The system of tributaries under monsoonal influence
The main catchment area of this system is the Ethiopian Highlands with
their high rainfall in summer. The Sobat, which receives three-quarters of its
water from the south-western Ethiopian Highlands and only one-quarter from
the south, discharges 13.3 billion m3
 of water into the White Nile annually,
the Blue Nile about 52 billion m3
, and the Atbara 12 billion m3
. Before the
construction of the Sadd el-Ali the annual mean run-off of the Nile at Aswan
was 84 billion m3
. Calculations of the annual discharge of the Nile into the
Sadd el-Ali storage lake today are based on this amount.