Regional Lake Victoria Environment Report Water Quality and Ecosystems Component Lake Victoria Environmental Management Project CHAPTER 3 Water Balance

J Okongu, S Sewagudde, R Mngodo, F Sangale, F Mwanuzi, R Hecky
One of the principal objectives of the Water Quality and Ecosystem Management Components is to find the reasons for the changes observed in the lake water quality and quantity in order to establish causes of change in the lake ecosystem and to identify remedial measures. To identify the reasons for the changes one requires a knowledge of the changes in the pollution loadings to the lake, which, in turn, depends on the discharges into the lake from the catchments and the atmosphere and the
more » ... w to River Nile i.e. hydrology and meteorological characteristics in and around the lake. Hydro-metrological data for the period running 1950-2004 were analysed and form the basis for computing the pollution loadings (catchment and atmospheric) into the lake and as well as calculation of the lake water balance. Continuous rainfall and evaporation records were applied and data gaps filled were necessary. Full records of land discharges were obtained from rainfall records using the NAM model. Model performance was evaluated on the ability to simulate the total flow rather for catchments, rather than the peak and minimum flows, for pollution estimation. Results indicate that Tanzania's land unshared catchment annual discharge contribution to Lake Victoria is approximately 5,430 BCM 1 , while Uganda's one is approximately 1,062 BCM and Kenya's is approximately 9,271 BCM which in percentage are 21.4%, 4.2% and 37% respectively. Rivers Mara and Kagera that are shared between Kenya and Tanzania and Tanzania, Rwanda, Burundi and Uganda respectively had total flows of 1,151 BCM and 8,215 BCM accordingly representing 4.6% and 32.7% of the total catchment discharges. The mean annual rainfall over the Ugandan side of the lake is about 62,539 BCM, Tanzania is about 60,682 BCM and Kenya is about 4,541 BCM. These constitute 48.9%, 47.5%, and 3.6% respectively of the total mean annual lake rainfall into Lake Victoria. There was a 10.7% decrease in rainfall over the Lake in the period 1972-1993. However in the period 1994-2004 there was a 2.2% increase in the amount However there was a 14.7% decrease in catchment inflows into the lake and a 1.64m drop in water level in the period 1998-2004.