3.9 Rainwater harvesting in ponds

Technical description

In the southern region of Bangladesh, the salinity of both surface and ground waters is beyond the tolerable limits for human and agricultural uses, including watering of animals as well as plants. From time immemorial, pond water, replenished by rainfall, is used for drinking and other purposes, including for fish cultivation. The ponds are specially constructed with bunds so that surface water cannot enter the ponds. This feature keeps the bed of the pond relatively free of sediment. Traditionally, trees have been planted on all four sides of the ponds to provide additional protection of the pond area. In the Sunderban region, animals are permitted to drink from the ponds during the night.

Extent of Use

In southern portion of Hatya Island (Figure 18), and other remote islands in the Bay of Bengal, rainwater, collected in ponds, meets nearly 80% of the drinking water requirements during the monsoon season. The water levels in the ponds fall during the dry season, and the water becomes turbid and more saline due to the deposition of marine aerosols, but the water continues to be used by the local people who have no other source of water, although some households bring water from distant areas. The pond water is used for bathing, washing of clothes and other household purposes through out the year. Where tubewells exist, women are generally carry the water from the wells to the point of use. In contrast, even in areas where religious norms are strictly adhered to, men may be found to carry water from the ponds.

Operation and Maintenance

Maintenance of the ponds involves repairing the bunds, cleaning and removing sediment, and general upkeep of the surrounding area.

Level of Involvement

This technology is generally implemented at the community level.

Costs

Capital costs depend on the materials and methods used in constructing and lining the ponds. Typical pond construction costs range between $350 and $500 for a 15m x 15m pond. Maintenance costs are typically about $12 per year.

Effectiveness of the Technology

In Bangladesh, the ponds fill with rainwater during the monsoon season, which, on Hatya Island, extends from April to August. At the conclusion of the monsoons, the water level in the ponds falls rapidly during January and February, and the water becomes salty. Nevertheless, a large number of people depend on the ponds for their domestic water supply. Hence, this technology appears to be very effective in meeting local needs.

Suitability

This technology is suitable in areas with abundant rainfall to fill the ponds naturally.

Advantages

This technology is relatively inexpensive and can be implemented using local technological skills and community cooperation.

Disadvantages

There is a potential for the pollution of the pond water by animals. Also, if the ponds are poorly maintained, there may be seepage losses. Because the ponds are open, they are subject to evaporative losses.

Cultural Acceptability

Some communities do not drink water from this type of storage pond because it is considered unholy and contaminated.

Further Development of the Technology

This technology may be considered to be fully developed. However, cost effective methods need to be introduced to reduce water losses and pollution to improve its effectiveness.

Information Sources

Mohammed Aslam, Saleh Ahmed Chowdhury, Alamgeer Faridul Hoque, and S.R. Sanwar, Intermediate Technology Group, House 32, Road 13A, Dhanmondi, Dhaka, Bangladesh, Tel. 880 2 811 934, Fax: 880 2 813 134, E-mail: [email protected].