English (UK)

In the recent months, Lake Victoria has experienced rising water levels that have caused significant flooding impacts on Lake shoreline communities in Kenya, Tanzania and Uganda as well as downstream communities near Lake Kyoga and Lake Albert. NBI Secretariat’s, Dr Modathir Zaroug shares some insight in the following article.

 Lake Victoria is the largest lake in Africa and the second largest freshwater lake in the world1. It has a surface area of around 69,000 km2. The Lake is shared by Kenya (6%), Uganda (43%) and Tanzania (51%) 10; while Burundi and Rwanda are also part of its catchment area which covers 184000 km2 (Fig.1). All the five countries are members of the Nile Basin Initiative (NBI).

The main outlet of Lake Victoria, is in Jinja (Uganda) and it is here where the White Nile, which is part of the mighty River Nile starts flowing northwards through Uganda. The White Nile keeps changing names as it journeys through South Sudan and Sudan2&3.

Lake Basin Climate

The diurnal, seasonal and inter-annual variability of Lake Victoria (and East Africa generally) climate results from a complex interaction between the Inter-tropical Convergence Zone (ITCZ), El Niño/Southern Oscillation (ENSO), Quasi-biennial Oscillation (QBO), large scale monsoonal winds, meso-scale circulations and extra-tropical weather systems. These interactions are responsible for the anomalous behavior of the Lake system and many studies have been carried out to understand the influence of these systems on water balance of the Lake. The Lake surface receives a high amount of rainfall of above 1500 mm, which represents about 85 percent of the water entering the Lake; the remaining inflow comes from the 23 rivers that drain the catchment. The annual evaporation rate from the Lake surface is about 1350 mm.

Recent rainfall variations

In the recent months, Lake Victoria has experienced rising water levels that have caused significant flooding impacts on the Lake Shoreline communities in Kenya, Tanzania and Uganda as well as downstream communities near Lake Kyoga and Lake Albert. Based on analysis using the CHIRPS v2.0 dataset, the rainfall in Lake Victoria exhibited above average rainfall since May 2019. A considerable increase in rainfall in 2019 was experienced in October by 79%, in November by 56%, and in December by 74% compared to the long term average. Observed rainfall in 2020 increased in January by 83%, in February by 25%, in March by 43%, and in April by 33% compared to the long term average. However May rainfall was less than the long term average (Fig. 2).

The cause of these major increases in rainfall in Lake Victoria Sub-basin was due to a positive Indian Ocean Dipole (IOD) phase5,6,7 caused by warmer sea temperatures in the western Indian Ocean region, with the opposite in the east (Fig. 3). This resulted in the higher-than-average rainfall and floods in eastern Africa. The positive IOD occurs when the westerly wind weakens and the easterly wind forms and allows warm water to shift towards Africa. The 2019 IOD was the most extreme event over the past 40 years8.

 

Historical and current water level variation

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 levels rose by 2.5 m by 19649 which was the historical highest level before 2020.   From 1964 onwards, Lake levels continued falling with some isolated increases (e.g. in 1982 and 1997/98) until they reached a minimum of 10.4 m in 2006. Due to the recent heavy rains however, by May 2020 the water level had exceeded the historical mark and hit 13.42 m.

Discharge analysis

The discharge at Jinja, where the White Nile commences, has been recorded since 190010&11. The average annual flow is approximately 32 billion m3 (1948 to 2014) at Jinja station in Uganda9. The recent rise of water level led to release of around 2400 to 2600 m3/sec, which is around 207 to 225 million m3/day respectively. By aggregating this amount to 30 days, the flow is currently 6.2 to 6.7 billion m3/month compared to a long term average of 2.3 billion m3/month.

Implications on upstream and downstream Nile Basin countries

  • Shoreline communities and businesses in low lying areas in Uganda and Kenya have been severely affected, and some communities displaced. In addition, some infrastructure have been damaged or made inaccessible while crops were washed away.
  • Hydro power generation at Owen Falls Dam in Uganda has been affected by a number of floating islands resulting in temporary blackouts that sometimes affect Uganda. The high velocity of the flow from Lake Victoria tributaries and the wind pushed the floating islands towards the only outlet. High persistent outflows are also causing river bank erosion downstream and risking collapse.
  • Downstream communities were also affected especially around Lake Kyoga. The picture (in Fig. 4) shows a Uganda Government hydrological measuring instrument that has been submerged.
  • Higher discharges are expected this year if the high rainfall episodes continue and water levels remain very high. This will have major impacts in South Sudan and Sudan and the countries should prepare for the potential negative impacts over the coming year.

Addressing the common challenge of rising Lake Victoria water level

The Nile Basin Initiative (NBI) continues to support efforts of its Member States to jointly address their water resources management challenges. Some of the interventions by NBI are;

  • Generating knowledge and information products that support the countries and people of the Nile Basin to better manage their shared water resources. These include the Nile Basin Water Resources Atlas, the state of basin reports, quarterly basin monitoring bulletins, strategic water resources analysis, ongoing development of the Nile Basin River Flow Forecasting System, and studies of projected hydrological scenarios for the Nile Basin under climate change12.
  • The Nile Basin Regional HydroMet Project currently under implementation will establish the first Regional HydroMet System for the Nile Basin, which will support monitoring and data collection efforts of Member States and the region at large.
  • The common platform for dialogue enables countries to engage, consult and deliberate with each other and other Nile stakeholders including joint planning and management of water and related resources in the Nile Basin including Lake Victoria.

References

1- https://www.worldatlas.com/aatlas/infopage/lakevictoria.htm

2- ILEC, I. and International Lake Environment Committee, 1999. Survey of the State of World Lakes.

3- https://www.nilebasin.org/about-nile-basin/1-the-course-of-the-river-nile

4- Kizza, M., Westerberg, I., Rodhe, A. and Ntale, H.K., (2010). Estimating areal rainfall over Lake Victoria and its basin using ground-based and satellite data. Journal of Hydrology. In review: 21pp.

5- Conway, D., 2002. Extreme rainfall events and lake level changes in East Africa: recent events and historical precedents. In The East African great lakes: limnology, palaeolimnology and biodiversity (pp. 63-92). Springer, Dordrecht.

6- Webster, P.J., Moore, A.M., Loschnigg, J.P. and Lebden, R.R. (1999) Coupled Ocean-Atmosphere Dynamics in the Indian Ocean During 1997-98. Nature 401, 356-360.

7- Saji, N.H., Boswami, B.N., Vinayachanran, P.N. and Yamagata, T. (1999) A Dipole Mode in the Tropical Indian Ocean. Nature 401, 360-363.

8- https://www.climate.gov/news-features/blogs/enso/meet-enso%E2%80%99s-neighbor-indian-ocean-dipole

9- Nile Basin Water resources Atlas.

10- Kite, G.W., 1982. Analysis of lake victoria levels. Hydrological Sciences Journal, 27(2), pp.99-110.

11- Conway, D. and Hulme, M., 1993. Recent fluctuations in precipitation and runoff over the Nile sub-basins and their impact on main Nile discharge. Climatic change, 25(2), pp.127-151.

12- https://www.nilebasin.org/information-hub/technical-documents.