How to harvest rainwater in Tharparkar

Post Source: Dawn economic and business review

By F. H. Mughal

WHILE it rained heavily in 2010 in Tharparkar, the stored rainwater dried up relatively quickly due to high rate of evaporation and inadequate storage facilities. What happened there with regard to rainwater storage was bound to happen with the prevalent practices. The Thar people need to understand some basics of harvesting rainwater. Since, Tharis are too poor to undertake this venture, some well-organised NGOs need to step in to help them to avoid this unfortunate episode of losing precious rainwater.

 

Rainwater harvesting systems have advantage of delivering water directly to households. Roof-water harvesting is accumulation of rainwater falling on the roof. The water is collected in tanks or jars. Since the tank is located right next to the house, the household women are free from the trouble of fetching it from distant source. The tank is normally at the ground level. If an underground design is used, the tank should be placed more than 15 meters uphill from any pit latrine. The roof has to have material that does not absorb the rain, nor pollutes the run-off. Tiles or concrete-lined roofs are suitable for the purpose. The larger the roof, the greater will be the runoff. Because of evaporation and other losses, only 90 per cent of rainwater that falls on the roof is collected.

If the roof area is not big, enough water will not be accumulated to meet the household needs. In such cases, the roof must be extended. In Tharparkar, the adjoining area of a house is mostly vacant. Elevated platforms can be build with the help of some cheap material (strong enough to take the load) to collect rainwater. The roof must be lined in all cases. If the roof is horizontal, rainwater is conveyed from the roof to the storage tank through down pipes. If the roof is inverted V-shaped, or has some other shape, open channels are constructed to carry water from the edge of roof to the downpipes. In Sindh, water spouts are a common sight, which can be used to convey water from roof to the tank. For the poor households, as in Tharparkar, who are unable to afford rainwater harvesting system, bowls and buckets may be placed under the spouts to collect the roof water. The water may then be transferred to some other container.

These informal arrangements may compromise with the hygienic aspect of the collected water. Unless the receiving bowls or buckets are placed on chairs, water can be contaminated by ground splash. Likewise, if water is stored in drums, frequent dipping of cups for taking out water will contaminate it. Besides, water in drums will allow growth of algae and mosquitoes within a few days after it is filed unless they are covered.

While rainwater harvesting provides a safe and convenient source of water of limited quantity, the quantity of water may be less than the water needed. The rainwater stored, therefore, needs to be managed effectively to ensure its availability in dry season. In this context, contaminated water, from other sources, may be used to meet other requirements (e.g., for toilet cleaning, and for suppressing dust around the house).

Roof water harvesting differs from other water supply modes as in that there is no need to transport water, since it is used within a few metres of where it falls as rain. Collecting rainwater from showers seems sensible in areas struggling to cope with potable water needs. Rainwater is one of the purest sources of water available as it contains negligible impurities. The water storage component (e.g., tanks, cisterns and jars) of the rainwater harvesting system entails some costs. As is the normal practice, depending on the size of household and the affordability aspects, water is stored in 1,000-3,000 litres household tanks, with 2,000 litres tanks being most common. Since, the preferable material is ferrocement (wire-reinforced) or RCC (reinforced cement concrete), some costs will be involved. However, there is a large scope of reduction in costs, when the storage tanks are produced on a large scale, based on economies of scale.

Ferrocement is the technology of choice for many rainwater harvesting programmes. The tanks are relatively inexpensive and with a little maintenance last for an indefinite period.

In Thailand, the rainwater jar programme launched in the 1980s proved to be very successful. Some 300 million ferrocement rainwater storage jars and tanks were constructed between 1980 and 1991. The programme involved a broad range of stakeholders, including households, communities, NGOs, universities and the private sector with support from the government at local, provincial and national levels. The programme operated with revolving funds provided by the government to generate resources from the households and the communities.

As the project gained momentum, the programme shifted from government initiative to commercial jar-manufacturing enterprises. During 1986 alone, approximately 1.7 million jars were built in northeast Thailand. Increasingly, the private sector took on the production responsibility, as it was able to turn out good quality jars at lower prices due to economies of scale. While rainwater is relatively free from impurities, its quality may deteriorate during harvesting, storage and household use. During the dry season, wind-blown dust, tree leaves and bird droppings get collected on the roof. This is a major source of contamination. Poor hygiene in storing and abstracting water from tanks can also cause water contamination. However, risks from these hazards can be minimised by good hygiene and sensible practices.

Rainwater is slightly acidic and low in dissolved minerals. As such, it is relatively aggressive. Rainwater can dissolve heavy metals and other impurities from catchments materials and storage tank. This has implications for the health of the Tharis, because, if zinc, galvanised iron or asbestos sheets are used as roofing material, and if lead and copper joints or fittings are used in piping, then, coupled with the relatively high average ambient temperatures in Tharparkar (above 40 degrees C), these materials could leach from roof and piping. This will pose a serious health hazard. These materials (zinc, copper, asbestos and lead) should not be used. Iron, however, is not a health hazard at moderate levels (5 milligrams per litre), but will give a rusty colour to stored water.

Metallic tanks and oil drums hasten evaporation. For Tharparkar, ferrocement or RCC tanks are advisable. If ponds are used to collect rainwater, they should be deep to minimise surface evaporation. Additionally, trees should be planted around ponds for shade. They will also serve as windbreak system.

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