
(or 21mm) and apply
the balance (2500 litres
or 9mm) one week
later. The soil is still
moist from the first lot
so the second lot goes
further. Then I have to
wait for more rain.
I wish I had twice the
capacity, as that would
allow me to endure eight
weeks with no rain,
which I could probably
stretch to 10 – quite a
long time for coastal
Sydney. But if I did, I’d
need 170mm of rain to
fill the tanks. In 2006,
we had enough rain to
fill the tanks in January
and again in June but for
the four months from
February to May we only
had a total of 158mm
(the average for my area
is 493mm) so in all that
time the tanks would
never have been full.
I know this because
I have a rain gauge
which I check every
morning. I keep a daily
record on my computer
(but you can also do it
on graph paper). When
I know how much rain
we have had I can better
judge when to use my
tank water.
bby
Burke’s Backyard 47