place of Domestic Roofwater Harvesting in water supply strategies
must think were crazy. We let the rain fall off our roofs onto our soil, it washes
the soil away and flows to the bottom of the hill. We then climb down the hill
and carry it back up to drink.
Many water professionals are worried about the increasing difficulty of finding
and improving water sources. Domestic roofwater harvesting provides an innovative
solution to meeting water needs and can be implemented quickly and modularly.
Renewed interest in the technology is reflected in the water policies of many
developing countries, where it is cited as a source of household water.
The basic system consists of a tank to capture rainwater that falls on the
roof and guttering to bring it to the tank. More sophisticated systems also have
Rainwater systems are decentralised and independent of topography and geology.
They deliver water directly to the household, relieving the burden of water carrying,
particularly from women and children. Implementation is similar to managing the
installation of on-site sanitation and once systems are in place they are owned
by the householders who can manage their own water supply.
Roofwater harvesting does have limitations. It is not suited to being used
as a stand-alone water supply solution in any but the most water-stressed situations
as the increase in tank capacity necessary to bridge a long dry season can be
prohibitively expensive. The storage provided by a tank does, however give households
good security against short-term failure of alternative sources.
Good niches for roofwater harvesting include:
- groundwater is either difficult to secure or has been rendered unusable by
fluoride, salinity or arsenic
- the main alternatives are surface water sources
- management of shared point sources has proved a problem
- the carriage of water is a particular burden on household members; or householders
are prepared to invest in higher water convenience.
Climate: Semi Arid; Rainwater used as main source
Petrolina is in the semi arid belt of Northeastern Brazil. Rainfall is low
and varies greatly year-on-year. A solution to the water-scarcity problem is the
use of large (10,000-20,000 litre) tanks that can store enough water to last a
frugal household until the next rains. The tanks are usually provided by NGOs
as the large structures necessary in this very arid area cost over $200 and are
unaffordable for the local population.
Badulla, Sri Lanka
Climate: Tropical, Bimodal ; Rainwater used as main source
The town of Badulla is located in a hilly area of Sri Lanka. Groundwater sources
are few and tend to be at the bottom of the hills. To reduce the burden of carrying
water the local authority provided 5,000 litre ferrocement tanks, at a cost of
about $150 which are used for most household water supply. The tanks are now being
adopted nationwide for use in areas where access to other protected water sources
Climate: Tropical, Bimodal; Rainwater used as suplimentary source
Rakai is in the southern hills of Uganda. It has a bimodal rainfall pattern
and hence a dry season of only 2 months. A local women's group was trained in
tank making by a Kenyan women's group and have made a large number of small (700
litre) jars to supplement their water use, particularly in the wet season when
they provide the bulk of water needs. The sub $70 cost of the systems are financed
by a self sustaining revolving fund.
Khon Kaen, Thailand
Climate: Tropical, Monsoon; Rainwater used as primary source
Northeast Thailand was the scene of one of the world's largest roofwater harvesting
disseminations. The technology of choice was the 1-2,000 litre "Thai jar"
The project passed through several stages with reducing outside intervention,
eventually becoming a commercial market producing jars in large numbers for less
than $30. This encouraged rapid penetration of rainwater jars and today most houses,
rich or poor, have at least one.
Social and gender considerations are central factors influencing the dissemination
and uptake of domestic roofwater harvesting. Women are usually the primary fetchers
of water, a task which consumes much time and energy on a daily basis. However,
decisions to undertake investments in the home, such as installing domestic roofwater
harvesting systems, are usually undertaken by men with women assuming responsibility
for utilising the system. Hence both need to be involved in discussions regarding
water source options. Provision has to be made to ensure that poorer households
and those headed by women have access to technical information and credit to facilitate
adoption of domestic roofwater harvesting when it is appropriate.
In terms of organic and inorganic pollutants, untreated rainwater, withdrawn
from well-maintained tanks fitted with inlet filters, is generally well within
WHO standards and is superior to most groundwater. Microbiological contamination
(indicated by levels of E.coli) is in the "low risk" category of WHO
water quality standards, surpassing the quality of most traditional water sources
and many improved sources. Such contamination falls further with storage. No additional
treatment is usually needed. However if higher quality water is required, standard
household treatments such as boiling, chlorination or SODIS are effective on stored
Requirements for Roofwater Harvesting
For roofwater harvesting to be viable there are a number of environmental
- Rainfall should be over 50mm/month for at least half of the year (unless other
sources are extremely scarce)
- Local roofs should be made from impermeable materials such as iron sheets,
tiles or asbestos
- There should be an area of at least 1m2 near each house upon which
a tank can be constructed
- There should be some other water source, either ground water or (for secondary
uses) surface water that can be used when the stored rainwater runs out
Implementing Roofwater Harvesting
The implementation of domestic roofwater harvesting involves training, contracting
and quality controlling a number of local craftspeople to undertake many small
building works. This is very different from many other water supply modes where
a large centralised project is the norm. It is, however, similar to providing
on-site sanitation at the individual household level.
Maintenance of roofwater harvesting systems is usually passed on to the householders
who, after a period of training and supervision, usually maintain the system well,
motivated by their direct ownership of the system. This is in contrast to communal
water supply systems which can prove difficult to maintain when responsibility
is too diffuse and personal politics interfere.
Sources of further information
John Gould and Erik Nissen-Petersen, “Rainwater
Catchment Systems for Domestic Supply”, 1999, IT
University of Warwick Development Technology
UNEP Sourcebook of Alternative Technologies for Freshwater Augmentation
Technical Briefs (from the WELL site)
Rainwater Harvesting Guide (pdf)
on the use of rainwater tanks (Australia) (pdf)
This document was prepared by the
Development Technology Unit of the University of Warwick, UK and its partners.