In temperate climates the rainfall is generally distributed over all the months of the year; in the tropics the great bulk of the rain often falls in a few months.
The fall at any one place varies greatly from year to year. To obtain really reliable figures concerning any place, observations at that place should extend over a period of thirty to thirty-five years. The figures of the mean annual fall will then probably be correct to within 2 per cent. The degree of accuracy to be expected in results deduced from observations extending over shorter periods is as follows:—
| No. of years | 25 | 20 | 15 | 10 | 5 |
| Error per cent. | 3 | 3¼ | 5 | 8 | 15 |
These figures were deduced by Binnie (Min. Proc. Inst. C.E., vol. cix.) from an examination of rainfall figures obtained over long periods of time at many places scattered over the world. The errors may, of course, be plus or minus. They are the averages of the errors actually found, and are themselves subject to fluctuations. Thus the 15 per cent. error for a five-year period may be 16 or 13, the 8 per cent. error for a ten-year period may be 8½ or 7½, with similar but minute fluctations for the other periods.
Binnie’s figures also show that the ratio of the fall at any place in the driest year to the mean annual fall, averages ·51 to ·68, with a general average of ·60, and that the ratio in the wettest year to the mean annual fall averages 1·41 to 1·75, with a general average of 1·51. For India the general averages are ·50 and 1·75. These figures are useful as a means of estimating the probable greatest and least annual fall, but they are averages for groups of places. The greatest fall at any particular place may occasionally be twice the mean annual fall. At some places in India, in Mauritius, and at Marseilles it has been two and a half times the mean annual fall. The least annual fall may, in India, be as low as ·27 of the mean. In England the fall in a dry year has, once at least, been found to be only ·30 of the mean annual fall. The mean fall (average for all places) in the three driest years is, from Binnie’s figures, about ·76 of the mean annual fall. The figures given above, except when a particular country is mentioned, apply to all countries and to places where the rainfall is heavy, as well as to those where it is light. But in extremely dry places the fluctuations are likely to be much greater. At Kurrachee, with a mean annual fall of only 7·5 inches, the fall in a very wet year has been found to be 3·73 times, and in a very dry year only ·07 times the mean annual fall.
In the United Kingdom the probable rainfall at any place in the driest year may be taken as ·63 of the mean annual fall. For periods of two, three, four, five and six consecutive dry years, the figures are ·72, ·77, ·80, ·82, and ·835. These figures are of importance in calculations for the capacity of reservoirs ([Chap. XIII., Art. 2]).
When accurate statistics of rainfall are required for any work, the rainfall of the tract concerned must be specially studied and local figures obtained for as many years as possible. Very frequently it is necessary to set up a rain-gauge, or several if the tract is extensive or consists of several areas at different elevations. Sometimes there is only a year or so in which to collect figures. In this case the ratio of the observed fall to that, for the same period, at the nearest station where regular records are kept, is calculated. This ratio is assumed to hold good throughout, and thus the probable rainfall figures for the new station can be obtained for the whole period over which the records have been kept at the regular station. The volumes of the British Rainfall Organisation contain a vast amount of information regarding rainfall. For a large area there should be one rain-gauge for every 500 acres, for a small area more. In the case of a valley there should be at least three gauges along the line of the deepest part—one at the highest point, one at the lowest, and one midway as regards height—and two gauges half way up the sides and opposite the middle gauge (Ency. Brit., Tenth Edition, vol. xxxiii.). Some extra gauges may be set up for short periods in order to see whether the regular gauges give fair indications of the rainfall of the tract. If they do not do so some allowances can be made for this.
2. Available Rainfall.—The area drained by a stream is called its “catchment area” or “basin.” The available rainfall in a catchment area is the total fall less the quantity which is evaporated or absorbed by vegetation. The evaporation does not chiefly take place directly from the surface. Rain sinks a short distance into the ground, and is subsequently evaporated. The available rainfall does not all flow directly into the streams. Some sinks deep into the ground and forms springs, and these many months later augment the flow of the stream and maintain it in dry seasons. The available rainfall of a given catchment area is known as the “yield” of that area.
Estimation of the available rainfall is necessary chiefly in cases where water is to be stored in reservoirs for town supply or irrigation. The ratio of the available to the total rainfall depends chiefly on the nature and steepness of the surface of the catchment area, on the temperature and dryness of the air, and on the amount and distribution of the rainfall. The ratio is far greater when the falls are heavy than when they are light. Again, when the ground is fairly dry and the temperature high—as in summer in England—nearly the whole of the rainfall may evaporate; but when the ground is soaked and the temperature low—as in late autumn and winter in England—the bulk of the rainfall runs off. In the eighteen years from 1893 to 1900 the average discharge of the Thames at Teddington, after allowing for abstractions by water companies, was in July, August, and September 12 per cent. of the rainfall—6·9 inches—in its basin, and in January, February, and March 60 per cent. of the fall which was 5·9 inches. The total fall in the year was 26·4 inches. Some rivers in Spain discharge, in years of heavy rainfall, 39 per cent., and in years of light rainfall 9 per cent. of the rainfall (Min. Proc. Inst. C.E., vol. clxvii.). The discharge of a river is not always greatest in the month, or even the year, of greatest rainfall.
The table opposite gives some figures obtained by comparison of rainfall figures and stream discharges. The case of the area of 2208 acres near Cape Town is described in a paper by Bartlett (Min. Proc. Inst. C.E., vol. clxxxviii.), and it is shown by figures that part of the rainfall in the rainy season went to increase the underground supply which afterwards maintained the flow in the dry season.