September 7, 1905] 



NA TURE 



467 



irrigation is where the canal or the river has its source of 

 supply in a great lake. For, be the rainfall ever so heavy, 

 the water surface in the lake will not rise very much, nor 

 will it greatly sink at the end of a long drought. Where 

 there is no moderating lake, a river fed from a glacier 

 has a precious source of supply. The hotter the weather, 

 the more rapidly will the ice melt, and this is just when 

 irrigation is most wanted. 



Elsewhere, if crops are to be raised and the rain cannot 

 be counted on, nor well irrigation be practised, water 

 storage becomes necessary, and it is with the help of water 

 storage that in most countries irrigation is carried on. 



Water Storage. 



To one who has not given the subject attention, surprise 

 is often expressed at the large volume of water that has 

 to be stored to water an acre of land. In the case of rice 

 irrigation in India, it is found that the storage of a million 

 cubic feet does not suffice for more than from 6 to 8 acres. 

 For the irrigation of wheat about one-third this quantity 

 is enough. It would never pay to excavate on a level plain 

 a hollow large enough to hold a million cubic feet of water. 

 It is invariably done by throwing a dam across the bed of 

 a river or a valley and ponding up the water behind it. 

 Many points have here to be considered : the length of 

 dam necessary, its height, the material of which it is to 

 be constructed, the area and the value of the land that 

 must be submerged, the area of the land that may be 

 watered. The limits of the height of a dam are from 

 about 150 to 15 feet. If the slope of the valley is great it 

 may be that the volume which can be ponded up with a 

 dam of even 150 feet is inconsiderable, and the cost may 

 b= prohibitory. On the other hand, if the country is very 

 fiat, it may be that a dam of only 20 feet high may require 

 to be of quite an inordinate length, and compensation for 

 the area of land to be submerged may become a very large 

 item in the estimate. I have known of districts so flat 

 that in order to irrigate an acre more than an acre must 

 be drowned. This looks ridiculous, but is not really so, for 

 the yield of an irrigated acre may be eight or ten times 

 that of an unirrigated one ; and after the storage reservoir 

 has been emptied it is often possible to raise a good crop 

 on the saturated bed. 



The advantage of a deep reservoir is, however, very 

 great, for the evaporation is in proportion to the area of 

 the surface, and if two reservoirs contain the same volume 

 of water, and the depth of one is double that of the other, 

 the loss by evaporation from the shallow one will be double 

 that of the deep one. In India, from time immemorial, 

 it has been the practice to store water for irrigation, and 

 there are many thousands of reservoirs, from the great 

 artificial lakes holding as much as 5000 or 6000 millions of 

 cubic feet, down to the humble village tank holding not a 

 million. There are few of which the dam exceeds 80 feet 

 in height, and such are nearlv always built of masonry 

 or concrete. For these it is absolutely necessary to have 

 sound rock foundations. If the dam is to be of earth, the 

 quality of the soil must be carefully seen to, and there 

 should be a central core of puddle resting on rock and 

 rising to the maximum height of water surface. If the 

 dam is of masonry, there may perhaps be no harm done 

 should the water spill over the top. If it is of earth, this 

 must never happen, and a waste weir must be provided, if 

 possible cut out of rock or built of the best masonry, and 

 large enough to discharge the greatest possible flood. More 

 accidents occur to reservoirs through the want of sufficient 

 waste weirs or their faulty construction than from any other 

 caus". 



As important as the waste weir are the outlet sluices 

 through which the water is conveyed for the irrigation of 

 the fields. If possible they should be arranged to serve at 

 the same time as scouring sluices to carry off the deposit 

 that accumulates at the bottom of the reservoir. For, 

 unless provided with very powerful scouring sluices, sooner 

 or later the bed of the reservoir will become silted up. and 

 the space available for water storage will keep diminishing. 

 As this happens in India, it is usual to go on raising the 

 embankment (for it does not pay to dig out the deposit), 

 and so the life of a reservoir may be prolonged for many 



NO I 87 I, VOL. 72] 



years. Ultimately it is abandoned, as it is cheaper to 

 make a new reservoir altogether than to dig out the old 

 one. 



Italian Irrigation. 



For the study of high-class irrigation there is probably no 

 school so good as is to be found in the plains of Piedmont 

 and Lombardy. Every variety of condition is to be found 

 here. The engineering works are of a very high class, 

 and from long generations of experience the farmer knows 

 how best to use his water. 



The great river Po has its rise in the foothills to the 

 west of Piedmont. It is not fed from glaciers, but by rain 

 and snow. It carries with it a considerable fertilising 

 matter. Its temperature is higher than that of glacial 

 water — a point to which much importance is attached for 

 the very valuable meadow irrigation of winter. From the 

 left bank of the Po, a few miles below Turin, the great 

 Cavour Canal takes its rise, cutting right across the whole 

 drainage of the country. It has a full-supply discharge of 

 3800 cubic feet per second ; but it is only from October to 

 May that it carries anything like this volume. In summer 

 the discharge does not exceed 2200 cubic feet per second, 

 which would greatly cripple the value of the work were it 

 not that the glaciers of the Alps are melting then, and the 

 great torrents of the Dora Baltea and Sesia can be counted 

 on for a volume e.xceeding 6000 cubic feet per second. 



Lombardy is in no respects worse off than Piedmont for 

 the means of irrigation ; and its canals have the advantage 

 of being drawn from the lakes Maggiore and Como, exer- 

 cising a moderating influence on the Ticino and Adda rivers, 

 which is sadly wanted on the Dora Baltea. The Naviglio 

 Grande of Lombardy is drawn from the left bank of the 

 Ticino, and is used largely for navigation, as well as irriga- 

 tion. It discharges between 3000 and 4000 cubic feet per 

 second, and nowhere is irrigation probably carried on with 

 less expense. From between Lake Maggiore and the head 

 of the Naviglio Grande a great new canal, the Villoresi, 

 has been constructed during the last few years with head 

 sluices capable of admitting 6700 cubic feet per second, of 

 which, however, 4200 cubic feet have to be passed on to 

 the Naviglio Grande. Like the Cavour Canal, the Villoresi 

 crosses all the drainage coming down from the foothills to 

 the north. This must have entailed the construction of 

 very costly works. 



Irrigation in Northern India. 



It is in India that irrigation on the largest scale is to 

 be found. The great plains of Northern India are peculiarly 

 well adapted for irrigation, which is a matter of life and 

 death to a teeming population all too well accustomed to a 

 failure of the rain supply. 



The Ganges, the Jumna, and the great rivers of the 

 Punjab have all been largely utilised for feeding irrigation 

 canals. The greatest of these, derived from the river 

 Chenab, and discharging from 10,500 to 3000 cubic feet per 

 second, was begun in 1889, with the view of carrying water 

 into a tract entirely desert and unpopulated. It was opened 

 on a small scale in 1892, was then enlarged, and ten years 

 after it irrigated in one year 1,829,000 acres, supporting a 

 population of 800,000 inhabitants, colonists from more con- 

 gested parts of India. 



The Ganges Canal, opened in 1854, at a time when there 

 was not a mile of railway, and hardly a steam engine 

 within a thousand miles, has a length of about 9900 miles, 

 including distributing channels. It was supplemented in 

 1878 by a lower canal, drawn from the same river 13U 

 miles further down, and these two canals now irrigate 

 between them 1,700,000 acres annually. On all these canalr. 

 are engineering works of a very high class. The origins' 

 Ganges Canal, with a width of bed of 200 feet, a depth 

 of 10 feet, and a maximum discharge of 10,000 cubic len 

 per second, had to cross four great torrents before it could 

 attain to the watershed of the country, after which it could 

 begin to irrigate. Two of these torrents are passed over 

 the canal by broad super-passages. Over one of them the 

 canal is carried in a majestic aqueduct of fifteen arches, 

 each of 50 feet span ; and the fourth torrent, the most 

 difficult of all to deal with, crosses the canal at the same 

 level, a row of forty-seven floodgates, each 10 feet wide, 

 allowing the torrent to pass through and out of the canal. 



