Storage and retention of water in the various lakes and wetlands in the Nile basin are of particular importance as they regulate and dampen the flows which results in an important role for local rainfalls as well as evaporation resulting in large water losses. The River Nile basin has numerous lakes, and water bodies, including some of the biggest freshwater lakes and man-made reservoirs in the world. Although the total area of open water in the Nile basin is vast, about 90 000 km2, it represents less than 3% of the basin’s total area (NIS 2013). The major lakes of the basin are found in the equatorial region, apart from Lake Tana which is to be found in the Ethiopian highlands. A summary of the Key characteristics of the Lakes is shown in the table.
|Lake||Surface Area(km2)||Volume(km3)||Maximum depth(m)||Mean Depth(m)||Shoreline length(m)||catchment Area(km2)||Altitude(m)||Country location|
|Victoria||68800||2750||84||40||3440||184000||1134||Uganda, Kenya, Tanzania|
Lake Victoria, the largest lake in the Nile basin, is shared by Kenya, Uganda and the Tanzania; although Burundi and Rwanda are also part of its catchment area which covers 184 000 km2 (ILEC 1999). Annual average rainfall on the lake is 1 500 mm, which represents about 85 per cent of the water entering the lake; the balance comes from the rivers that drain the catchment. The annual evaporation rate from the lake surface is about 1 260 mm (Fahmy 2006). The main outlet for Lake Victoria is the White Nile at Jinja linking to Lake Kyoga.
Lake Albert lies along the shared border of Uganda and the DRC. It is about 160 km long and 30 km wide, with a maximum depth of 58 m and a surface elevation of 615 masl (ILEC 1999). Evaporation over the lake is estimated at 1 200 mm per annum and rainfall is 710 mm (Fahmy 2006). Lake George and Lake Edward Lake George has a surface area of 250 km2 and a catchment area of 9 705 km2. Lake Edward has a surface area of 2 325 km2 and its catchment’s basin area is 12 906 km2 (ILEC 1999). Lake George empties into Lake Edward via the Kazinga Channel. Queen Elizabeth National Park in Uganda extends from the eastern shores of Lake George and together with the adjacent Virunga National Park in the DRC completely surrounds Lake Edward. River Semliki receives flows from these two lakes and with runoff from its own catchment sends about 4 BCM of water to Lake Albert every year (Fahmy 2006).
Lake Tana, found in the Amhara region in the north-western Ethiopian highlands, is the largest freshwater lake in Ethiopia. It is sited in a wide depression and has a surface area ranging between 3 000 and 3 600 km² depending on the season. It is about 84 km in length and 66 km wide, with a maximum depth of 14 m and an elevation of 1 788 m (Wale 2008, ILEC 1999). Lake Tana is fed by four main rivers: the Gilgel Abay, Ribb, Gumara and Magech; and discharges at Bahir Dar through the Blue Nile. The four inflowing rivers contribute 93 per cent of the lake’s inflow (Anbah and Siccardi 1991). The average flow from Lake Tana was estimated at 3.8 BCM/year swelling to 54 BCM by the time it reaches Khartoum as a result of contributions from the Rivers Dinder and Rahed (Fahmy 2006).
Lake Tana WS Elevation
Inter-annual flow variability
Over the last decades, the Nile Equatorial Lakes water system has experienced important inter-annual variability with sudden changes (rise or drop of water levels and flows) which would persist for some years because of storage. Regulation of Lake Victoria’s outflow at Jinja, Uganda, has a clear effect on the lake’s water levels (Sutcliffe and Petersen 2007) and less direct impacts on many of the lake’s other ecosystem functions. These effects are also experienced by Tanzania and Kenya, who share the lake with Uganda, and to a lesser extent by all of the downstream countries in the basin.
Regulation of Lake Victoria
The quantity of water released from Lake Victoria through the Nalubale/Kiira power plants is constrained by an international
Observed water levels and discharge from the Lake Victoria from 1895 to 1998
treaty which stipulates that the outflow should simulate the natural flow of the Victoria Nile, based on a ten-day average flow, as a function of lake level. This so-called ‘agreed curve’ safeguards the environmental integrity of the lake and guarantees water supplies to downstream users. However, a more flexible interpretation of the agreed curve – e.g. annual releases that follow the annual agreed curve release volumes – might make it possible to increase power production without serious impacts on the lake or the river downstream
The Agreed Curve allows for a release of 693 m3/s at a gauge level of 1,134 m while the discharge would increase to 2,400 m3/s at a gauge level of 1,137 m. The installed generation capacity of the two plants at Jinja (387 MW) cannot be achieved until a level of 1,137 m is reached. Levels above 1,137 m should be avoided since this could damage the dam and power plants, while technical considerations related to cavitation restrict operations at the Kiira facility when the lake level falls below 1,134 m. The effective range of lake level fluctuation for power production is therefore around 3 m. Because greater volumes can be released and more power generated when lake levels are high it is important to keep the lake level as high as possible and avoid a drawdown to low levels .
Recent investigations have found dramatic and sometimes rapid changes in the lake’s level over the past two centuries (Sutcliff e and Peterson 2007, Nicholson and Yin 2000). The level of Lake Victoria remained fairly stable around 1,134 m up to 1960 but it rose rapidly thereafter to reach 1,136 m in the mid-1960s. Since then the level of the lake gradually declined, but heavy rains during the 1997 El Niño raised the level by more than 1 m, followed by a sudden drop around 2004. The lake level rise in the early 1960s was a result of abnormally heavy rains; in the last six months of 1961, 2323 mm of rain were recorded, nearly 100% higher than its average value. Very high rainfall was recorded during the first six months of 1962 (1884 mm/year, about 50-60% above average), and 1963 (1739 mm), and 1964 (1739 mm). As a result the lake rose by 2.5 m during those years.
The fall in lake level in the first decade of the 21 century was the result of poor rains and excessive releases of water at Owen Falls. After this sudden rise that changed the base flow downstream of the lake for years, the flows decreased steadily with gradually falling lake levels until in 2005, caused probably by over-release from the Nalubaale/Kiira dam hydropower stations, the lake levels dropped considerably leading to a now decreased flow after the increased flows that resulted from the increased hydropower operation. During these hydrological changes Lake Victoria as well as the consecutive lakes and swamps have functioned as a buffer so that the strong changes in upstream hydrological regime have less impact downstream.
The basic determinants of Lake Victoria’s water regime, direct rainfall on the lake and evaporation, are difficult to measure and not yet fully understood. The average difference between rainfall and evaporation over its 69,000 km2 surface area is quite narrow, and its hydrological regime is therefore very sensitive to changes in climate. This is demonstrated by the considerable fluctuation in net basin supply. The water balance of the lake is also affected by inflow from the basin and irrigation developments could reduce this inflow and contribute to falling lake levels.
Assumptions about future water levels are necessary in planning Nile dams and their current and future operation. The future viability of hydropower from the Victoria Nile is generally as uncertain and variable as the climate. The lake’s water level may also affect other ecosystem services, such as fisheries, wetlands, invasive species, water quality (Kiwango and Wolanski 2008) and malarial mosquito habitat (Minakawa and others 2008).