Rain Water Harvesting or Collection
Rain water is a free source of nearly pure water. It
can be used to supply potable (drinkable) water
and non-potable water. For non-potable uses, like
watering landscapes, it is ready for use as it falls from
the sky. For potable uses, rain water must be treated
to remove or kill disease organisms that may be present.
For centuries the world has relied upon rainwater
harvesting to supply water for household, landscape, and
agricultural uses. Before city water systems were
developed rainwater was collected (mostly from roofs) and
stored in cisterns or storage tanks.
The central plazas of towns were not only gathering
places, but also the collection surfaces for underground
tanks where water was stored for use by adjacent shops
and homes. Historic structures collected rainwater from
their roofs for household use. These systems may no
longer be in use, but they are clear evidence of the
reliance placed on rainwater .
Today, many parts of the world, including Hawaii and
the entire continent of Australia, promote rainwater as
the principal means of supplying household water. On many
Caribbean islands where rainwater is the most viable
water supply option, public buildings, homes, and resorts
all collect rainwater to supply their needs. In Hong
Kong, rainwater is collected from skyscrapers to supply
water needs.
Rainwater harvesting promotes self-sufficiency and
fosters an appreciation for water as a resource. It also
promotes water conservation.
Rainwater harvesting also conserves energy as the
energy input needed to operate a centralized water system
is bypassed. Many systems require only a small pump to
create water pressure in household pipes.
Local erosion and flooding from impervious cover
associated with buildings is lessened as a portion of
local rainfall is diverted into collection tanks.
Rainwater is one of the purest sources of water
available. It's quality almost always exceeds that of
ground or surface water. It does not come into contact
with soil or rocks where it can dissolve minerals and
salts nor does it come into contact with many of the
pollutants that are often discharged into local surface
waters or contaminate ground water supplies. However,
rainwater quality is influenced by where it falls.
Rainfall in areas where heavy industry or crop dusting is
prevalent may not have the same purity as rain falling in
other areas.
Rainwater is soft. It can significantly lower the
quantity of detergents and soaps needed for cleaning.
Soap scum and hardness deposits do not occur. There is no
need for a water softener as there often is with well
water. Water heaters and pipes are free of the deposits
caused by hard water and should last longer.
There are several questions to answer to decide if
rain water collection is for you.
For centuries throughout the world, people have relied
on rainwater harvesting to supply water for household,
landscape, livestock, and agricultural uses. Before
large, centralized water supply systems were developed,
rainwater was collected from a variety of surfaces - most
commonly roofs - and stored on site in tanks known as
cisterns. With the advent of large, reliable community
treatment and distribution systems and more affordable
well drilling equipment, rain harvesting systems have
been all but forgotten, even though they offer a source
of pure, soft, low sodium water. A renewed interest in
this time-honored approach has emerged due to:
- the escalating environmental and economic costs
of providing water by centralized water systems
or by well drilling;
- health concerns requarding the source and
treatment of polluted waters; and
- a perception that there are cost efficiencies
associated with reliance on rainwater.
An amazing amount of water can be collected. The rule
of thumb is 600 gallons of water per inch of rain per
thousand square feet of catchment area. Here is the basic
formula for calculating the potential amount that can be
collected:
catchment area of building1
x inches of rain x 600 gallons
1000
|
1 Calculate the catchment area using the
footprint of the building plus the length of the
overhangs. In other words if your building is 25 feet by
50 feet the catchment area is 1,250 square feet.
Sample calculation
1,250 sq ft of catchment area
x 3 inches of rain x 600 gallons per inch
1000
|
= 2,250,000
1000
|
| = 2,250 gallons of water
collected from three inches of rain |
Not all the rain that falls can actually be collected.
Several factors affect collection efficiency:
- A small amount of rain, 3/100s to 1/10th of an
inch, will be needed to wet the roof and fill the
roof washer.
- Some of the rain will overshoot the gutters or
spill out of gutters during heavy downpours.
- Once storage tanks are full any additional rain
will not be collected.
Efficiency is usually presumed to be 75% to 90%
depending on system design and capacity.
How much rain will fall in a given year is unknown.
Rainfall data for previous years can be used to predict
how much rain may fall.
The first step in determining reliability of rainfall
is to find out the average rainfall in your area. This
may be all you need to know if you are relying on
rainwater as a supplement to another source.
If rainwater is your sole source of water, you need to
know more than average rainfall. You need to know what
the least amount of rainfall in a year can be and you
need to know when the rain falls. If part of the year is
very dry, can enough water be collected during the rainy
months to last through the dry months? If only the
minimum amount of rain falls in a year will that be
enough to meet needs?
Least Amount of Rainfall
It is important to examine the worst case scenario. A
handy rule of thumb to determine the likelihood of low
rainfall is to take the average amount of rainfall for an
area and divide it by two. For example, for an averages
32 inches of rain a year: divide 32 inches by two and you
get 16 inches. per. year.
- Assume 1,250 square feet of catchment area
- Assume 16 inches of rain in the year
- Assume 75% collection efficiency
1,250 x 16 x .75 x 600
1000
|
| = 9,000 gallons of water a
year |
On the other hand in a year of average
rainfall (32 inches) you can collect
1,250 x 32 x .75 x 600
1000
|
= 18,000 gallons of water a
year
For centuries throughout the world, people
have relied on rainwater harvesting to supply
water for household, landscape, livestock, and
agricultural uses. Before large, centralized
water supply systems were developed, rainwater
was collected from a variety of surfaces - most
commonly roofs - and stored on site in tanks
known as cisterns. With the advent of large,
reliable community treatment and distribution
systems and more affordable well drilling
equipment, rain harvesting systems have been all
but forgotten, even though they offer a source of
pure, soft, low sodium water. A renewed interest
in this time-honored approach has emerged due to:
- the escalating environmental and economic
costs of providing water by centralized
water systems or by well drilling;
- health concerns requarding the source and
treatment of polluted waters; and
- a perception that there are cost
efficiencies associated with reliance on
rainwater.
 Wooden collection tank
|
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