372 THE FERTILITY OF SOILS IN THE TROPICS 



to more than twenty times that discharge early in Sep- 

 tember, after which it begins to fall. 



This water, which passes Wadi Haifa, has fallen as 

 rain some six weeks or two months previously on the 

 Lake Plateau, draining into the Nyanzas Victoria and 

 Albert and on the mountains of Abyssinia. The map 

 shows these areas and the main feeders of the Nile, the 

 Bahr el Jebel supplied from the Lake Plateau, and the 

 Sobat, Blue Nile, and Atbara draining the Abyssinian 

 hills. The role of these two rainfall areas must be care- 

 fully distinguished. In flood, from July to the end of 

 the year the Blue Nile and Atbara are the only water 

 supplies which count, the Sobat assists supplies after the 

 top of the flood has been passed, whilst the water which 

 comes from the White Nile is of supreme importance in 

 the low stage when the flow of the other rivers in many 

 years almost or entirely ceases. 



Taking the rivers in order from south to north the 

 Bahr el Jebel draws its constant supply from the great 

 natural reservoir Lake Albert, augmented during flood by 

 numerous mountain torrents. The river then enters the 

 famous region of the sudd, which it leaves with a volume 

 which is very nearly constant throughout the year owing 

 to causes into which I need not here enter. 



Downstream of the sudd region the river is joined 

 by the Sobat, a river of considerable proportions during 

 flood, but almost dry at low stage. The lower dotted 

 line in Diagram II shows the volume of water passing 

 Malakal after the junction of the Sobat and Bahr el 

 Jebel. This water passes on to the junction with the 

 Blue Nile at Khartoum, the discharge at Khartoum being 

 somewhat greater during the falling stages owing to 

 gains by drainage from the banks. At the time of flood, 

 on the other hand, the great rise in the Blue Nile holds 

 back the waters of the White Nile with the result that 

 the actual amount of water joining the Nile from the 

 latter river is very greatly reduced and in some years 

 there is probably a net upflow just before the top of the 

 flood with a correspondingly increased downflow when 

 the flood has begun to fall. 



The Blue Nile receives its water from the rainfall over 



