NATURE 



513 



THURSDAY, SEPTEMBER 19, 1907. 



IRRIGATION ENGINEERING. 

 Irrigation : its Principles and Practice as a Branch 

 of Engineering. By Sir Hanbury Brown, 

 K.C.M.G. Pp. XV + 301. (London: Archibald 

 Constable and Co., Ltd., 1907.) Price lOi'. net. 



IRRIG.ATIOX is of such paramount importance to 

 e.xtensive tracts in India, and to the whole of 

 Egvpt, that it is in these countries that irrigation 

 works have been carried out on the largest scale, and 

 therefore the author, who has gained his experience 

 in this branch of engineering in both these countries, 

 is particularly well qualified for dealing with this 

 subject; and by drawing his examples of works in 

 illustration from both sources, and especially from 

 Egypt, he has been able to present the principles and 

 practice of irrigation in their grandest and most 

 modern aspects. The whole object of irrigation is to 

 supply water for agriculture to lands which are either 

 devoid of adequate rainfall or on which the rain does 

 not fall at a suitable period for the crops ; and owing 

 to the very unequal and irregular distribution of the 

 rainfall in certain warm, and especially tropical, coun- 

 tries, large and extensive engineering works are often 

 required to store and convey the abundant rainfall to 

 arid regions at a distance. The simplest sources of 

 water for irrigation are rivers which overflow their 

 banks in .the flood season, and inundate the adjacent 

 low-lying lands, such as the Nile in Egypt, the Tigris 

 and Euphrates in Mesopotamia, and the Indus in Sind. 

 -As, however, the land near the river bank is higher 

 than the plains at the back, owing to the chief deposit 

 of silt from the muddy flood-waters occurring when 

 the overflow from the river loses its velocity on over- 

 topping the banks, inundations would only occur with 

 high floods, and would spread irregularly over the 

 land according to variations in level. 



To provide against these natural defects, and to 

 obtain a uniform distribution of the fertilising silt as 

 well as of the water, the basin system of irrigation 

 was introduced,, in which the land is divided by banks 

 into a series of basins, and the water is admitted into 

 the large low-lying basins at the back through sluices 

 at the side of an embanked canal drawing its supply 

 from the river above the highest up-stream basin of 

 the set ; w-hilst the smaller basins at a higher level 

 adjoining the river are fed from a high-level canal 

 deriving its water from the river further up-stream. 

 By these arrangements the filling of the several basins 

 in each set can be regulated with uniformity, and 

 advantage is taken of a high flood to secure a large 

 deposit of mud serving as manure. The mud is 

 gradually deposited from the still water in the basins, 

 and when the river falls the clear water is discharged, 

 and the seed is laid in the damp mud. The basin 

 system has been very extensively developed in Egypt ; 

 whereas in India canals draw the water from the 

 rivers in flood-time, and irrigate the land by means of 

 branch canals leading the water into field channels. 

 Irrigation by inundation, however, onlv provides for 

 winter crops; whilst the more valuable summer crops, 

 NO. 1977, VOL. 76] 



such as sugar-cane, cotton, and rice, need water at 

 definite intervals when the rivers are low, and pro- 

 tection from inundation during the flood season. This 

 perennial irrigation enables two crops to be raised in 

 the year, and more than doubles the value of the land ; 

 and the water stored by the .\ssuan dam has provided 

 for the conversion of 450,000 acres from basin irri- 

 gation into perennially irrigated land. Besides rivers, 

 wells, lakes, and artificial reservoirs afford sources for 

 the supply of water for irrigation. 



In India, wells irrigating 13 million acres are 

 second only in importance to canals from rivers, 

 which irrigate 17 million acres; whilst tanks 

 enclosed by embankments, serving as primitive stor- 

 age reservoirs collecting the rainfall and local drain- 

 age, irrigate S million acres; and the high 

 concrete reservoir dam across the River Periyar, in 

 the very rainv district of Travancore, stores up water 

 which is discharged by a tunnel through the Ghats 

 for irrigating the arid district of Madura, on the 

 eastern side of the mountain range. Wells are used 

 for irrigation in Egvpt. but do not occupy at all the 

 same important position as in India; though in olden 

 times Lake Moeris provided a natural reservoir filled 

 with water in flood-time, and supplementing the dis- 

 charge of the Nile at its low stage, for which also 

 the equatorial lakes act as regulators at the present 

 time ; whilst the reservoir formed by the Assuan dam 

 supplies water for summer irrigation when the flow of 

 the Nile is deficient. In the chapter on sources of 

 supply it is stated that the storage required to 

 .secure the volume of water needed for the irrigation 

 of summer crops for the whole of Egypt, during the 

 hundred davs in the year during which the flow of the 

 Nile is at its minimum, is six thousand million cubic 

 metres, of which one-sixth has been provided by the 

 existing .Assuan reservoir. The author enumerates 

 five methods of increasing the storage, the first of 

 which, namely, the raising of the .Assuan dam, has 

 been decided upon, since the book was written, for 

 doubling the present supply ; whilst the second pro- 

 posal of building a second darn higher up the Nile 

 has been rejected as impracticable. Little attention 

 has been bestowed on the third project of forming a 

 reservoir in the Wadi Rayan depression near the site 

 of Lake Moeris ; and a reservoir of this kind at the 

 side of the river, when periodically filled up with 

 muddy Nile water, would appear destined to be filled 

 up before long with deposit. 



The two last schemes for increasing the supply, 

 advocated by Sir William Garstin, consist in effecting 

 a great reduction in the loss from evaporation of the 

 flow of the White Nile in passing through the swamps 

 of the Sudd region, by diverting the discharge from 

 the swamps into a straight cut, not less than 210 

 miles long, from Bor to the confluence of the River 

 Sobat at Taufikia below the Sudd region, and com- 

 bining this increased discharge with the utilisation of 

 the .Albert Nyanza for storage, during the four months 

 of flood discharge, by regulating its outflow, so as to 

 make up the deficiency during the remaining eight 

 months. The new cut would have the great value of 

 permanently increasing the flow of the Nile through- 

 out the 3'ear below the confluence of the Sobat, but 



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