Page:History of the Periyár project. (IA historyofperiyar00mack).pdf/22

6 "The numerous tanks supplied by the channels were many of them first class reservoirs originally, but are now so silted up as to be capable of storing not much more than half the quantity of water they were designed to hold ; they occupy a great deal of valuable land, and, in the attempt to get as much water as they require, the ryots of one tank frequently cause injury to their neighbours above them, damming up their escape calingulahs and flooding land by the extended waterspread.

"The tanks having become shallow in proportion to the area of the waterspread, there is enormous waste of valuable water by evaporation. I calculate that this waste amounts to at least 30 per cent of the water stored.

"The character of the Vaigai makes the tanks system essential. For some reason or other the quantity of water received into the channel of the river bears a very small proportion to the rainfall on its catchment basin. The average annual rainfall registered at Periyaculam and Madura is 32 and 41 inches, respectively. On the Cumbum valley (where no register has been kept) it is at least as much as at Periyaculam ; in the Wursanaad valley it is probably less. Taking it at 33 inches only over the whole catchment basin above the Peranny it would amount to 3,600 millions of cubic yards per annum, and supposing that only one-third of this found its way into the streams and rivers so as to be available for irrigation there would be more than enough (with due allowance for the enormous waste on tanks) for three times the extent of paddy crop now raised.

"Yet it is affirmed by good authority that, in an average year, not a drop of Vaigai water reaches the sea; but this I think is hardly sufficiently well established to be accepted as the fact. My belief is that sufficient water for more than double the present area of irrigation does flow down the Vaigai, but that three-fourths of the annual supply passes down the river at three times the rate at which all the channels together can draw off water from it, so that if the big freshes could be detained, so as to spread over, say, 60 days, instead of running off at three times the rate in 20 days, it would be found that there would be water enough (if the tanks could contain what it would be necessary to store) for double the area of rice-crops.

"However this may be, there is no doubt as to the main fact that the supply of water obtained from the Vaigai is so precarious and scanty that even in good years the paddy crop barely covers 22,000 acres annually, although the existing tanks and channels command land enough and have sufficient hydraulic capacity, for the irrigation of fully double that extent