RESERVOIRS. These may be divided into two classes, "impounding reservoirs" and "service reservoirs," the latter being concerned with the distribution of water (see WATER SUPPLY).
Impounding to the fact that the flow of streams and rivers varies greatly throughout the year, it is necessary to provide works to store water if any substantial use is to be made of the annual discharge. Such works are known as impounding reservoirs, their function being to store water when the stream flow is ample for the purpose of augmenting the natural flow in dry weather.
The urgency for the construction of such reservoirs must have become apparent in very early times in countries where the climatic conditions were such that the streams ran dry for a portion of the year, and records exist of one being made in Ceylon as early as 504 B.C. Anciently reservoirs were formed by an embankment across the valley through which a stream flowed, and were sometimes of vast extent, the Padavil-Colan Tank in Ceylon, for instance, having an embankment I I m. long and, in parts, 70 f t. high.
Storage.—Having selected a catchment area capable of yield ing sufficient water, the capacity of the reservoir has next to be determined. This will depend upon the incidence and intensity of the rainfall and the loss by evaporation and absorption, con ditions which vary within wide limits. In countries subject to long periods of drought, the necessary capacity will be greater than in those enjoying a temperate climate, and in India, for instance, where the rain falls only during monsoon periods, two years' storage of the daily quantity may be necessary. Few records exist of the flow of streams in the British Isles taken over a sufficiently lengthy period to be of service, and recourse has generally to be made to the annual rainfall records, from which the annual discharge of the stream is deduced. Long period rainfall gaugings show that the rainfall of the driest year is about two-thirds, the mean fall of the two driest years about three-quarters, and the rainfall of the three driest consecutive years about four-fifths, of the average annual rainfall. Notwith standing the wide variation of climatic conditions, these propor tions hold fairly well over a large portion of the land surface of the globe (see "The Variations of Rainfall," by A. R. Binnie, Proc. Inst. C.E., vol. 109).
As storage increases in relation to the average flow of a stream, the maintainable yield increases in a decreasing ratio until a maximum is reached where there would be little advantage in further increase, and in the British Isles the economic limit is generally taken as that capacity which would he sufficient to equalize the flow of the three driest consecutive years.
The average annual rainfall of the three driest consecutive years, being approximately four-fifths of the average annual rainfall; and the average annual loss by evaporation and absorp tion being about 14 inches; the average annual discharge of the stream during the three driest consecutive years would be that due to average rainfall-14 inches running off the catchment area, which may be denoted by f. The formula known, from its author, as the Hawksley Formula gives the number of days storage which should be provided to maintain this In many cases it is not necessary to provide so large a storage, as some quantity may be required which is less than the average flow of the stream during the three driest consecutive years.
Fig. I gives the relation between the maintainable yield and the capacity to be provided for catchment areas in the British FIG. 1 Isles having a mean annual rainfall varying from ioo to 3o in., and is due to the investigations of Dr. G. F. Deacon. The capacity of the reservoir in gal. per ac. of catchment area is shown on the base line, and the yield of the reservoir in gal. per ac. per diem is given by the length of the vertical line between that capacity and the curve of average rainfall, the yield in gal. per ac. per diem being read from the vertical scale at the left hand side. The storage required for any particular average rain fall to balance the average stream discharge during different series of consecutive dry years, is given by the diagonal lines which intersect the curve of rainfall on the diagram.
The diagram gives the capacity above the lowest draw-off level of the reservoir, and as it is undesirable to abstract muddy water for supply, this level should be well above the bottom of the reservoir. The loss by evaporation from a water surface is greater than the loss on the catchment area, and in the British Isles the depth of the reservoir should be about 6 in. more than would be required to give the gross storage, whereas in tropical countries the allowance may be as much as 6 ft.