3/8 THE FERTILITY OF SOILS IN THE TROPICS 



After the top of the flood all the feeder rivers fell 

 away rapidly, and it was early seen that the water supply 

 during the summer of 1914 would be very short, par- 

 ticularly if the spring rain should fail. To provide the 

 maximum amount of water the overflow sluices of the 

 Aswan dam were boarded up and the volume of the 

 reservoir was increased by about 150 million cubic metres, 

 or roughly 6 per cent. 



Diagram III shows the amount of water which it has 

 been possible to supply to Egypt. The lower dotted 

 curve shows the volume reaching Wadi Haifa corrected 

 for the loss by evaporation and seepage between Wadi 

 Haifa and Aswan; whilst the stepped curve shows the 

 volume which it has been possible to supply with the 

 aid of the Aswan reservoir. The canals are all closed 

 for clearances during January, and little water is then 

 required, and during this period water was taken from 

 the river to fill the dam to its utmost capacity. When 

 the canals are reopened a large amount of water ds 

 required for a few days, after which the necessary supply 

 is then small. This accounts for the sharp rise at the 

 end of January and the reduced supplies in February, 

 during which month the reservoir was again filled. In 

 March and April there is again a very heavy demand, 

 as the cotton is then sown, and a heavy watering is 

 given. Once the cotton is in the ground the water 

 requirements are reduced, but thereafter, as the summer 

 heat increases, a gradually increasing amount of water 

 is required. The possible water supply has sufficed for 

 the cotton crop, but in addition to that there is a demand 

 for rice which is sown during May. 



In a normal year, the area in Lower Egypt under 

 cotton is 1,600,000 feddans, and under rice 230,000 

 feddans. Rice requires about three times as much water 

 as cotton, about 70 cubic metres per acre per day, against 

 22 cubic metres, so that with rice the Delta requires 

 48 million cubic metres a day, whilst without it 32 mil- 

 lions a day suffice. 



To provide the rice area with water a further 200 cubic 

 metres per second are required, which would have necessi- 

 tated the Aswan discharge curve taking the form shown 



