 Rainwater
Availability:
The potential yield of a rainwater harvesting system is largely
dependant upon the available surface area for collection and
upon location. The East coast receives an average annual rainfall
of approximately 850 litres/m2 whilst the west coast receives
more than 1000 litres/m2. The nature of the collection surface
is also important in determining potential yield. When collecting
rainwater from a well maintained, pitched roof with regularly
cleaned seamless gutters a yield of approximately 90% can
be expected. The yield from a flat roof with poorly maintained,
leaking gutters will be much less than this. Table 1 below
estimates rainwater yields from an average roof on both the
East and West coasts. The table assumes that 90% of available
rainfall is collected and that a further 10% is used by the
self cleansing facility of the primary screening chamber.
Click
here to see table - Typical Average Daily Yields (Litres /
Day)
Optimum Tank Size:
Table 2 below lists the optimum tank size for ‘grey
water’ type only applications (estimated at 75 litres
/ day / person). Table 3 lists the optimum tank size assuming
that the water is used for all purposes apart from direct
consumption (estimated at 150 litres / day / person). Both
tables assume an East coast rainfall intensity. For West coast
locations read the tank size from the column with the next
largest roof area (ie. For a 75 m2 roof area read from the
100 m2 column, for a 200 m2 roof area read from the 250 m2
column). The listed values are optimum tank sizes capable
of capturing an average of 90% of available rainfall and assume
a mains top up facility is available to cover shortfalls.
It becomes increasingly difficult to increase the yield beyond
90% and to achieve a capture rate of 99% a much larger tank
is required. Table 4 lists the recommended tank size to achieve
an average yield of 99% for various roof sizes, again assuming
an East coast rainfall intensity. As before, for West coast
locations read the tank size from the column with the next
largest roof area. Tanks sized to achieve a 99% yield are
generally used only in applications where mains top up is
not available and a premium is placed on harvested water.
Click
here to see table - Optimum Tank Size for ‘Grey water’ use
only
Click
here to see table - Optimum Tank Size for ‘Grey water’ and
bathing
Click
here to see table - Maximum Practicable Tank Size (Litres)
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