Technical brief

Rainwater harvesting
Part of a series ofWaterAid technology
briefs.
Available online atwww.wateraid.org/
technologies
January 2013

Introduction
Where there is no surface water, where
groundwater is deep or inaccessible due to hardground conditions, or where it is too salty, acidic orotherwise unpleasant or unfit to drink, another sourcemust be sought. In areas that have regular rainfall, themost appropriate alternative is the collection of rainwater,
called ‘rainwater harvesting’.
Falling rain can provide some of the cleanest naturally
occurring water that is available anywhere. This is
not surprising, as it is a result of a natural
distillation process that is at risk only fromairborne particles and from man-madepollution caused by the smoke andash of fires and industrial processes,
particularly those that burn fossil fuels.
Most modern technologies for obtaining
drinking water are related to the
exploitation of surface water from rivers,
streams and lakes, and groundwater fromwells and boreholes. However, these sources
account for only 40% of total precipitation.
It is evident, therefore, that there is considerable scope for the collectionof rainwater when it falls, before huge losses occur due to evaporation andtranspiration and before it becomes contaminated by natural means or
man-made activities.
The term ‘rainwater harvesting’ is usually taken to mean the immediatecollection of rainwater running off surfaces upon which it has fallen
directly. This definition excludes run-off from land watersheds into
streams, rivers, lakes, etc. WaterAid is concerned primarily with theprovision of clean drinking water; therefore, the rainwater harvesting
projects we support are mainly those where rainwater is collected fromroofs, and only to a lesser extent where it is collected from small ground,or rock, catchments.

Technical brief

Rainwater harvesting
Advantages of rainwater harvesting✓ Relatively cheap materials can be used for construction of containers andcollecting surfaces✓ Construction methods are relatively straightforward✓ Low maintenance costs and requirements✓ Collected rainwater can be consumed without treatment, if a clean
collecting surface has been used
✓ Provides a supply of safe water close to homes, schools or clinics,
encourages increased consumption, reduces the time women and children
spend collecting water, reduces back strain or injuries from carrying heavy
water containers
Disadvantages of rainwater harvesting✗ Supplies can be contaminated by bird/animal droppings on catchment
surfaces and guttering structures unless they are cleaned/flushed beforeuse✗ Poorly constructed water jars/containers can suffer from algal growth andinvasion by insects, lizards and rodents. They can act as a breeding groundfor disease vectors if they are not properly maintained

How it works

Fig 1: Guttering materials

Roof catchments
Rainwater can be collectedfrom most forms of roof. Tiledroofs, or roofs sheeted withcorrugated mild steel etc are
preferable, since they are theeasiest to use and give thecleanest water. Thatched or palm
leafed surfaces are also feasible,
although they are difficult toclean and can often taint the
run-off. Asbestos sheeting or
lead-painted surfaces should be
avoided.
The rainwater is collected inguttering placed around the eavesof the building. Low cost guttering
can be made up from galvanised
mild steel sheeting (a thickness ofaround 22 gauge), bent to form a‘V’ and suspended by galvanised
wire stitched through the thatch or
sheeting, as shown in Figure 1.

The guttering drains to a down-pipe,
which discharges into a storagetank. The down-pipe should be madeto swivel so that the collection of
the first run-off can be run to waste(the first foul flush), preventing
accumulated bird droppings, leaves,
twigs and other vegetable matter,as well as dust and debris, fromentering the storage tank. Sometimes
a collecting box with a mesh strainer(and sometimes with additional filter
media) is used to prevent the ingressof potential pollutants.
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Technical brief

Rainwater harvestingFig 2: Possible guttering to tankconnection

Alternatively, a foul flush box, which
can be drained separately, may befitted between the down-pipe and thestorage tank.
The run-off from a roof is directly
proportional to the quantity of rainfall
and the plan area of the roof. For every
one millimetre of rain, a square metreof roof area will yield one litre of water(disregarding evaporation, spillage
losses and wind effects). The gutteringand down-pipes should be sized so as
to be capable of carrying peak volume
of run-off; in the tropics this can occur
during high intensity storms.
Storage tanks
The capacity of the storage tank isbased upon several design criteria:
rainfall patterns and volume, theduration of the dry period and theestimate of demand. Sometimes
sophisticated calculations are
involved, but these tend not to take
into account human behaviour andthe willingness to use water if it isavailable and not conserve it for futureuse, in the hope that the dry spell willsoon be over.
The following simple calculation canbe used to approximate the potential
supply of rainwater from a collecting
surface. This can help to determine
the capacity of storage tanks:

WaterAid/Caroline Irby

S = R x A x Cr
S = Mean rainwater supply in m3
R = Mean annual rainfall in mm/year
A = Surface area of catchment in m2
Cr = Run-off coefficient
The run-off coefficient accounts forlosses due to splashing, evaporation,
leakage and overflow and is normallytaken to be 0.8 (80%).
The provision of the storage tank is
the most costly element of a rainwater
harvesting project, usually about
90% of the total cost. Storage canrange from small containers madefor other purposes, for example oil
drums, food cans etc, up to largetanks of 150 cubic metres or more, at
ground level, or sometimes beneathit. These larger tanks are made ofconcrete or ferrocement and are usedas storage for schools, clinics or otherinstitutions with large areas of roof.

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Technical brief

Rainwater harvesting
Domestic storage tanks
Tanks for household use can be madecheaply in a variety of ways. ‘Basket
tanks’ are baskets made of bamboo,
originally intended for carrying or
storing maize, which have beenplastered internally and externally, in
two stages, with sand/cement mortar.
Storage of up to two cubic metres
can be provided by such baskets.
Corrugated galvanised mild steel
sheeting, bent and welded or bolted
into a circular plan, and often coatedwith sand/cement mortar, can provide
similar storage capacity, but at agreater cost.
Ferrocement tanks
Larger tanks can be made offerrocement, which substitutes
chicken wire for the bamboo
reinforcement of the basket tank.
These are cheaper to construct thantanks made of masonry, blockwork,
reinforced concrete etc, and do not
require the rendering with waterproof
cement mortar that masonry andblockwork often need.

Above ground level, tanks areconstructed with a plain or reinforced
concrete base, cylindrical walls of
ferrocement and a roof of ferrocement,or sometimes mild steel sheeting.
The construction of ferrocement walls
is carried out by first assembling acylindrical mesh of chicken wire and/or fence wire reinforcement, withor without the aid of formwork. On
to this, a cement-rich mortar of 3:1
sand:cement is applied by troweland built up in layers of about 15
millimetres to a finished thicknessof between 30 to 100 millimetres,
depending on wall height andtank diameter. Thicker walls may
have two layers of mesh. The meshhelps to control local cracking andthe higher walls may call for theprovision of small diameter verticalsteel reinforcing bars for bending
resistance. Sometimes barbed fence
wire is wound spirally up the wall toassist with resistance to ring tension
and stress distribution.

Eight year old Falida
fills her jerry can at
one of the two new
4000 litre capacity
rainwater harvestingtanks at her schoolin Kitayita, Uganda.
Previously, water
had to be collectedfrom an unprotectedsource half akilometre away.
These were the firstjars constructed in
the community andthe exercise wasused as a workshop
to train local masonsin jar construction.

WaterAid/Caroline Irby

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Technical brief

Rainwater harvestingFig 3: Wrapping the reinforcing wire

WaterAid/Chris Leake

A rainwater harvesting tank at a school in Guirhora Kello, Burkina Faso.
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Technical brief

Rainwater harvestingFig 4: Example of design plans for a 23m3 ferrocement tank

Effective curing of the mortar between
the trowelling of each layer is very
important and affects the durabilityof the material and its resistance tocracking. Mortar should still be green
when the next layer is placed. Thismeans that the time gap between
layers should be 12-24 hours. The
finished material should then be
cured continuously for up to tendays under damp hessian, or othersheeting. A ferrocement tank is easy
to repair and, if the mortar has been
properly applied and cured, should
provide long service as a waterretaining structure at a fraction of thecost of a reinforced concrete structure.

Rock catchments
Just as the roofs of buildings can beused for the collection of rainwater,rock outcrops can also be used ascollecting surfaces. Indeed, if accessto the catchment area by animals,children etc can be prevented, aprotected catchment can collect waterof high quality, as long as its surfaces
are well flushed and cleaned before
storage takes place.
A significant proportion of Gibraltar’s
water is obtained from sloping rockcatchments on the Rock. At the footof the slopes, collecting channels
drain into pipes that lead to tanks
6Technical brief

Rainwater harvesting
excavated inside the rock. Some
artificial collection surfaces have also
been formed; cracks and voids in rocksurfaces have been filled in and alarge, soil covered, sloping area has
been covered in corrugated mild steel
sheeting supported on short piles
driven into the subsoil. This is an
example of what may be possible on asmaller domestic or village scale.
Sometimes it proves difficult toprevent the collected water frombeing polluted. If so, it is sensible touse this water for purposes that do
not require a potable water supply,
such as house cleaning, laundry,
horticulture etc, and reserve fordrinking water, cooking and personal
hygiene the better quality water thathas been collected from a clean roof.

References
Gould G and Nissen-Petersen E (1999)
Rainwater catchment systems.
IT Publications, London
Nissen-Petersen E (2007) Water fromroofs. Danida
Pacey A and Cullis A (1986) Rainwater
harvesting: The collection of rainfall
and run-off in rural areas.
IT Publications, London
Thomas T H and Martinson D B (2007)
Roofwater harvesting: A handbook forpractitioners. IRC International Waterand Sanitation Centre. Available at:www.irc.nl/page/37471

Use can also be made of other formsof ground catchment where, although
the collection coefficient can be
as low as 30%, useful volumes ofwater can be collected and used for
agriculture and animals.

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Technical brief

Case study

Rainwater harvesting

Construction of household rainwater harvesting jars in Uganda

WaterAid has been working to improve household water supplies in a Ugandan village where 3,000
people lack access to safe water. The community is heavily affected by HIV/AIDS and there are many
widows and single occupancy households. The terrain here is hilly, often with very steep gradients,
making it difficult for old or infirm people to access safe water sources.
WaterAid/Caroline Irby

Local masons have been trained in theconstruction of rainwater harvestingjars. These jars are made from locallyavailable materials and have a capacityof 1,500 litres which is equivalent to 75
jerry cans of water. The objective has
been to help the community constructon-site water supplies, close to home,removing the need for old or infirm
people to travel long distances acrossdifficult terrain to collect water.

The jars have a long life and once
constructed can provide a stablewater source for many years. There are
two dry seasons and two wet seasonsin this part of Uganda and the jars
augment supply over the dry seasons, Anna Maria Nanvubya and husband Zevilio Nakuzabasajja
although the water inside may not last Kwafu constructing a rainwater harvesting jar at their home
for the duration of a whole dry season. in Kitayita village, Uganda.
Materials requiredfor construction• Locally madebricks for base(50)
• Three bags ofsieved sand
• One bag of
normal cement• One
kilogrammeof waterproof
cement• A wooden
mould that canbe reused tomake a numberof jars• One metre
copper pipe
• One brass tap

Construction methodology
50 bricks are used to assemble a stable platform upon which the jar isconstructed.
A one metre long copper pipe is shaped and laid in the brick base. This
will channel water from the jar to the tap fitting.
A reusable wooden mould is assembled from 12 component pieces on topof the brick base.
The outside of the mould is smeared with mud for approximately three hours.
It is left to stand for three days after which it is plastered with normal cement.
The jar is left to dry for four days, giving time for the cement to set.
After four days the mould is removed by extracting individual pieces fromthe mouth of the jar. The layer of mud inside the jar is also removed.
The inside of the jar is then sealed using waterproof cement.
Community members provide guttering for their roofs and a plastic basin
which is perforated to act as a filter at the top of the jar.
Some jars have lockable tap chambers attached to their base to prevent
theft of water.
All jars are made in situ as they are fragile to transport. One jar costs
around £35 to make. This cost can be shared between five households.
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Water source
options:
See howSee
tubewells
boreholesharvesting
compares to otherwater source
optionsWatersourceoptions:
how andrainwatercomparesto otherwater source options

Technical brief

Rainwater harvesting

Bacteriologicalwater quality

Situation in which technology ismost applicable

High

Good if springcatchment isadequately protected

Reliable spring flow required
throughout the year

Low

Medium/high –depending on method
used to abstract water.
Water can be abstracted
from the sand andgravel upstream of thesand dam via a well or
tubewellGood if area upstreamof dam is protected

Can be constructed acrossseasonal river beds onimpermeable bedrock

Low – local labourand materials used

Low

Medium/high –Good if area upstream
depending on methodof dam is protected
used to abstract water.
Water can be abstracted
from the sand, gravel orsoil upstream of the subsurface dam via a well or
tubewellCan be constructed in sediments
across seasonal river beds onimpermeable bedrock

Infiltration
galleries

Low – a basic
infiltration gallery
can be constructedusing local labourand materials

Low

Medium/high –depending on method
used to abstract waterGood if filtration
medium is wellmaintained

Should be constructed next tolake or river

Rainwater
harvesting

Low – low costLowmaterials can beused to build storage
tanks and catchment
surfaces

Medium – dependent on
size of collection surface
and frequency of rainfall

Good if collection
In areas where there are one or
surfaces are kept clean two wet seasons per year
and storage containers
are well maintained

Hand-dug wellcapped with arope pump

Low

Medium – spare parts
required for pump

MediumGood if rope andpump mechanisms aresealed and protectedfrom dust. Area around
well must be protected

Where the water table is not
lower than six metres – althoughcertain rope pumps can lift
water from depths of up to 40
metresHand-dug wellcapped with ahand pump

MediumMedium – spare parts
required for pump

MediumGood if area around
well is protected

Where the water table is not
lower than six metres

Tubewell or
borehole
capped with ahand pump

Medium – welldrilling equipment
needed. Boreholemust be lined

Medium – hand pumps
need spare parts

MediumGood if area aroundborehole/tubewell is
protected

Where a deep aquifer must beaccessed

Gravity supply

High – pipelines and Low
storage/flow balancetanks required

High

Good if protectedspring used as source

Stream or spring at higher
elevation – communities served
via tap stands close to the home

Borehole
capped withelectrical/
diesel/solar
pumpHigh – pump andstorage expensive

High – fuel or powerHigh
required to run pump.
Fragile solar cells need tobe replaced if damaged

Good if source isprotected

In a small town with a large
enough population to pay forrunning costs

Directriver/lakeabstraction
with treatment

High – intake must
be designed andconstructedHigh – treatment andpumping often required.
Power required for
operation

Good following
treatment

Where large urban populationmust be served

ReverseosmosisHigh – sophisticated High – power required for Highplant and
operation. Replacement
membranes required membranes required

Good

Where large urban populationmust be servedHouseholdfilters

High – certain filters
can be expensive topurchase/produce

Filters can be fragile.
Replacement filters canbe expensive or difficult
to source

Low

Good as long as
regular maintainanceis assured

In situations where inorganic
contaminants are present
in groundwater sources orprotected sources are notavailable

SODIS (solar
disinfection)

Low – although
clear bottles can bedifficult to source inremote areas

Low

Low

Good

In areas where there is adequate
sunlight – water needs to be
filtered to remove particulatematter that may harbour
pathogens before SODIS can be
carried out effectively. SODIS
is not appropriate for use withturbid water

Water source

Capital cost

Running cost

Yield

Springprotection

Low or medium if
piped to community

Low

Sand dams

Low – local labourand materials used

Sub surfacedams

= most preferable= preferable

High= least preferable



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Technical brief

Rainwater harvesting

WaterAid transforms lives by improving accessto safe water, hygiene and sanitation in the world’s
poorest communities. We work with partners andinfluence decision-makers to maximise our impact.
Registered charity numbers:
Australia: ABN 99 700 687 141
Sweden: Org.nr: 802426-1268, PG: 90 01 62-9, BG: 900-1629
UK: Registered charity numbers 288701 (England and Wales) and SC039479 (Scotland)
US: WaterAid America is a 501(c)(3) non-profit organization

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