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Despite the great area of the Middle East, there are only three rivers, the Nile, the Euphrates, and the Tigris, that can be classified as large rivers by world standards. Of these, the Nile, which is the world's longest river, receives most of its discharge from precipitation falling well outside the Middle East on the upland plateau of East Africa and the highlands of Ethiopia (fig. 2.20).
The Nile is the whole life of Egypt. The country owes its existence to the river, which provides water for agriculture, industry, and domestic use. Cultivation is dependent on irrigation from the river.
Fig. 2.19B Salinity-control land-reclamation system for the lower Indus valley (Source: Buras 1967)
Fig. 2.20 The Nile River basin
The proposal to build a single large dam at Aswan for multiple objectives including flood control, year-to-year water storage, and hydro-power generation was put forward by Adrein Danionson in 1949 as an alternative to a "century storage" scheme. Construction of the high dam started in 1960 and was completed in 1970. Before the dam was built and went into operation, the Nile floods brought silt containing potassium and phosphorous but also could leach away any accumulated salts. The fine-grained alluvial soils of the Nile valley do not drain easily and need artificial drainage. Due to the hot, arid climate, irrigation water evaporates quickly, leaving behind its salt, causing salinization.
The water levels in the Nile have been falling for nine years since the early 1980s. In 1985-86 there was a three-metre drop in the level of Lake Nasser, the reservoir behind the Aswan dam, and in 1986-87 it fell from 195.6 m to 184.7 m (WPDC 1988). Egypt has been attempting to avert a national crisis by three strategies: rationalization, river development, and groundwater development. The reservoir storage has been recovered by steps with intensive rainfall and inflow in the early 1990s. This long-term fluctuation depends on the large-scale basin hydrology in the humid through the arid zones.
2.4.1 The river basin
The Nile is 6,690 km long, extending through 35 degrees of latitude as it flows from south to north. Its basin covers approximately one-tenth of the African continent, with a catchment area of 3,007,000 km², which is shared by eight countries: Egypt, Sudan, Ethiopia, Uganda, Kenya, Tanzania, Rwanda, and Zaire. Its main sources are found in Ethiopia and the countries around Lake Victoria.
All along the Nile's course from its most remote source, the Cagier River in Central Africa, to the Mediterranean, people are affected to some extent by the river or its water. With a few exceptions, the water resources in the headwater areas of the system are not yet much developed. The main development has taken place in the countries situated in the semi-arid and arid zones such as Sudan and Egypt. The upstream countries, however, are now considering Nile resourcedevelopment projects in their territories.
The hydrographic and hydrological characteristics vary greatly over the basin. Rainfall in the headwater areas is abundant though seasonal. On the other hand, from about Sudan the river runs through arid country.
The river system has two main sources of water: the Ethiopian highlands and the equatorial region around Lake Victoria. More than 60% of the river flow arriving in Egypt originates in the Ethiopian highlands by way of the Sobat, Blue Nile, and Atbara Rivers, with the bulk of this water coming down during the summer. The remainder of the flow arrives by way of the White Nile, which has its most remote source in Burundi. This source is a tributary of the Kagera, which enters Lake Victoria near the border between Uganda and Tanzania. In the equatorial region, the Nile system consists of a number of great lakes, connected either by rocky sections or swamps. The White Nile, called Bahr el-Jebel after leaving the lake area, enters Sudan through rocky gorges and then flows through a large swamp area (the Sudd region) in southern Sudan, where it is joined by the Sobat from the east and the Bahr el-Ghazal, which occasionally receives water from Lake Chad from the west. In the Sudd region a huge quantity of water evaporates or is transpired from aquatic vegetation. Only a small part of the Bahr el-Ghazal flow ever reaches the Bahr el-Jebel, and practically all of its water disappears in the swamps. Although the contribution of the White Nile to the total annual flow at Aswan is only 30%, it is most important because of its timing: during the dry season from February to June its flow is large compared with that of the Blue Nile.
The Nile flows for half of its course through country with no effective rainfall. The rainfall of the Nile basin indeed is scanty compared with other major rivers in Africa such as the Zaire, Niger, and Orange Rivers. For the size of the Nile basin with its catchment area of 3,007,000 km², the annual discharge is as small as 99.5 x 109 m³, which is equivalent to 4.3% of the annual runoff. Rainfall is heavy in the headwater areas. The annual average rainfall in the lake plateau basin is about 750 mm (50 inches). The heaviest rainfall occurs at Kalungala on the island of Bugola, where it averages 2,250 mm (90 inches) per year. Other places of high rainfall are Bukoba on Lake Victoria and Gore in Ethiopia, where the average is about 2,000 mm (80 inches) per year. The distribution of rainfall is shown on the map in fig. 2.20.
The dependence of Egypt on the Nile has led to intensive studies of quantities of water carried by the main stream and its tributaries throughout the year. The long-term record of the annual flow discharge at Aswan from 1871 to 1965 is shown in fig. 2.21. The annual average for the 95 years is estimated to be 91.2 x 109 m³, which was the basis of design for the year-toyear storage at the Aswan high dam. The change in the patterns of flow of the main Nile at Aswan and Rosetta before and after the construction of the Aswan dam are indicated in fig. 2.22. The upper graph in fig. 2.23 shows the average discharge of the main Nile at Aswan unaffected by reservoirs, the top line showing the discharge of the main river and other lines the component discharges. The peak discharge of 712 million m³ per day, recorded on 8 September, was made up as follows:
Fig. 2.21 Mean annual flow discharge at Aswan, 1871-1965
Fig. 2.22 Mean annual flow discharge at Aswan and Rosetta, before and after consruction of the Aswan high dam (Source: Beaumont et al. 1988)
Fig. 2.23 Flow diagram of the main Nile at Aswan and Khartoum (Source: Hurst 1952)
The minimum discharge of about 45 million m³ per day, on 10 May, was made up as follows:
During the high-flow stage from July to January, the largest proportion of the flows is contributed by the Blue Nile, and the least by the Atbara, while the White Nile is the more important source of supply during the low-flow stage from February to June. The Atbara carries much less water than the Blue Nile, but the effect of a peak may be considerable, owing to the shorter journey with very little damping. The Atbara contributes nothing from January to June.
The average discharge of the main Nile at Khartoum and the portions contributed by the Blue and White Niles are shown in the middle graph in fig. 2.23. On average 84% of the water of the main Nile comes from Ethiopia and 16% from the lake plateau of Central Africa. An interesting point is seen at the confluence of the Blue and White Niles: when the Blue Nile is rising rapidly, the White Nile discharge is ponded up and reduced, and only when the rise slows down does the White Nile discharge begin to increase. To make this clear a dotted line is interpolated showing the White Nile contribution as it would be if it were unaffected by the Blue Nile. When the Blue Nile falls, the White Nile discharge is increased by the water which has been ponded back. The effect of the Blue Nile is therefore to make a natural reservoir of the White Nile, and this effect is increased artificially on a great scale by the Gebel Aulia dam, which is situated about 45 km from Khartoum, some little distance up the White Nile.
About 800 km upstream from Khartoum, the White Nile is joined by the Sobat, 90% of whose water comes from Ethiopia. About half the discharge of the White Nile comes from the Sobat and the other half from the Bahr elJebel through the swamps of the Sudd region. The average discharge of the White Nile is about 28 x 109 m³ per year, of which the Sobat produces 13.5 x 109 m³, and the rest, except for a negligible amount contributed by the Bahr el-Ghazal, comes from the Bahr el-Jebel or its branch, the Bahr el-Zeraf. This remainder is shown as discharge from the swamps in the third graph in fig. 2.23.
Because of the regulating effect of the large swamps of the Sudd region on the Bahr el-Jebel, the discharge of the latter varies very little throughout the year (see the bottom graph in fig. 2.23). When a rise occurs upstream, most of the water flows out of the river into the swamps, and only a very small part of the increase is felt in the stream below. When the river falls, there is a tendency for the marshes to drain back into the river, but large areas are below river level and cannot drain back. The water that enters is lost by evaporation and by transpiration from the luxuriant vegetation, and in fact very little returns to the river, with the result that the Bahr el-Jebel loses nearly half its water in the swamps.
The heavy evaporation and general dryness of the climate in Egypt and most of Sudan have various implications, including a substantial evaporation loss from the surface of large reservoirs. The evaporation loss from Lake Nasser (the Aswan high dam reservoir) is estimated to be about 10 x 109 m³ per year, which is about 10% of the net storage volume of 90 x 109 m³. In Egypt evaporation is at a maximum in June and a minimum in December and January. In northern Sudan it is the same, but May has practically the same evaporation as June. In the vicinity of Khartoum the maximum is in April and May, owing to the reduction later caused by monsoon rains, of which this area is on the fringe. In southern Sudan the minimum evaporation is in the months of July and August, at the height of the rains. The evaporation in Egypt and Sudan, on the whole, follows the temperature. Potential evaporation at selected points in the Nile basin is shown in table 2.2 and the estimated monthly evaporation from Lake Nasser in fig. 2.24.
Table 2.2 Mean daily potential evaporation at selected points in the Nile basin
|Potential evaporation (mm/day)|
|Cairo and neighbourhood||5.5||2.8|
|Northern Sudan (Halfa Atbara)||15.1||7.6|
|Khartoum and neighbourhood||15.5||7.8|
|Central Sudan (Dueim to Roseires)||12 6||6.3|
|Southern Sudan (Malakal and south swamp)||6.8||3.4|
Source: Hurst 1952.
Fig. 2.24 Monthly evaporation rates for Lake Nasser (Source: Hurst 1952)
2.4.3 Water-resource development of the Nile system
The national-economy and social objectives of developing Nile resources may vary from country to country. Certain Nile projects in the upper parts of the basin could be also advantageous for the more downstream countries. The timing of such projects could have a significant effect on the development of the resources of the basin as a whole
The Nile is a geographical unit, and projects for its full development must also form a unity, the parts of which must work together. The basic idea of a 1950s scheme, an account of which follows, was year-to-year storage, or "century storage" in Hurst's (1952) terminology. The key projects in Hurst's master plan were the Owen Falls dam, Lake Kioga barrage, Lake Albert dam, Jonglei diversion canal, Lake Tana dam, Fourth Cataract dam, Aswan high dam, and Wadi Rayan reservoir (see table 2.3). The hydraulic works in the Nile basin are shown on the map in fig. 2.25.
Table 2.3 Existing dams the Nile basin
|Dam||Location||Storage capacity (109 m³)||Installed capacity (MW)||Evaporation loss (109 m³)|
|Owen Falls||Uganda||Victoria Nile||-a||120|
|Jebel Aulia||Sudan||Bahr el-Jebel||3.6||2.8|
|Tis Abbay||Ethiopia||Blue Nile||-a||9.6|
|High Aswan||Egypt||main Nile||90b||1,815||10|
|Nag Hammadi||Egypt||main Nile||-c|
Source: Deekker 1972.
a. Outflow regulation with lake
storage, or run of the river.
c. Barrage for irrigation intake.
A major step in achieving collaboration among the Nile basin states was initiated in 1967 when the five countries Kenya, Tanzania, Uganda, Sudan, and Egypt started a hydro-meteorological survey of the basins of Lakes Victoria, Kyoga, and Albert, with the assistance of the UN Development Programme.
Fig. 2.25 Hydraulic works of the Nile basin (Source: Beaumont et al. 1988)
2.4.4 Jonglei diversion canal project
The Jonglei canal scheme to divert water from the Bahr el-Jebel above the Sudd region to a point farther down the White Nile, bypassing the swamps (see fig. 2.25), first studied by the government of Sudan in 1946, would make significantly more water available for use downstream. Plans were developed in 1954-59, and work on the project began in the 1970s, but it has been held up for many years by political instability in Sudan (Collins 1988).
The original plans could now be reviewed, taking into account the completion in 1970 of the Aswan high dam, with its gross storage capacity of 168.9 x 109 m³ and maximum spillway capacity of 11,000 m³/sec, and our present increased knowledge of the hydrology of the Blue Nile and the hydrometeorology of the Albert-Victoria catchment. The water from the southwestern tributaries (the Bahr el-Ghazal system) for all practical purposes does not reach the main river and is lost through evaporation and transpiration in the swamps. It should be possible to reduce these losses and to lead at least a part of the water to the main river. This procedure would require international collaboration, because storage and regulation would become necessary in Lake Albert to reduce flood levels in Sudan. Furtherrnore, storage would probably be needed on the Blue Nile in Ethiopia because the reservoir of the Aswan high dam by itself would not be large enough to regulate the combined floods of the Bahr el Ghazal, Bahr el-Jebel, and Blue Nile. It is estimated that the Jonglei canal project would produce 4.8 x 109 m³ of water per year, including 2.4 x 109 m³ for the first-stage project and 2.4 x 109 m³ for the second-stage project. There are, however, complex environmental and social issues involved, which may limit the scope of the project in practical terms.
2.4.5 Hydro-power potential
The hydro-potential of the Nile system is enormous, but the energy demand in the Nile basin countries is still small at present, with the exception of Egypt. According to the Ministry of Information of Ethiopia in 1966, the hydropotential of the whole Nile system was preliminarily estimated to be about 8,000 MW (Deekker 1972). The most promising river for hydroelectric power development is the Victoria Nile, of which the hydro-potential is preliminarily estimated to be 1,843 MW in total, including six potential stations such as Bujagali (180 MW), Busowoko (150 MW), Kalagala (125 MW), Kamdinia (234 MW), Aingo (490 MW), and Murchison (664 MW) (Deekker 1972).
2.4.6 The Aswan high dam
The importance of energy production at the Aswan high dam to the Egyptian economy is perhaps about equal to that of making more water available for irrigation, taking into account the huge saving in crude oil for energy production.
The construction of the Aswan high dam has affected the entire economy of Egypt, allowing reliable irrigation throughout the year and satisfying about 40% or less of the country's energy demands. The flow of the Nile River below the Aswan high dam is fully regulated. Releases from the dam are authorized by the Ministry of Irrigation and are based on seasonal irrigation needs in the Nile delta. The nature of Egypt's climate, as well as the established cropping patterns, require a very high release in the summer months when agricultural production is at its peak. Monthly irrigation release requirements are shown in table 2.4. Hydro-power generation at the dam has always been viewed as a residual benefit since the potential varies extensively between summer and winter months because of the uneven distribution of downstream irrigation requirements.
Egypt's irrigation practices require nearly 55 x 109 m³ of water from the Nile every year, which is the amount allotted to Egypt by the 1959 Nile Waters Agreement with Sudan. Sudan was allotted 18.5 x 109 m³ by the same agreement but has been using only 16.5 x 109 m³ per year in the 1970s. Monthly flows at various points along the Nile River have been recorded since the late 1800s. From 1871 to 1976 the flow records at Station Aswan were adjusted to account for Sudanese abstractions and evaporation losses.
The water stored in the upper rule storage zone is considered to be live storage and can be released to meet irrigation demands or to prevent flooding. Reservoir storage is discretized into 18 states, varying from a maximum of 168.9 x 109 m³ (183-m elevation) to 89.2 x 109 m³ (168-m elevation).
According to an inventory of the world's hydro-power in 1989, the power station at Aswan has a rated capacity of 1,815 MW with an installed capacity of 2,100 MW, which accounts for about 40% of the national power supply (Mermel 1989).
Table 2.4 Monthly irrigation release requirements (109 m³) from the Aswan high dam
Source: Thomoson and Marks 1982
2.4.7 Waterlogging and salinization problems of the Nile delta
Before the implementation of year-to-year storage at the Aswan high dam, the Nile floods brought silt to the fields of Egypt containing potassium and phosphorous and could also leach away accumulated salts.
The fine-grained alluvial soils of the Nile valley do not drain easily and need artificial drainage. Because of the hot-arid climate, irrigation water evaporates quickly, leaving behind salt which causes primary salinization. Consequently, farmers had to apply more water to wash the accumulated salts into the ground below the root zone. Deep percolation thus caused a rise in the water table to a few decimetres below the surface level soon after the change to perennial irrigation. The soil then became waterlogged. When the water table is less than two metres deep, capillary forces lift it to the surface, where the salts accumulate after evaporation. This is known as secondary salinization. Thus, to avoid primary salinization it is essential to ensure quick infiltration of irrigated water, and to avoid secondary salinization the water table must be kept low.
In 1982 almost all the irrigated area in Egypt was potentially affected by salt, and at least half of the area (12,000 km²) is already more or less affected. Egypt's agriculture potential and Nile delta salinity are mapped in figs. 2.26 and 2.27. About 400 km² are provided with drainage systems each year at a cost of US$200 per hectare. Nevertheless, this is not sufficient to stop salinization. Farmers are unwilling to make this investment, and the government authorities have difficulty in keeping open the drainage channels that are essential for proper functioning of the tile drainage underneath the farmlands (Meybeck et al. 1989; Beaumont et al. 1988).
Regulating the flow of the Nile at the Aswan high dam has immensely increased the costs of agriculture for the irrigated lands on the Nile delta by requiring artificial fertilization, drainage systems, and water lifting. On the other hand, some costs have been reduced, such as those for clearing the irrigation channels of silt.
2.4.8 Egypt's water crisis and the Aswan high dam
Until high flows on the Blue Nile in 1990 saved the situation, Egypt was facing a national crisis as a result of nine years of falling water levels in the. Nile. and Lake Nasser Countermeasures taken to avert the water crisis comprised the following three strategies (WPDC 1988):
>> rationalization - improving irrigation systems to save 86 million m³ of water per year, and recycling agricultural drainage to recover 196 million m³ of water per year;
>> river development - building a major new dam on the Nile at Rashid, near Alexandria, to reduce the Nile's flow into the Mediterranean;
Fig. 2.26 Egypt's agricultural potential. Irrigation schemes: (1) Tahrir, (2) Maryut, (3) Nubariya desert, (4) northern delta, (5) south of Lake Manzala, (6) western Sinai, (7) Faiyum depression, (8) El-Minya, (9) Kena, (10) Radesia Wadi Abbady, (11) Kom Ombo. (Source: Beaumont et al. 1988)
Fig. 2.27 Nile delta salinity (map area indicated by box in fig. 2.26) (Source: Ab-Zeid and Biswas 1990)
>> groundwater development - exploiting underground reservoirs to develop nonrenewable groundwater in the deep sandstone aquifers.
Details of the groundwater development plan for the deep sandstone aquifers in the New Valley are given in section 2.7.4.
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