RECENT FLUCTUATIONS IN PRECIPITATION AND RUNOFF OVER THE NILE SUB-BASINS AND THEIR IMPACT ON MAIN NILE DISCHARGE

Substantial fluctuations in precipitation and runoff have occurred ove r the Nile Basin in recent decades. Ten-year mean flows of the Blue Ni le (Khartoum gauge) during the 20th century have ranged from 42.2 to 5 6.7 km(3) and for the White Nile (Malakal gauge) from 25.5 to 36.9 km( 3). These fluctuations have been responsible for changes in decade-mea n Main Nile discharge of up to +/-20% which have had important consequ ences for water resource management in both Egypt and Sudan. This pape r provides a review of the Nile Basin hydrology incorporating, for the first time, analyses of the relationships between precipitation and r unoff fluctuations in the instrumental period for each of the eight ma jor sub-basins within the Nile Basin. These sub-basins possess very di fferent physical, climatic and hydrological characteristics. Over 90% of the Main Nile discharge originates from only four of the sub-basins ; Lake Victoria, Blue Nile, Atbara, and the Sobat. Interbasin correlat ions of 40 y (1945-84) precipitation and runoff annual time series ide ntify two broadly homogeneous regions; the Ethiopian highlands (Blue N ile and Atbara) and the Lake Victoria and Equatorial Lakes (White Nile ). These regions posses contrasting precipitation regimes whose intera nnual variations are uncorrelated in time and which are therefore asso ciated with different atmospheric circulation anomalies. The observed relationships between catchment precipitation and runoff, however, are not straightforward and the sensitivity of runoff to precipitation fl uctuations varies from basin to basin. Some of the water resource mana gement implications of these fluctuations for Egypt are discussed. Wit h water demand in Egypt alone set to increase 17% by the year 2000, it is critical that the role of future climate change in Nile water mana gement is thoroughly assessed based on a correct modelling of the dive rse hydrological characteristics of the various Nile sub-basins.