Rainwater Harvesting and Stormwater Reuse for Arid Environments - Design with the Desert: Conservation and Sustainable Development - page 370

Cistern lid with vent to be secured and tight
enough to keep out insects and animals

Connection to
municipal water/
makeup water

To pump and
pressure tank

Hole in pipe to prevent
siphon effect

Overflow inlet
(minimum 12 in. below
municipal water services)

Downspout
from gutters

Floating or stationary
pump inlet

First-flush
diversion device
Optional turbulence
calming device to
prevent remixing of
sediment

Drain valve or hose bib

Minimum water
level 6-12 in.

Overflow to French drain or
surface water drain (cover end
with netting or wire mesh or
flat valve to keep out insects
and animals)

Anaerobic zone

FIGURE 21.4
Typical residential rainwater harvesting cistern.

require an inflow smoothing filter or turbulence dissipater depending on the proximity of
the rainwater inlet and pump intake or the amount of time rainwater is left to settle before
the pumping is initiated.
The storage system overflow may act as a distribution system that delivers excess water
to an adjacent landscape. All overflows exposed aboveground should have some means of
stopping pests from entering the storage system. Fine screens may be placed over the pipe
ends and, in areas of high rain quantities, water traps—similar to sinks and toilets—may
be used.
For landscape irrigation, stored rainwater may go through additional filters before it is
directed into an irrigation pump and distribution lines. This may be necessary to avoid
clogging the irrigation system. For a potable water system, the water must go through a
purification process (Figure 21.5).

21.4 Water Balance Analysis
A water balance analysis or water budget allows a designer to determine how much rain-
water can be collected by the project catchment area, including rooftop and ground level
areas. A water budget provides a supply and demand analysis on a monthly basis and pro-
vides water quantities for sizing cisterns. It will also determine how much, if any, supple-
mental water is needed to augment a system. The budget will allow a designer to redesign
a project to increase or reduce the catchment area to meet the water demands of a landscape
or the amount of potable water desired. An alternate water source may be needed for a
few years in addition to rainwater to supplement water needs until plants are established.
Eventually, harvested and natural rainfall may be adequate for plant needs.