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The Nile Basin

The Nile River, with an estimated length of over 6800 km, is the longest river flowing from south to north over 35 degrees of latitude. It is fed by two main river systems: the White Nile, with its sources on the Equatorial Lake Plateau (Burundi, Rwanda, Tanzania, Kenya, Zaire and Uganda), and the Blue Nile, with its sources in the Ethiopian highlands. The sources are located in humid regions, with an average rainfall of over 1000 mm per year. The arid region starts in Sudan, the largest country of Africa, which can be divided into three rainfall zones: the extreme south of the country where rainfall ranges from 1200 to 1500 mm per year; the fertile clay-plains where 400 to 800 mm of rain falls annually; and the desert northern third of the country where rainfall averages only 20 mm per year. Further north, in Egypt, precipitation falls to less than 20 mm per year.

The total area of the Nile basin represents 10.3% of the area of the continent and spreads over ten countries (Map 4 and Table 20).

For some countries, like Zaire, the Nile basin forms only a very small part of their territory. Other countries, like Burundi, Rwanda, Uganda, Sudan and Egypt, are almost completely integrated into the Nile basin.' However, all the waters in Burundi and Rwanda and more than half the waters in Uganda are produced internally, while most of the water resources of Sudan and Egypt originate outside their borders: 77% of Sudan's and more than 97% of Egypt's water resources as shown in Table 6. Moreover, these latter two countries already use nearly all of the water currently allocated to them, as shown below.

Table 20: Nile basin: areas and rainfall by country

Rivers and discharges

The most distant source from the sea is the Luvinzora River in Burundi, a tributary of the Kagera River. The Kagera River forms the border between Rwanda and Tanzania, then between Uganda and Tanzania and then flows into Lake Victora, the second-largest freshwater lake in the world with an area of about 67000 km². Total flow into the lake is about 20 km³/year, of which 7.5 km³ from the Kagera River, 8.4 km³/year from the forest slopes in the north-east (Kenya), 3.2 km³/year from the drier Serengeti Plains in the south-east (Tanzania) and from 1 to 2 km³/year from the swamps in the north-west (Uganda).

The level of Lake Victoria is extremely sensitive to moderate changes in rainfall over the lake and its tributaries. Average lake rainfall and evaporation are the main factors affecting the lake balance and are more or less equal. As evaporation varies little from year to year, high rainfall gives rise to a disproportionate surplus and also greatly increases the tributary flows which are themselves relatively more variable than the rainfall. The rise in lake level during 1961-64 of about 2 metres seems to be the result of a higher rainfall during that period over the lake and its basin. This surplus then influences the outflow which declines only gradually over a longer period of years [41].

The only outlet of Lake Victoria is at Ripon Falls (Owen Falls Dam) in Uganda. Then begins the Victoria Nile which flows through Lake Kyoga into Lake Albert, also called Lake Mobutu Sesse Seko. This lake also receives water from the Semliki River, which originates in the Mufumbiru mountains in Zaire and flows through Lake Edward to Lake Albert. The combined waters of the Semliki and the Victoria Nile leave Lake Albert at the northern end and become the Albert Nile, which then flows into Sudan.

Uganda is a humid country with numerous lakes and wetlands and with internal renewable water resources globally estimated at 39 km³/year. However, the total annual flow into the country (at Ripon Falls and from Zaire) is about equal to the total annual outflow to Sudan, which means that a lot of water disappears within the country through evaporation and evapotranspiration from the lakes and wetland.

Entering Sudan, the Albert Nile becomes the Bahr el Jebel. It flows into the Sudd region, the great wetlands which are a maze of channels, lakes and swamps in southern Sudan, and which also receive water from the Bahr el Gazal River, originating in south-west Sudan.

The most remarkable topographic feature of the Sudd area is its flatness: for 400 km, from south to north, the slope is a mere 0.01 % and much of it is even flatter. The soils of the whole area are generally clayish and poor in nutrients. Rain falls in a single season, lasting from April to November and varying in the Sudd area from about 900 mm in the south to 800 mm in the north. As the rainy season coincides with, though is slightly shorter than, the flood seasons of the rivers, there is land of water and mud for half of the year and, away from the rivers, land of desert-like dryness for the other half. The main natural channels flow through a swamp area waterlogged throughout the year, and are then flanked by grasslands flooded at high river and exposed when the river level drops. Because of the important rainfall in the Equatorial Lake Plateau during the 1960s and 1970s the permanent swamp area increased from 2700 km² in 1952 to 16200 km in 1980 [42].

Less than half of the water entering the Sudd region flows out of it into the White Nile. The rest disappears through evaporation and evapotranspiration. The quantity entering the Sudd region varies greatly over the years, mainly depending on the rainfall in the upper catchment area, and hydrological measurements have shown that the greater the flow of water into the Sudd, the greater the percentage of water 'lost' in evaporation (Table 21 [42]).

Table 21: Average annual discharges at different locations in the Sudd region

In order to bypass the Sudd region and to direct downstream a proportion of the water considered lost each year by spill from the river and evaporation in the swamps, the construction of the Jonglei Canal had been planned. This water could then have become available for irrigation and other uses downstream in Sudan and Egypt. Construction of the canal began in 1978 for a planned total length of 360 km, but work stopped in November 1983 after 240 km because of the civil war. By that time it had also become clear that these 'losses' create resources in pasture and fisheries and that the canal causes enormous human and environmental problems in the area. The issue is now how much water can be drained from the Sudd through the construction of the Jonglei Canal without serious and irreparable damage to the local environment and economy and its potential expansion [195].

The Sobat River, that flows into the White Nile just upstream of Malakal, is fed by the Baro and Akobo Rivers and others with catchment areas situated mainly in the southern Ethiopian foothills.

The Blue Nile and its main tributaries, the Dinder and the Rahad, rise in the Ethiopian mountains and around Lake Tana. The confluence of the White Nile and the Blue Nile is at Khartoum. Further downstream is the Atbara tributary, the last important tributary of the Nile system, again deriving from the Ethiopian plateau north-east of Lake Tana and forming the border between Ethiopia and Eritrea before entering Sudan. There are no important tributaries further downstream in Egypt.

The contribution of the rivers of the Ethiopian catchment area (Blue Nile system) to the Nile is about twice the contribution of the rivers of the Equatorial Lake Plateau catchment area (White Nile system), but it is characterized by the extreme range in discharges between the peak and low periods, while the flow from the Equatorial Lake Plateau is more uniform. At its peak the former provides nearly 90% of all water reaching Egypt, the latter only 5%. During the months with low flow the contributions are nearer 30% and 70% respectively [29].

Figure 13: Average discharges of the Blue Nile and the White Nile

As already mentioned, variations in rainfall over the years can cause quite considerable variations in discharges and lake levels. This seems to be more explicitly the case for the White Nile River system. For this reason, average discharge figures might vary greatly depending on the period under consideration, as shown in Table 22 [29, 210, 44].

Table 22: Variations in discharges at different locations on the Nile

In addition to variations due to rainfall, the discharges might vary also due to water abstractions, mainly for irrigation purposes.

Irrigation potential and water requirements

Both Burundi and Rwanda are characterized by a rolling topography with a continuous pattern of hills and valleys, with lakes and marshy lowlands at the bottom of the valleys. Improving the drainage network in part of the swamp areas, combined where possible with an irrigation network, would allow year-round cultivation, which is important for these small, but very densely populated countries. The total area of these valley bottoms in the Nile basin is estimated at 105000 ha for Burundi [78] and 150000 ha for Rwanda [176].

For Tanzania the irrigation potential has been estimated at 30000 ha, but this would require the construction of considerable water conveyance works [199]. In addition to this, at the beginning of the century settlers from Germany, the then colonial power in the country, proposed a plan to transfer water from Lake Victoria to the Vembere Plateau in the Manonga River basin in central Tanzania to irrigate between 88000 and 230000 ha of cotton. Though this project is still on the table, it would be very expensive. The transfer would be effected by gravity as the plateau lies below the water level of the lake [199].

The Lake Victoria basin in Kenya covers only 8.5% of the total area of the country but it contains over 50 % of the national freshwater resources. The national water master plan identified an irrigation potential of 180000 ha based on 80% dependable flow [125]. As part of the plan, dams and water transfers to other (sub) basins are proposed. At present only about 6000 ha are irrigated. Moreover, in Kenya there has been lengthy debate as to whether, given adequate technology, Lake Victoria basin water should be transferred to arid areas of the country for irrigation. It is considered that perhaps the most appropriate location for such an experiment would be the Kerio Valley (located in the Rift Valley, see section The Rift Valley), for which a special development authority has been established by the Kenyan Parliament. The feasibility of such a project is a question of engineering and several observers consider it possible. Such an undertaking would use significant quantities of water. Projects of this kind are analogous to the irrigation of the Vembere steppe proposed in Tanzania (see above).

The Nile basin in Zaire covers less than 1 % of the area of the country. The area is hilly and does not really lend itself to irrigation. This area is rather densely populated with most people engaged in cattle rearing and fishery activities around Lake Albert [46]. It is considered that about 10000 ha could be developed for irrigation [*].

Uganda has large swamp areas covering about 700000 ha. The irrigation potential is estimated at 202000 ha, requiring, however, major works such as storage, river regulation and large-scale drainage [209]. At present only 5550 ha are irrigated.

The irrigation potential in the Nile basin in Ethiopia has been estimated at more than 2.2 million hectares [106]. The irrigated area was about 23000 ha in 1989.

Table 23: Water resources, irrigation potential and areas under irrigation in the different Nile sub-basins in Ethiopia

The seasonality of the flows in Ethiopia is very high, as shown in Figure 14. This means that very considerable regulation would be necessary for their full utilization. The risk of rapid siltation of the reservoirs because of the steep slopes is a real problem. Construction of dams would augment the quantity of water available, because of a loss of only 3 % by evaporation as against a loss of almost 16% in the Aswan reservoir. Egypt, however, would no longer be the beneficiary of additional water in years of high flood, which would then be stored and regulated in the Blue Nile reservoirs instead of Aswan.

The irrigation potential in the Nile basin in Eritrea has been estimated at between 60000 and almost 300000 ha, though these figures are based on very limited studies [100]. Most of it would be in the Tekeze-Setit basin, which Eritrea shares with Ethiopia. The Mereb-Gash basin has mainly spate flows and its water reaches the Atbara River in Sudan only during extremely high floods. In this review the average irrigation potential has been estimated at 150000 ha [*].

Irrigation potential in Sudan has been estimated at over 4.8 million hectares [193], but this figure does not take into consideration the available water resources. The irrigated area was about 1.6 million hectares in 1979 [195] and 1.9 million hectares in 1990 [196]. There are plans to increase irrigation to about 2.8 million hectares by the year 2000, almost all to be irrigated by Nile water [195].

The figures in Table 24 for irrigated area in 1979 and 1990 correspond to the area equipped for irrigation. The actual irrigated area in 1990 was about 1.2 million hectares, or about 63 % of the total equipped area of 1.9 million hectares. About 16.8 km³ of water was used, corresponding to 14000 m³/ha [196]. Despite this relatively high value, water management is a problem, for example water supply on the old established cotton schemes of Gezira-Managil was and is about 12% below crop requirements at crucial points in the growth cycle. At the same time, as much as 30% of the water delivered is not used by crops. In large state-run irrigation projects, like Gezira-Managil and Khashm al Girba, average water deliveries to the command area are between 9700 and 12600 m³ per cultivated hectare per year. Sugar cane, a very water-consuming crop, uses between 28000 and 40000 m³ per ha per year [195].

Table 24: Irrigated land use in Sudan [195, 196]

Considering an availability of 25 km³ of water for irrigation in 2015 (see Table 25) and a water requirement of 14000 m³/ha, only about 1.8 million hectares could be irrigated as opposed to the proposed 2.8 million hectares.

The water balance of Sudan at present, and as proposed over the next 20 years can be summarized as follows [196]:

Table 25: Estimated water balance of Sudan in 1995 and 2015 [196]

(1) Under the Nile Water Agreement between Sudan and Egypt, the quantity of water allocated to Sudan is 18.5 km³/year at Aswan, which corresponds to 20.55 km³ further upstream.

(2) The total amount of water becoming available through the construction of the Jonglei Canal is estimated at 8 km³ in 2015, of which 50% for Sudan and 50% for Egypt under the agreement between the two countries. Egypt considers 2 km³ to be available by the year 2000 as shown in its water balance in Table 27. Work on the canal is currently stopped as explained at the beginning of this section.

In Egypt the agricultural land use in 1990, almost all irrigated, was as follows [95]:

Table 26: Agricultural land use in Egypt [95]

It should be noted that each time new land is reclaimed it is of a lower quality than the already cultivated land. The best soils in Egypt cover an area of only about 1 pillion ha [20], while the best plus suitable soils cover an area of about 3.6 million ha. Adding the still more marginal land, the maximum area for agriculture could be 4.8 million ha [20]. The remaining soils are unsuitable for agriculture.

Taking into consideration water saving and possibilities of re-use, the water balance of the Nile basin in Egypt in 1993 and 2000 is presented in Table 27.

Taking an average water requirement of 13000 m³/ha per year in the Nile Valley and Delta in this study, about 4420000 ha could be irrigated using the 57.4 km³/year of Nile water.

As can be seen from Table 28, the sum of the irrigation potential of the countries leads to a water deficit of over 26 km³/year, (column 7) without considering possibilities of reusing water as indicated by Egypt and Sudan in their water balance, but after deducting the water 'losses' in the Sudd region.

Table 27: Estimated water balance of Egypt in 1993 and 2000

This deficit corresponds to an area of almost 2.2 million hectares, considering an average water requirement in the region of 12000 m³/ha per year [*]. This leads to an irrigation potential for the basin as a whole of 8 million hectares instead of the nearly 10.2 million hectares.

However, even these 8 million hectares are still a very optimistic estimate and should be considered as a maximum value, requiring very important storage works and optimum water use.

Table 28: Nile basin: irrigation potential. water requirements, water availability and areas under irrigation

NOTES:

For the sake of simplicity it was supposed that if a certain quantity of water is abstracted upstream, this same quantity is subtracted from the resource downstream, except in cases where more information was available.

Tanzania:

(6) Equal to inflow (7.00) minus water requirement upstream countries (1.04 + 1.88).
(7) Equal to outflow (10.70) minus water requirement upstream and within Tanzania (1.04+1.88+0.33).

Uganda:

(6) Equal to inflow (28.70) minus water requirement upstream countries (1.04+1.88+0.33+1.53+0.10).
(7) Equal to outflow (37.00) minus water requirement upstream and within Uganda (1.04+ 1.88+0.33+ 1.53+0.10+ 1.62).

Sudan:

(1) Not included the possibility of irrigation within the Sudd area (area about 1600000 ha).
(4) Total inflow from Uganda and Ethiopia.
(5) Attribution to Egypt according to 1959 agreement after deduction evaporation Aswan.
16) Equal to inflow (117.1) minus water requirement equatorial plateau countries (6.50) and Ethiopia (19.98).
(7) Equal to outflow (90.63) minus losses in Sudd (21) and water requirement within Sudan (38.50).

Egypt:

(4) Attribution to Egypt according to 1959 agreement after deduction evaporation Aswan.
(6) Equal to outflow from Sudan after potential deductions (31.13) minus water requirements (57.46).

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