li>6 EGYPTIAN AGRICULTURE. 



than O'lO metre, and if extra drainage is thrown on 

 them their capacity doubles with a head of 0*40 metre. 

 The loss of 0*30 metre extra head will not do much harm 

 if maintained for a short period. If, however, a syphon is 

 calculated for a head of 0'25 metre, to double its capacity 

 the head must rise to 1 metre and land will be damaged if 

 not swamped. It is false economy to curtail the size of a 

 syphon. The excavation of trench, deviation @f canal 

 where necessary, concrete bed and masonry heads cost 

 nearly as much for a small syphon as for a large one. 

 An iron pipe 1*5 metre diameter weighs only 40 per cent 

 more than one of 1 metre diameter but its sectional area 

 is 2*25 times as great, and it would carry with a 0*20 

 metre head an amount of water which could only be forced 

 through the smaller pipe with a head of 1 metre. 



The amount of drainage water which has to be provided 

 for depends on various points but it should not be less 

 than 50 cubic metres per feddan per 24 hours where there 

 is considerable rainfall. The main drain of a block of say 

 5,000 feddans should be as large in sectional area as the 

 canal which supplies that block. If all the 5,000 feddans 

 were under reclamation at one time, the drain would have 

 to be much larger, for whilst a canal runs full a drain 

 should only carry water to J or J its depth. On 5,000 

 feddans, part w r ould be reclaimed and giving only 10 cubic 

 metres per feddan of drainage, part would be under 

 process of washing and giving 100 cubic metres, and 

 part would probably be untouched. 



Ordinary crop irrigation requires 25 to 30 cubic metres 

 per feddan per 24 hours, and the drainage resulting from 

 this is 8 to 10 cubic metres, but in reclamation much more 





