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Virda: A Ingenious Method of Rain Water Harvesting |
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written by Srinivas Chokkakula and Sunil R Patel |
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11586 | :
Water Conservations | :
Honey Bee Practices |
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‘Virdas’ are rain water harvesting wells found in Kutch. (For a full description, see Honey Bee Vol 5 No2). Here we put forward some hypotheses on how the ‘virda’ gets charged with fresh water. We had focussed our study in the same village Dhordo described in the previous issue. Dhordo village has six tanks and though all the tanks have ‘virdas’ in them, only those in three tanks are in use. In the main tank, in which ‘virdas’ were a failure; the water is saline. Two of the other tanks are slightly away from the village and the ‘virdas’ in these tanks are not in use. We were told that these ‘virdas’ were old and so were abandoned. The general slope of the ground is towards the main tank, which is at one end of the village. If a section is taken on the line connecting the main tank and the farthest of the tanks, the other tanks will be seen at different levels of the slope with three of the tanks on one side of the line and one on the other side. This line approximately represents the slope direction towards main tank. The depth of the ‘virdas’, which indicates the point beyond which the saline water strikes varies from one tank to other and follows an order. The depth of the ‘virdas’ in the tank which is at higher level is deeper than those in the tank at lower level. The water level in ‘virdas’ in the tank adjacent to the main tank is 2.0 metres (6 ft) from the bottom of the tank and the total depth of the ‘virda’ is more than 3.0 metres (10 ft), whereas those in the tank farthest from main tank, the depths are beyond 3.0 m (10 ft) and more than 4.6 metres (15 ft) respectively. If it is assumed that the saline water table is in a horizontal plane for the whole village (which could be a valid assumption over such small distances), the above observation implies that as we go towards the main tank, the slope is downwards and consequently the depth of the saline water table is also decreasing . This may be one reason why the saline water is struck at shallow levels in the main tank and the ‘virdas’ are unsuccessful. Each tank contains many ‘virdas’. The cattle troughs are constructed in the tank itself and in many cases each of them is fed by more than one ‘virda’. This is done by connecting a newly opened one to the same cattle trough after the old one is exhausted. It is also observed that a new ‘virda’ is constructed adjacent (about 1.0m to 1.5 m away) to the exhausted one. Hypotheses and Questions Since the saline water is beyond a certain level, it could be taken that the saline water table is somewhere just below the bottom level of ‘virda’. This implies that the ‘virda’ is being charged by the water in the top layers of the ground above this level. If it is so, we need to answer the following questions. (i) How can the water in the top layers of the ground be fresh water when the whole land is visibly saline? (ii) This phenomenon of surrounding soil in the top layers charging the ‘virda’ with fresh water should be localized. If so, could the soil in the immediate surroundings, hold such a large amount of water? (The discharge rate of a’virda’ is about 200 litres per day for two to three months.) And also, in this case the newly constructed ‘virda’ adjacent to the exhausted one should not work. But, how is it possible that they are also working as efficiently as the older ones? (iii) The other possibility is that there is a constant flow of ground water towards the downward slope (in this case, towards the main tank) and this constant flow must be the reason for an adjacent ‘virda’ to be successful. Then, the first ‘virda’ constructed itself should continuously yield fresh water for the reason that the flow is persistent. An explanation for this can be that the flow of the fresh water in the top layers of the soil is much slower compared to the rate of extraction of fresh water from ‘virda’, thus resulting in saline water ingress throughupconing. And the exhaustion of a ‘virda’ can be a localized and temporary phenomenon. (In the absence of empirical data regarding the water quality, soil properties and other geological information, it is difficult to come to a conclusion. The arguments were put forward to create a discussion on the technique. We request the readers to respond with their arguments and information regarding similar techniques. :Ed.) |
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Honey Bee, 5(3):7, 1994 |
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SRISTI Library