MONITORING THE NILE BASIN USING SATELLITE OBSERVATIONS

The Nile cuts across diverse climatic zones with its sources in the humid regions and encountering arid conditions as it flows downstream across the arid regions in the desert of Sudan and Egypt with changes in annual and seasonal rainfall over the ten major sub basins of the Nile basin region.
The NBI aims at promoting evidence-based decision making and has analyzed rainfall distribution, actual evapotranspiration distribution and changes in water levels in the large lakes during 2017 as compared to the long term average using satellite observation data. The results are presented in this second issue of quarterly basin monitoring bulletins.
Minimum rainfall is seen to be less than 50mm/year in the arid areas in the northern part of the basin and the maximum rainfall estimates are observed in the equatorial lakes region in the areas around Lake Victoria and the Ethiopian Highlands at over 1000mm/yr. The high altitude areas of the basin such as the Ethiopian highlands, Rwenzori Mountains in Uganda and Mount Elgon are considered to be the water towers in the basin receiving rainfall over 1500mm/yr.
In 2017, monomodal rainfall pattern was experienced during June - September (JJAS) in the Ethiopian plateau especially in the Blue Nile and Tekeze subbasins, April - October in Bahr el Ghazal and May - October in White Nile, Baro Akobo Sobat, and Bahrel Jebel subbasins.
Bimodal rainfall pattern was experienced during March - May (MAM) and September -November (SON) in the equatorial lakes region especially in the Lake Victoria, Lake Albert, and Victoria Nile sub-basins. The highest seasonal rainfall occurs over the Blue Nile and the lowest over Tekeze Atbara.
During the second wet season of 2017, Lake Victoria Subbasin exhibited more rainfall as compared to the long term average by 30% in Sep, 33% in Oct and by 23% in Nov. During the second wet season of 2017, Lake Albert subbasin exhibited more rainfall compared to long term average rainfall, especially during July ( by 35%), August ( by 29%), and September ( by 20 %).
During the second wet season of 2017 ,Victoria Nile subbasin exhibited more rainfall compared to long term average rainfall during August (37%) and September (23%). During the wet season of 2017 Bahr el Ghazal subbasin exhibited more rainfall from May to September (May 16%, Jun 29%, Jul 13%, Aug 9%, and Sep 22%) compared with the long term average rainfall. During the wet season of 2017, Bahr el Jebel subbasin exhibited more rainfall compared to long term average rainfall during August (37%) and September (35%). During the wet season of 2017, Baro Akobo Sobat subbasin exhibited more rainfall compared to long term average rainfall during September and October. Rainfall over White Nile subbasin during 2017 was more compared to long term average rainfall, especially during May (35%), June (25%), and September (14%). Rainfall over the Blue Nile subbasin during 2017 was above the long term average rainfall, especially during the rainy season in Jul ( by 9%), Aug ( by 7%), and Sep ( by 27%). Tekeze Atbara subbasin during 2017 exhibited more rainfall compared to long term average rainfall during May ( by 41%), July ( by 7%), and August ( by 8%). The long term average rainfall over Main Nile subbasin during July (-24%) was more than the rainfall of 2017 for the same month. However, August (14%) showed more wetting during 2017 compared to the long term average.
The rainfall anomaly during DJF 2017 exhibited dry condition over the equatorial region (Lake Victoria, Lake Albert, Victoria Nile subbasins). The rainfall anomaly during MAM 2017 experienced wet condition over the central parts, and dry condition over the boundary parts. The Blue Nile subbasin exhibited more rainfall during the low season MAM.
The rainfall anomaly during rainy season in the northern hemisphere in JJA 2017 exhibited much rainfall over Tekeze Atbara, Blue Nile, White Nile, and north part of Bahr el Ghazal subbasins. The dry season (JJA) over the equatorial subbasins recorded more rainfall.
The rainfall anomaly during SON 2017 experienced more rainfall over the eastern part of Baro Akobo Sobat, Victoria Nile, and Lake Victoria subbasins. Analysis of Actual evapotranspiration estimates shows variation in spatial and temporal distribution among the ten major subbasins. Lake Albert sub-basin seems to have the highest Evapotranspiration to Rainfall ratio (almost 90%) of all water generating sub-basins. AET/Rainfall ratios are highest in the drier areas of northern Sudan and Egypt reflecting the fact that any moisture available evaporates. In the wetter basins of the Nile Equatorial Lakes region, the ratios are about 0.5. In the vast plains of South Sudan and around Lake Albert, the ratios are close to 1.0
The NBI aims at providing a shared understanding of patterns of the water cycle components using satellite data observations so as to promote evidence based decision making.