THE FERTILITY OF SOILS IN THE TROPICS 375 



low stage. The slope of the water-surface is less than 

 that of the land, and by the time Beirut is reached is 

 above land-level, and watering by free-flow becomes pos- 

 sible. The Delta Barrage fulfils a similar purpose, the 

 canals which water the Delta taking off upstream of it. 

 As a matter of fact the canal system of the Delta is not 

 at present designed to secure free flow to more than a 

 quarter of the Delta, and the water in the canals in the 

 remainder flows below land-level, and has to be pumped 

 through one or two before it can be applied to the land. 

 The main feature of the perennially irrigated area from 

 the point of view of this paper is that its agriculture 

 depends entirely on the quantity of water which is avail- 

 able, and not on the level at which that water is flowing 

 in the river. 



It is very different in the case of the basin lands, about 

 a million and a quarter acres in extent, which receive a 

 watering once a year by flooding at the time of high 

 Nile, the only supply at other times of year being that 

 which is obtained -from wells. For the supply, canals are 

 dug taking direct from the river and when the river rises 

 they fill. The slope of the water surface in the canal 

 is less than that of the land and some distance below 

 the head the water can be poured on to the land. 

 Theoretically, the greater part of this basin land could 

 be given its flooding even in bad years, but the canal 

 system would be enormously costly and the annual upkeep 

 would be prodigious if years such as the one we have 

 just gone through were to 'be provided for. If the Nile 

 falls below a proper level the irrigation of the area be- 

 comes difficult and in years such as 1913-1914 it becomes 

 impossible to provide water to large stretches of country. 



The approximate amount O'f water required (in millions 

 of cubic metres per day) during the summer when there 

 is a deficiency of supplies in the natural river is shown 

 in the table below : 



Deficiency 

 Haifa 

 March ... .. ..50 ., 77 .. 27 



April ... 

 May ... 

 June ... 

 July (1-15) 



65 ... 60 



55 - 52 ... 



65 ... 65 ... 



70 ... no ... 40 



