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Dew Ponds


THE legend that dew has anything particular to do with dew ponds seems to die very hard. The essence of a dew pond surely is that it should be watertight, which those who make them seem to know quite well. Except in very abnormal years such as 1921 (and I suppose 1933 and perhaps 1934, though I have not seen the figures) the rainfall in England for the year largely exceeds the loss by evaporation from a water surface, and in an average year an empty watertight pond will have accumulated about 5 inches of water from January 1 to the end of April, will lose 2 inches during the next three months and will then progressively become deeper until at the end of the year it has 11 inches. Evaporation averages about 15 inches and never exceeds 20 inches, so that making allowance for the probability that it is a good deal more in exposed ponds than in the relatively sheltered tanks in which it is measured, it would seem that 2 feet of water in the early spring would see any watertight pond safely through any summer. It is also desirable that there should be no standing vegetation, which downland farmers know, though they attribute its bad effect to the roots perforating the floor of the pond rather than to transpiration, which is extraordinarily effective. Last summer (1934) a cement tank in my garden with Potamogeton crispus and Lemna lost 5 inches of water, while a precisely similar tank a few feet away which had three good clumps of Alisma plantago lost 19 inches and went dry.

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