Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for Freshwater Augumentation
2.1.2 Rock and Roof Catchments
Rock catchments are simple systems for the collection of rainwater. Siting of
these structures should take into account ease of access of the users and the
geological structure of the site. The best sites are found on the lower reaches
of bare rock inselbergs, where runoff losses to the soil, vegetation and
structures is minimised. Storage may be provided in dams or open tanks.
Roof catchments are suitable for individual household use, and use in schools
and other institutions where sufficient impermeable roof cover exists. To
collect rainwater from roof catchments, gutters and ground storage tanks are
required (Figure 28). "First flush" water from each shower should be
prevented from entering the storage facility to reduce the degree of pollution
of the stored water by dust, leaves and bird droppings washed from the roof top
into the reservoir. Underground tanks may also be used (Figures 29 and 30).
Figure 28. Lateral view of a rainwater catchment system.
When calculating the size of the storage for rock or roof catchments, the
demand for water, and the length of the dry periods, must be considered. The
required catchment area also depends on the amount and variability of rainfall.
In most cases, however, the available area is often the limiting factor due to
local conditions. Figure 29. Rock catchment with underground storage tank.
Extent of Use
This technology is extensively used in arid and semi-arid areas of Africa,
such as Mauritania, Benin, Burkina Faso, Uganda and Kenya.
Operation and Maintenance
Limited regular maintenance of gutters, and removal of leaves and other
debris from the catchment surface, is required. Cleaning of the tanks is
necessary before and after the first rains. All of these activities can be
handled by the community. Water is drawn by bucket or taps fitted to the storage
Level of Involvement
This technology is installed and operated primarily by local communities,
sometimes using hired labour. Technical advice from government, NGOs, or private
sector agencies may be required. Once the technical training of locals has been
completed, the roof catchment system installation and management can be left in
the hands of the householders.
In 1994, a typical roof catchment system in Benin, constructed of ferrocement,
cost $346/7 m3 storage, $496/12 m3 storage, or $800/24 m3 storage. In Burkina
Faso, Uganda and Kenya, costs ranged from $852/20 m3 storage, constructed of
ferrocement, to $1 016/30 m3 storage constructed of masonry.
Effectiveness of the Technology
Rain water catchment systems have been successfully utilized by people all
over the world for many centuries. Presently, rain water is collected from many
types of surfaces to provide water for domestic, livestock, agricultural and
fish-farming use. Rain water is also used as a supplement to piped water
supplies. The effectiveness of rain water collection systems depends on the type
of roofing material used. For example, thatched grass gives lower yields than
corrugated iron sheets.
Figure 30. Artificial roof catchment with ground water storage
No environmental benefits have been reported.
This technology has good potential in areas of rugged and steep terrain.
It is more feasible in high rainfall areas, because rain can fill the
storage reservoirs more frequently. On the other hand, it is quite
suitable for arid and semi-arid areas where rain water is the most
accessible water source. It also has good potential for community
The advantages of using this technology are that water is provided at the
point of consumption, and there is good potential for community-based
management of the collection systems (with low operating and maintenance
costs). Relatively good quality water can be obtained using this
Disadvantages of this technology include difficulties in controlling the
water quality, an high per capita cost of development, and a lack of reliability
as a source of water. It cannot serve large users, although, it is usually
adequate to provide a low level of service, suitable for family use.
No negative cultural factors have been observed.
Further Development of the Technology
There is very little that needs to be done to further develop this
Ministry of Land Reclamation Regional and Water Development,
Post Office Box 30521, Nairobi, Kenya.
CREPA, Ouagadougou BP. 7112, Ouagadougou, Burkina Faso, tel
310359, fax: 310361.
The Institute of Agricultural Engineering, Post Office Box
BW330, Borrowdale, Harare, Zimbabwe.
AGRITEX, Post Office Box CY 639, Causeway, Harare, Zimbabwe.
Ministry of Agriculture, Private Bag 003, Gaborone, Botswana.
Ministry of Agriculture, Post Office Box 92, Maseru 100,
Coordenacao Geral dos Projectos Integrados, Prorural, Ruo da
Resistencia 1746, Maputo Mozambique.
Direction de l'hydrologie et d'hydraulique, Programme
d'Hydraulique Pastorale, Mauritane, Tel. 251611, Fax 251602.
Hissen-Petersen, E. and M. Lie 1992. Harvesting Rain Water in Semi-arid
Kenya-Finland Western Water Supply Programmes 1990. Water Supply
Development Plan 1990-2005. Ministry of Land Reclamation Regional and Water
Direction de l'hydrologie et d'hydraulique 1978. Programme d'Hydraulique
Pastorale: survie du betail en Mauritane, 356 p.