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The following points highlight the three modern water harvesting techniques of arid regions. The techniques are: 1. Virda 2. Dew Water Harvesting System 3. Greenhouse Water Harvesting.
Technique # 1. Virda:
Virda is an interesting and unique indigenous water harvesting system developed in arid and saline region of Kachchh, Gujarat. Basically, virdas are shallow wells dug in low depressions of lakes or large tanks. Previously, these rainwater harvesting structures were found all over the Banni grasslands, a part of the Great Rann of Kachchh in Gujarat. Currently, presence of such structures in Banni area is rare.
These systems were built by the nomadic Maldharis, who used to roam the Banni grasslands. Now settled, they persist in using virdas. These structures harvest the rainwater during monsoon season because of undulating topography of the area with depressions on the land surface.
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The Maldharis usually study flow of water during the monsoon to identify these depressions and make their virdas there in post-rainy season. Thus, the structure usually stores rainwater till next monsoon season. This unique water harvesting system speaks volumes of the ingenuity and adaptive capacity of the local people.
The virda structures use a technology that separate potable freshwater from un-potable salt water. After rainwater infiltrates the soil, it gets stored at a level above the salty groundwater because of the difference in their density. A structure is built to reach down (about 1 m) to this upper layer of accumulated rainwater. Between these two layers of sweet and saline water, there exists a zone of brackish water.
As freshwater is removed, the brackish water moves upwards, and accumulates towards the bottom of the virda. As such there are no specific technical guidelines available for designing this water harvesting system. The Maldharis construct this structure by using their own traditional knowledge.
Technique # 2. Dew Water Harvesting System:
Dew condensing and harvesting is relatively a new method of water harvesting. Several attempts to improve dew water yield to serve as a potable water source were made during the 20th century but results of the most of them were disappointing. Dew water is indeed used by plants and small animals in arid and semi-arid environment, and was significant to sustain their activity.
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In arid coastal plains of Kachchh region, abundant dew is observed during 8-9 months of the year; during rest of the months, sky remains cloudy and dew does not form. Thus, small quantities of dew water may be harvested for relatively a long period compared to rainy period. The key component of dew water harvest is condensing roof, which consists of two surfaces sloping at 30° angle from horizontal and stand on a triangular support.
The water condensing on the surface at night is collected under gravity flow by a gutter and run to a container where it is stored. The condenser is coated with condensing foil, 0.35 mm thick, made of TiO2 and BaSO4 micro-spheres embedded in low density polyethylene with a food surfactant. The foil is thermally insulated from the condenser frame by a 20 mm thick Styrofoam plate. A proper design of condenser is essential to increase the water yield and to prevent heating by the ambient air.
Technique # 3. Greenhouse Water Harvesting:
Greenhouse water harvesting is a modern technique of harvesting rainwater and using the harvested water for agricultural production under controlled conditions of temperature and relative humidity. The technology of using the surface of greenhouses to collect rainwater was initially developed in China in 2005 by the Beijing Agricultural Technology Dissemination Station and the Soil and Compost Work Station.
The technology consists of a greenhouse with a special type of roof that collects rainwater. Water is guided through the rainwater collection flume at the bottom of the greenhouse into a deposit pool and pumped into an underground storage pool. The captured rainwater is used for irrigating the crops grown in greenhouse with efficient irrigation techniques, e.g. drip irrigation. It is suggested that an average greenhouse of this type may be about 85 metres long and 8 metres wide. The plastic roof measures about 900 m2, while the cultivable area under the roof may be about 500 m2.
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This technology has a number of advantages for arid region. Firstly, it taps freshwater, i.e. rainwater, free from salts and impurities. Secondly, it reduces the pressure on groundwater up to certain extent as the rainwater collected from roof of greenhouse is utilized for irrigating the crops under the greenhouse. Also, as the rainwater is collected on polyethylene sheets before it comes in contact with soil, the salts present in the soils do not have opportunity to get dissolved into the water. Furthermore, the technology is suitable for the arid region where rainfall is very low, and this technology harvests the rainwater very efficiently and allows agricultural production and increases livelihood options. The rainwater is of good quality for irrigation and suitable for micro-irrigation.
The chemical composition of rainwater is such that it rarely jams micro-irrigation pipes. The technology provides a reliable supply of water (especially important under erratic rainfall), and thus, stimulates the production of several harvests of a wider diversity of crops. This increases the benefits for farmers, and subsequently stimulates the local economy. After the structure is built by local builders, it is relatively simple to use and maintain.
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