CHAPTER II
RAINFALL
1. =Rainfall Statistics.=--The mean annual rainfall varies very greatly according to the locality. In England it varies from about 20 inches at Hunstanton in Cambridgeshire, to about 200 inches at Seathwaite in Cumberland; in India, from 2 or 3 inches in parts of Scinde, to 450 inches or more at Cherrapunji in the Eastern Himalayas.
Rain is brought by winds which blow across the sea. Hence the rainfall in any country is generally greatest in those localities where the prevailing winds blow from seaward, provided they have travelled a great distance over the sea. Rainfall is greater among hills than elsewhere, because the temperature at great elevations is lower. Currents of moist air striking the hills are deflected upwards, become cooled, and the water vapour becomes rain. This process, if the hills are not lofty, may not produce its full effect till the air currents have passed over the hills, and thus the rainfall on the leeward slopes may be greater than elsewhere, but on the inner and more lofty ranges the rainfall is generally greatest on the windward side.
Thus the rainfall may vary greatly at places not far apart. An extreme instance of this occurs in the Bombay hills, where the mean annual falls at two stations only ten miles apart are respectively 300 inches and 50 inches.
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.
+------------------------+-------------+-----------------+---------+---------------+-----------------------+ | Place. | Catchment |Period over which| Total | Available | Remarks | | | Area. | Observations |Rainfall | Rainfall. | | | | | Extended. |Observed.| | | +------------------------+-------------+-----------------+---------+---------------+-----------------------+ | | Acres. | | Inches. |Ratio to Total.| | |Nagpur, Central India | 4,224 |June to September| 44 | ·40 | | | | |(Monsoon period).| | | | | ” ” | ” | ” | 30 | ·27 | | |Mercara, South India | 48 | Whole year. | 119 | ·37 |Gravelly soil overlying| |King William’s Town, | | | | | granite. | | Cape Colony | 67,200 | ” | 27 | ·21 |Hills with forest and | |Near Cape Town, Cape | | | | | bush. | | Colony | 110 |May to October | 31·5 | ·51 |Bare hills. | |Near Cape Town, Cape | |(rainy season). | | | | | Colony | 2,208 | ” | 43 | ·40 | ” | |Melbury Moor, Devonshire| ... | Whole year. | 50·7 | ·54 | | |Newport, Monmouthshire | ... | ” | 40 | ·40 | | |Newport, Isle of Wight | ... | ” | 32 | ·40 | | |Basin of Nepean River, | 284 sq. | | | | | | N.S.W. | miles | ” | 44·3 | ·44 |Bare, broken ground. | |Basin of Cataract River,| | | | | | | N.S.W. | 70 sq. miles| ” | 54 | ·45 | ” ” | +------------------------+-------------+-----------------+---------+---------------+-----------------------+
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 | Remarks. | | | | Rainfall. | | +-----+---------+---------------+-------------------------------------+ | | 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 | ” ” February (15 ins.). | |1899 | 54·9 | ·43 | ” ” August (11·5 ins.). | |1900 | 26·1 | ·50 | ” ” 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.
It is usually said that forests act as reservoirs by preventing snows from melting. This is disputed in the paper above quoted, and it is stated that in the absence of forests the snow forms drifts of enormous depth, and these melt very gradually and act as reservoirs after the snow in the forests has disappeared.
5. =Heavy Falls in Short Periods.=--When rain water, instead of being stored or utilised, has to be got rid of, it is of primary importance to estimate roughly--exact estimates are impossible--the greatest probable fall in a short time. This bears a rough ratio to the mean annual fall. The maximum observed falls in twenty-four hours range, in the United Kingdom, generally from ·05 to ·10 of the mean annual fall--but on one occasion the figure has been ·20,--and in the tropics from ·10 to ·25. Actual figures for particular places can be extracted from the rain registers, but the probability of their being exceeded must be taken into account. The greatest fall observed in twenty-four hours in the United Kingdom is 7 inches, and in India 30 inches in the Eastern Himalayas.
But much shorter periods than twenty-four hours have to be dealt with. The following figures are given by Chamier (_Min. Proc. Inst. C.E._, vol. cxxxiv.) as applicable to New South Wales, and he considers that they are fair guides, erring on the side of safety, for other countries:--
Duration of fall in hours 1 4 12 24 Ratio of fall to maximum daily fall ¼ ½ ¾ 1
The above figures are probably safe for England. For India the case is far otherwise. The following falls have been observed there:--
+----------+-------+--------------+--------+ | Period. | Fall. |Rate per Hour.|Remarks.| +----------+-------+--------------+--------+ | |Inches.| Inches. | | |7 hours | 10 | 1·43 | | |4·5 hours | 7·7 | 1·7 | | |2 hours | 8 | 4 | | |1 hour | 5 | 5 | | |20 minutes| 1·6 | 4·8 | | |10 minutes| 1 | 6 | | +----------+-------+--------------+--------+
The falls of 1 inch in ten minutes were frequently observed near the head of the Upper Jhelum Canal, a place where the annual rainfall is not more than 30 inches (see also CHAP. XII., _Art. 1_). In some parts of the Eastern Himalayas, where 30 inches of rain has fallen in a day, it is possible that 8 inches may have fallen in an hour. In England 4 inches has fallen in an hour. The heaviest falls in short periods do not usually occur in the wettest years, and they may occur in very dry years. Nor do they always occur on a very wet day.