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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
in Some Countries in Asia>

3.2 Rainwater Harvesting for Drinking Water Supply

Technical Description

Rainwater has been used traditionally as the primary source of potable water in Tamilnadu (South India), with drinking water storage being provided by a construct known as an oorany. It is a very simple storage facility designed to store the locally available rainwater sufficient to meet the drinking needs of a community. Almost every village in the Ramanathapuram District has an oorany, which is an artificially constructed pond. The pond is created by excavation with the excavated soil used to form bunds around the pond. There is a sluice arrangement to admit the rainwater. The oorany is located in impermeable soils so that seepage losses are minimal, and are sized to meet the village's consumption requirements and accommodate losses such as those due to evaporation. The primary use of the stored water is for drinking, although, by enlarging the storage capacity of the oorany, irrigation requirements may also be serviced. Irrigation tanks have are common in Tamil Nadu and more so in Ramanathapuram.

The Village of Thattankudiyiruppu, in Tamilnadu State, India, is a typical community serviced by an oorany. The population of the village is 550. Accordingly, based upon the standard minimum rural drinking water requirement of 20 l per capita per day, the village requires a minimum of 4 015 m3 of drinking water per year. Given that the average annual rainfall of the district is 792 mm, and employing a moderate runoff coefficient of 25% , a 2.5 ha catchment area would be needed to capture the 5 000 m3 of water used by the village. Generally, the catchment area of an oorany is closer to 5 ha, which provides additional storage to account for, inter alia, interannual variability in rainfall. The hydraulic and water quality particulars of a typical oorany are:

i ) Length at top - 97.80 m
ii) Width at top - 32.60 m
iii) Length at bottom - 91.58 m
iv) Width at bottom - 26.08 m
v) Depth - 3.26 m
vi) Capacity - 9090 m3
vii) Catchment area - 6 ha
viii) Quality of water - Potable

Extent of Use

There are as many as 39 200 oorany or rainwater storage tanks have been constructed in Tamilnadu State. The tanks cater to the needs of agriculture, and supply water to irrigations schemes that occupy nearly 30% of the irrigable lands in the State.

Operation and Maintenance

Rainwater harvesting and storage in an oorany is very simple and easy to operate. Operation and maintenance involves periodic cleaning of the oorany, as well as ensuring that the catchment area is protected water from any pollution. Constant inspection of the surrounding area is essential to prevent abuse of the water source by the people.

Level of Involvement

The entire project is a community activity with little involvement by the government. The success of the technology has already encouraged agencies like the Vivekanand Centre to support construction of drinking water oorany in other villages elsewhere in the Ramanathapuram District with a minimum degree of external assistance.

Costs

The cost of construction generally includes the provision of a draw well located outside of the ooranybunds. This abstraction point generally has a diameter of 3 m and a depth equal to the depth of the oorany. A pipe connects the well to the oorany. Additional, site-specific arrangements are made to protect the water in the oorany from pollution. An oorany with a capacity of about 10 000 m3 costs about $7 000 to construct.

Effectiveness of the Technology

This traditional method of rainwater harvesting has been extremely successful. Most communities are able to raise the capital required to fund this type of venture, and are capable of operating and maintaining the system with little assistance from the government.

Advantages

The main advantage of the method is that it is a well-tried and tested method that has been practised for a number of years. It can be implemented and maintained by communities with little external assistance. Most communities have the necessary skills within their populations, as well as the necessary mechanical equipment that may be needed to construct the pond. Generally, there is little need for chemical treatment of the rainwater. There is also no dependence on energy sources to operate the system.

Disadvantages

If not properly constructed and maintained, the oorany may be subject to seepage. Water accumulating around the pond could provide mosquito breeding habitat with a concommitant threat to the public health. In addition, if the catchment serving the oorany is not maintained, the water stored in the tank may become contaminated with particulates, faecal material and other pollutants.

Cultural Acceptability

This technology is well accepted by people because of their long tradition of using this technology.

Further Development of the Technology

The construction of the oorany could be improved by the use of new materials to better seal the berms and reduce seepage losses.

Information Sources

Kanmani, S. and K. Karmegam 1987. First Regional Seminar on Technology on Drinking Water and Related Water Management for Southern States and Union Territories, Department of Civil Engineering, Anna University, Madras.

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