| Place. | Catchment Area. | Period over which Observations Extended. | Total Rainfall Observed. | Available Rainfall. | Remarks |
|---|---|---|---|---|---|
| Acres. | Inches. | Ratio to Total. | |||
| Nagpur, Central India | 4,224 | June to September (Monsoon period). | 44 | ·40 | |
| Nagpur, Central India | 4,224 | (Monsoon period). | 30 | ·27 | |
| Mercara, South India | 48 | Whole year. | 119 | ·37 | Gravelly soil overlying granite. |
| King William’s Town, Cape Colony | 67,200 | Whole year. | 27 | ·21 | Hills with forest and bush. |
| Near Cape Town, Cape Colony | 110 | May to October (rainy season). | 31·5 | ·51 | Bare hills. |
| Near Cape Town, Cape Colony | 2,208 | (rainy season). | 43 | ·40 | Bare hills. |
| Melbury Moor, Devonshire | ... | Whole year. | 50·7 | ·54 | |
| Newport, Monmouthshire | ... | Whole year. | 40 | ·40 | |
| Newport, Isle of Wight | ... | Whole year. | 32 | ·40 | |
| Basin of Nepean River, N.S.W. | 284 sq. miles | Whole year. | 44·3 | ·44 | Bare, broken ground. |
| Basin of Cataract River, N.S.W. | 70 sq. miles | Whole year. | 54 | ·45 | Bare, broken ground. |
The following statement shows how the available rainfall may vary from year to year. The figures are those of a catchment area of 50 square miles on the Cataract River, New South Wales (Min. Proc. Inst. C.E., vol. clxxxi.):—
| Year. | Rainfall. | Available Rainfall. | Remarks |
|---|---|---|---|
| Inches. | Ratio to Total. | ||
| 1895 | 34·1 | ·84 | Heavy rain falling on saturated area. |
| 1896 | 33·7 | ·28 | Evenly distributed fall. |
| 1897 | 44·7 | ·49 | Heavy rains in May. |
| 1898 | 56·4 | ·45 | Heavy rains in February (15 ins.). |
| 1899 | 54·9 | ·43 | Heavy rains in August (11·5 ins.). |
| 1900 | 26·1 | ·50 | Heavy rains in May and July. |
| 1901 | 37·4 | ·11 | Evenly distributed fall. |
| 1902 | 29·9 | ·06 | |
| 1903 | 41·7 | ·23 | No heavy fall. |
The manner in which the available rainfall may vary from month to month is shown in the following statement, which gives the figures for 1905 for the Sudbury River in Massachusetts:—
| Month. | Rainfall. | Available Rainfall. |
|---|---|---|
| Inches. | Percentage of fall. | |
| January | 5·3 | 48 |
| February | 2·2 | 24 |
| March | 3·2 | 142 |
| April | 2·7 | 104 |
| May | 1·3 | 40 |
| June | 5·0 | 16 |
| July | 5·5 | 6 |
| August | 2·7 | 8 |
| September | 6·9 | 31 |
| October | 1·5 | 18 |
| November | 2·1 | 23 |
| December | 4·0 | 40 |
| Total | 42·3 | Average 39·5 |
Rankine gives the ratio of the available rainfall to the whole fall as 1·0 on steep rocks, ·8 to ·6 on moorland and hilly pasture, ·5 to ·4 on flat, cultivated country, and nil on chalk. These figures are only rough. The figures for rocks and pastures are too high. The loss from evaporation and absorption is not proportional to the rainfall. It is far more correct to consider the loss as a fairly constant quantity in any given locality but increasing somewhat when the rainfall is great. The available rainfall in Great Britain has generally been overestimated. Sometimes it has been taken as being ·60 of the whole fall. More commonly the loss is taken to be 13 to 15 inches. This is correct for the western mountain districts, where the rainfall is about 80 inches and the soil consists chiefly of rocks partly covered with moorland or pasture. In other parts of the country, especially where flat, the loss is often 17 to 20 inches. All the above figures are, however, general averages. The proper estimation of the available rainfall at any place in any country depends a great deal on experience and judgment, and on the extent to which figures for actual cases of similar character are available. Regarding the “run-off” from saturated land during short periods, see [Chap. XII., Arts. 1] and [2.]
3. Measurement of Rainfall.—A rain-gauge should be in open ground and not sheltered by objects of any kind. The ordinary rain-gauge is a short cylinder. This is often connected by a tapering piece to a longer cylinder of smaller diameter. In this the rain is stored safely and is measured by a graduated rod. The measurement can be made more accurately than if the diameter was throughout the same as at the top. In other cases the water is poured out of the cylinder into a measuring vessel. If the rain-gauge was sunk so that the top was level with the ground, rain falling outside the gauge would splash into it and vitiate the readings unless the gauge was surrounded by a trench. Ordinarily the top of the gauge is from 1 to 3 feet above the ground. When it is 1 metre above the ground the rain registered is said to be on the average about 6 per cent. less than it should be, owing to the fact that wind causes eddies and currents and carries away drops which should have fallen into the gauge. The velocity of the wind increases with the height above the ground, and so does the error of the rain-gauge. Devices for getting rid of the eddies have been invented by Boernstein and Nipher (Ency. Brit., Tenth Edition, vol. xviii.), but they have not yet come into general use. The Boernstein device is being used experimentally at Eskdalemuir. It would appear that much splashing cannot take place when the ground is covered with grass, and that in such a case the top of the gauge could be 1 foot above the ground, thus making the error very small.
If the ground is at first level, then rises and then again becomes level, a rain-gauge at the foot of the slope will, with the prevailing wind blowing up the slope, register too much, and a rain-gauge just beyond the top of the slope will register too little (Ency. Brit., Tenth Edition, vol. xxxiii.).
4. Influence of Forests and Vegetation.—When the ground is covered with vegetation, and especially forests, the humus or mould formed from leaves, etc., absorbs and retains moisture. It acts like a reservoir, so that the run-off takes place slowly and the denudation and erosion of the soil is checked. The roots of the trees or other vegetation also bind the soil together. Vegetation and forests thus mitigate the severity of floods and reduce the quantity of silt brought into the streams. They also shield the ground from the direct rays of the sun and so reduce evaporation, and thus, on the whole, augment the available rainfall. Forests render the climate more equable and tend to reduce the temperature, and they thus, at least on hills, increase the actual rainfall to some extent.
If a forest is felled and replaced by cultivation, the ploughing of the soil acts in the same way as the humus of the forest, and the crops replace the trees; and it has been stated that in the U.S.A. the cultivation is as beneficial as the forests in mitigating floods and checking denudation of the soil (Proc. Am. Soc. C.E., vol. xxxiv.). But when forests are felled they are not, at least in hilly country, always replaced by cultivation. Measures to put a stop to the destruction of forests or to afforest or reforest bare land may enter into questions of the régime of streams or the supply of water. On the Rhine, increase in the severity of floods was distinctly traced to deforestation of the drainage area.