+-----------+-------------+-----------+----------+--------+---------+ | Number of | | |Column 2 ÷| | | | Driest | Net Capacity| | Column 3 | | | |Consecutive| of Reservoir| | or Number|Ratio of| | | Years the | for a | Daily | of Days’ |Rainfall|Available| | Flow of | Catchment | Yield of | Supply |to Mean |Rainfall.| | which | Area of 100 |Reservoir. | contained| Annual | | | is to be | acres. | | in the | Fall. | | | Equalised.| | |Reservoir.| | | +-----------+-------------+-----------+----------+--------+---------+ | | 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.