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2.1.2 Rock and Roof Catchments

Technical Description

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 tank.

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.

Costs

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 tank.

Disadvantages

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.

Cultural Acceptability

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 technology.

Information Sources

Contacts

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, Lesotho.

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.

Bibliography

Hissen-Petersen, E. and M. Lie 1992. Harvesting Rain Water in Semi-arid Africa, Nairobi.

Kenya-Finland Western Water Supply Programmes 1990. Water Supply Development Plan 1990-2005. Ministry of Land Reclamation Regional and Water Development, Nairobi.

Direction de l'hydrologie et d'hydraulique 1978. Programme d'Hydraulique Pastorale: survie du betail en Mauritane, 356 p.

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