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Contemporary Innovation in Traditional Water Harvesting |
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written by Babubhai Rajpurohit, Poornimabhai Prajapati, Srinivas Chokkakula |
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1538 | :
Water Conservations | :
Honey Bee Practices |
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SettingTakhuva is an arid village situated near Rajasthan border in Banaskantha district of Gujarat. Till five years ago, water used to be extracted from virdas similar to those in the Banni area of Gujarat (see Honey Bee Vol 5(2):5 and 5 (3):7). These virdas were dug at the bottom of a tank during the dry season and the water was sufficient only for domestic purposes for the whole village. The water that could be harvested was meagre. Since the output of these virdas was very low, the people were on the look out for ways of augmenting water supply. Soon the conventional form of virda was modified to harvest water for irrigation purposes by drilling lateral holes through the walls of virda (see figure). The technique of drilling lateral holes was learned from villages in the neighbouring state of Rajasthan. How to make a Virda for Irrigation PurposesThe virda in Banaskantha is similar to a conventional well, circular in shape and with a diameter ranging from 1 m to 2.5 m. It is dug up to the point beyond which it would become saline. This point is determined by monitoring the salinity of the water or soil moisture as the work on the well progresses. Another guiding principle used is to monitor the level of moist soil from where the water begins to ooze out. The depth of the well should not normally be 1.5 m below this point. Once the well is dug, lateral holes are dug at a level of about 0.5 m above the bottom of the virda. Holes are made with the help of a series of iron tubes, each 1.6 m long. Water is allowed under pressure into these tubes with the help of a hose pipe connected to a pump used to draw water out of the well. The jet of water through the tubes helps in loosening the soil and drilling a hole in the walls of virda. Force is also applied through jerks. This along with the force of the jet, enables the tube to move forward. As many as 20 to 25 such tubes are inserted, each one connected to the other. Once the hole is complete, the iron tubes are removed. The hole acts as a recharger for the well allowing fresh water from surrounding layers of the soil to seep into the virda. Usually, four to eight such holes are made in different directions. One person can drill one such hole in a day. Variations in DesignThe virda has been further modified according to the varying conditions of soil. In one form, the part of the virda where the recharging of water takes place is lined with concentric layers of a rope made of grass in order to prevent erosion of soil and to avoid caving in of the walls. The rope is plastered with clay and soil to keep it intact. Alternatively, concrete slabs are used to make the walls permanent. The bottom part from where the water oozes into the well is left bare. Whenever the water yield is reduced, lateral holes are drilled to increase the flow. Criteria for Determining the Rate of ExtractionA virda may fail because of two reasons; firstly, caving in of its walls over time; secondly, rapid extraction of water making water saline. According to one view, salinization occurs due to upconing of the saline water lying below the sweet water. The rate of extraction should therefore be limited to ensure that enough pressure is maintained by the water inside the well on the saline water table below. When a virda fails, it is abandoned and a new one is constructed. Farmers in Takhua believe that the extraction per day should be limited to the quantity of water needed to irrigate quarter of an acre of land, lest the well becomes saline.Lateral extraction of groundwater is certainly more sustainable in this case because of the risk of salinisation due to excessive groundwater extraction. The limited diffusion of this technology strengthens the case for knowledge network. (see editorial, Honey Bee vol 6(4), 1995). |
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Honey Bee, 7(2):12, 1996 |
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