The reservoir considered above does not, as already remarked, fully utilise the yield of the catchment area. In a wetter year there would be overflow and the yield from the reservoir would not be much increased. In order to equalise the flow of the two driest years the capacity of the reservoir must be increased, its yield being also increased, and so on for larger groups of years. By collecting information for large numbers of places in the British Isles, Deacon has prepared diagrams and tables which show the capacities and yields of reservoirs. The following table gives the figures for the case where the rainfall is 60 inches and the loss by evaporation and absorption 14 inches:—
| Number of Driest Consecutive Years the Flow of which is to be Equalised. | Net Capacity of Reservoir for a Catchment Area of 100 acres. | Daily Yield of Reservoir. | Column 2 ÷ Column 3 or Number of Days’ Supply contained in the Reservoir. | Ratio of Rainfall to Mean Annual Fall. | Available Rainfall. |
|---|---|---|---|---|---|
| Gallons. | Gallons. | Inches. | |||
| 1 | 166,000,000 | 1,475,000 | 113 | ·63 | 23·8 |
| 2 | 258,000,000 | 1,815,000 | 142 | ·72 | 29·2 |
| 3 | 329,000,000 | 1,987,000 | 165 | ·77 | 32·2 |
| 4 | 390,000,000 | 2,103,000 | 190 | ·80 | 34·0 |
| 5 | 441,000,000 | 2,187,000 | 201 | ·82 | 35·2 |
| 6 | 487,000,000 | 2,255,000 | 216 | ·835 | 36·1 |
The figures in the fifth column are those given in [Chap. II., Art. 1]. The figures in the last column show the corresponding available falls, after deducting the loss of 14 inches. It will be seen that, owing to this deduction, the available falls for the shorter periods are reduced in a greater ratio than the figures in the fifth column.
In arranging for the supply of towns in the British Isles it is usual to design the reservoirs so as to equalise the flow of the three driest consecutive years. Existing reservoirs, old and new, usually contain from 140 to 170 days’ supply, but some contain less. The above table shows that for the assumed fall of 60 inches and loss of 14 inches, the capacity of a reservoir, to allow for a six-year dry period, has to be 49 per cent. more than for a three-year dry period, while the daily supply from it is only 13 per cent. greater.
The following statement gives Deacon’s figures for mean annual rainfalls ranging from 30 to 100 inches. The columns marked R show the reservoir capacities in millions of gallons, and those marked S the daily yields of the reservoirs in thousands of gallons. The figures for other falls can be interpolated. For a fall of, for instance, 50 inches, the figures, whether of R or S, are practically a mean between those for falls of 40 and 60 inches.
| Number of Years whose Supply is to be Equalised. | F = 30. | F = 40. | F = 60. | F = 100. | ||||
|---|---|---|---|---|---|---|---|---|
| R. | S. | R. | S. | R. | S. | R. | S. | |
| 1 | 35 | 300 | 79 | 695 | 166 | 1475 | 345 | 3040 |
| 2 | 85 | 470 | 140 | 900 | 258 | 1815 | 495 | 3600 |
| 3 | 120 | 560 | 190 | 1050 | 329 | 1987 | 610 | 3900 |
| 4 | 150 | 620 | 230 | 1110 | 390 | 2103 | 710 | 4100 |
| 5 | 175 | 650 | 260 | 1170 | 441 | 2187 | 800 | 4230 |
| 6 | 195 | 680 | 290 | 1220 | 487 | 2255 | 887 | 4320 |
In all cases the loss is supposed to be 14 inches annually. If it is 15 or 13 inches, the reservoir capacity is less or more by about five, ten, or fifteen million gallons, according as the number of years in column 1 is 1, 3, or 6. And the daily yield is less or more by about 50,000 gallons.
With a low rainfall the advantage of a large reservoir is somewhat increased. The capacity of the six-year reservoir for a fall of 30 inches is 63 per cent. more than that of the three-year reservoir, but the supply is 22 per cent. greater.
The figures given above for reservoir capacities are suitable for the British Isles. They assume that the distribution of the rainfall is the least favourable that is at all likely to occur. Deacon states that the figures do not relieve the engineer of the exercise of judgment. As regards the British Isles, the chief questions on which judgment has to be exercised are whether to equalise the flow of three years or of another number, and how much to allow for loss. As already stated, three years is the period usually taken. The figures are suitable for most places in Europe, but in some places, e.g. on the Mediterranean coast, the distribution of the rainfall is somewhat less favourable than in the British Isles. In other parts of the world, and notably in or near the tropics, the distribution of the rainfall must be specially studied, and a diagram be prepared on the same principle as in the case of [fig. 58]. The diagram should be extended to cover the desired number of years. In hot countries loss by evaporation from the surface of the reservoir should be allowed for. In India during the hot dry months this loss may be half an inch in twenty-four hours.
In the article above quoted it is shown that if, as commonly happens, the consumption of water is, in summer, greater than the mean, and in winter less, the conditions are still more trying for the reservoir; and that in the case where the summer consumption is 13 per cent. greater than the mean, the capacity of the reservoir which impounds the water of the driest year must be 33 per cent., instead of 30 per cent., of the total supply impounded during the year. It would then contain 121 days’ instead of 110 days’ mean supply. In the table on page 170 the number of days’ supply is 113. From this it appears that the tables from which extracts have been given are calculated on the basis of a constant consumption. This, however, in the case where the number of years whose supply is equalised is greater than one, makes, owing to the increased size of the reservoir, no practical difference.