3.8 Open sky rainwater harvesting technical description

The open sky rainwater catchment is the least sophisticated form of rainwater harvesting, requiring minimal levels of investment to install, maintain and operate. For this reason, open sky rainwater harvesting is usually practised by relatively poor people who cannot afford appropriately-sized gutters or motkas in which to store water, In Bangladesh, a number of variants on this technology have been observed, ranging in cost and design from two corrugated iron sheets supported by four bamboo posts and slanted in such a way as to collect and direct rainwater into pots, motkas, etc., to plastic sheeting, suspended on four bamboo sticks, with an hole in the centre, weighted with a brick chip, to direct rainwater into a pot placed below the hole, as shown in Figure 18. When applied for agricultural use, the polyethylene sheeting may be placed in the open and slanted so that rainwater collecting on the plastic flows off the sheeting and downslope to the agricultural. Mosquito nets, bed sheets or even sarees (women's garments), inverted umbrellas, and open drums have been used as catchments to harvest rainwater, while fishermen and mariners living offshore for a considerable periods have traditionally harvested rainfall to replenish potable water supplies on their boats.

Figure 18. Open sky rainwater harvesting using of plastic sheeting in Shamnagar.

Operation and Maintenance

This technology requires little maintenance, except that devoted to the periodic cleaning of water storage pots, and few skills to operate.

Level of Involvement

This technology may be implemented at the household level.

Costs

Use of this technology can be had for a minimal investment in materials. In its relatively sophisticated version, that constructed of corrugated iron sheeting, typical costs would be between $5 and $9 for two corrugated iron sheets. In its least expensive form, that constructed of plastic sheeting, the cost would be about $0.50. Likewise, the cost of storing the harvested rainwater depends on the type of storage vessel used. Typical costs would range from about $0.25 to over $500: pitchers would cost about $0.25 each; small earthen jars (motkas), $1.20; medium-sized earthen jars, $5.00; large earthen jars, $10.00; cement underground storage tanks, $50 to $500; and, cement above ground storage tanks, $100 to $300.

Effectiveness of the Technology

This technology is effective in meeting the short term water requirements of people living in boats and of small families lacking access to other water supplies.

Suitability

This technology is limited to very small scale uses.

Advantages

This technology is simple to use, requiring few skills and minimal materials.

Disadvantages

It may be difficult to securely anchor the rainwater catchment materials during windy conditions or during storms, limiting the ability of the technology to harvest rainwater. Because of the limited surface area of the rainwater catchment, this technology does not provide a reliable source of water, and its use is restricted to the wet season.

Figure 19. Woman taking fresh water from a pond on Hatya Island.

Cultural Acceptability

There are no known cultural problems associated with the use of this technology.

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 2813 134, E-mail: [email protected].