Water Harvesting-Introduction
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Rain Water Harvesting is the process of collecting rainwater directly or recharging into the ground water storage. This helps to improve productive use of rainwater.

The main objectives of Rain Water Harvesting are:-

  • Increasing the availability of ground water during periods of requirement.

  • Preventing depletion of ground water reservoirs in areas of over exploitation.

  • Enhancing sustainable yield of aquifers.

  • Improving quality of ground water through dilution e.g. in coastal areas.

  • Decreasing menace of floods on local and regional scale.

  • Reducing pressure on storm drains in urban areas.

  • Enhancing the quality of environment

The Technique
Rainwater harvesting technology involves three principal components:

  • Catchment area

  • Conveyance system

  • Collection devices 

  • Catchment Area 

This could be a land surface or a rooftop.

a) Land surface catchments: This is the simplest way of collecting rainwater. It involves improving runoff capacity of the land surface through various techniques including collection of runoff with drainpipes and storage of collected water. It provides an opportunity to collect water from a larger surface area. By retaining the flows (including flood flows) of small creeks and streams in small storage reservoirs (on surface or underground) created by low cost (e.g. earthen) dams, this technology can meet considerable water demand. However, during dry periods there is a possibility of high rates of water loss due to evaporation and infiltration into the ground. The technique is mainly suitable for storing water for agriculture purposes.

b) Rooftop catchments: This is more suitable for meeting individual water needs. In the most basic form of this technology, rain water is collected in simple vessels or drains at the edge of the roof and conveyed by pipes to containers for settling particulates before being conveyed to the storage container for domestic use. The water needs to be disinfected if used for drinking purposes.

Conveyance Systems
Conveyance system is required to transfer the rainwater collected on the rooftop to the storage tank. This is usually accomplished by making connections to one or more down-pipes connected to the rooftop gutters. To selectively collect clean water down-pipe flaps are used. When it starts raining, the flap is left in the closed position, directing water to the down-pipe, and later, opened when relatively clean water can be collected. However, a disadvantage of using this type of conveyance control system is the necessity to observe the runoff quality and manually operate the flap. Hence, for convenience, sometimes all rainwater is collected and passed through a settling/filtration tank before final storage. These settling tanks usually have a layer of sand followed by layers of brickbats through which water passes before storage. The sand layer is cleaned or changed from time to time.

Collection Devices
a) Storage tanks: Storage tanks for collecting rainwater may be either above or below the ground. Certain precautions are required to be taken while using these storage tanks, e.g. provision of an adequate enclosure to minimize contamination from human, animal or other environmental contaminants, and a tight cover to prevent algal growth and the breeding of mosquitos. Commonly used storage tanks are, cylindrical ferrocement tanks and mortar jars. The ferrocement tank consists of a lightly reinforced concrete base on which is erected a cylinder with a 10 mm steel base. This cylinder is further wrapped in two layers of light wire mesh to form the frame of the tank. Mortar jars are large jar shaped vessels constructed from wire-reinforced mortar.

b)Rainfall water containers: These can be battery tanks (i.e. interconnected tank) made of pottery, ferrocement, or polyethylene. The polyethylene tanks are compact, have a large storage capacity, are easy to clean and have many openings, which can be fitted with connecting pipes.

Traditional Systems of Rain Water Harvesting
Various kinds of traditional Rain Water Harvesting systems are ponds, tankas, vav or baoli’s, stepwells, kunds, kuls, etc.  The traditional practices involves conserving rainwater at the place where it falls. In this process ground water is also recharged. It depends on local conditions, needs and nature of the land. These systems are community based and best managed in the areas of low rainfall and undulating land surface.

Modern Systems of Rain Water Harvesting

These include:
i) In case of towns where each house has a yard, small pits are constructed together with an injection well. The flow from the rooftop is directly towards the pit, which help in decanting the water. The decanted water is directly to flow down into the injection well, which is usually a borehole of 8” diameter consisting of perforated PVC pipe.

ii) In townships where built-up areas do not have individual courtyards. The rainfall can be stored by constructing water bodies at different locations depending upon land contours. Though these water storage bodies have high initial cost but they are helpful in sustaining the groundwater table.

Computation of artificial recharge from Rooftop rainwater collection Major factors to be taken in account for computation are:

1) Roof top area (of an individual house or multistoried building)

2) Average annual monsoon rainfall

Following formula is applied for calculating the recharge:

Area of (sq m)  x  Amount of (m) =  Volume of water

Catchment            Rainfall                    received (cu m)

For example, if the catchment area of rooftop is 100 sq m (of an individual house) and the recorded average annual monsoon rainfall of that particular place is 780 mm, then effective annual rainfall contributing to recharge is approximately 70% i.e. 550 mm. Hence total water available will be 55 cu m which is enough for 100 days for a five member family @ approx. 110 litres/day).

Advantages of Rain Water Harvesting

Rain Water Harvesting systems are advantageous to the community and the environment as they:

  • Promote self-sufficiency and foster an appreciation for water as a resource.

  • Promote water conservation

  • Provide good quality of water.

  • Easy to adapt

  • Conserve energy and water resources

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