THE FERTILITY OF SOILS IN THE TROPICS 379 



in the upper dotted line. It was obvious at an early date 

 that it was impossible to secure such a supply, and accord- 

 ingly the cultivation of rice was prohibited, with the 

 result that instead of being* starved the cotton crop has 

 received a proper measure of water. 



Looking at the question from the financial point of 

 view, a million cubic metres a day produces a cotton 

 crop worth a million pounds sterling, but a rice crop 

 which is only worth a tenth of that amount. This is a 

 somewhat crude way of looking at the question, but is 

 sufficiently reasonable to show that in time of dearth the 

 rice must be sacrificed. The minimum water require- 

 ments of the Lower Egypt cotton crop is about 32 million 

 cubic metres a day, and the share of water which it has 

 been possible to allot to that part of the country has been 

 33 million cubic metres, so that there has just been 

 sufficient water for proper irrigation. 



In a bad year, as has been indicated before, spring 

 rains in the Blue Nile area are of great value in alleviating 

 the situation. It has been established that, whilst the 

 rains which produce the flood come from the west, the 

 spring rains come from the east. In view of the great 

 importance of these rains, an investigation was carried 

 out during the past winter to see whether a statistical 

 connection could be found between the variations in the 

 intensity of these rains and variations in the meteorology 

 of places in and surrounding the Indian Ocean. 



It was found by analysis of the records for the fifteen 

 years for which they are available, that a surprisingly 

 good forecast of the rains could be made about six 

 weeks ahead. As a measure of the rains the rise in 

 level of the river at Roseires from March to May is 

 taken, and the 'full line in Diagram IV shows the actual 

 rise, whilst the dotted line indicates the rise computed 

 from the meteorology of the Indian Ocean in March. 

 It will >be seen that the agreement is on the whole very 

 good. On the receipt of telegraphic information at the 

 beginning of April, it was disappointing to find that the 

 meteorological indications were that the spring rains 

 would be very weak, the forecasted level at Roseires being 

 nearly a metre below normal. In the middle of April 



