The water balance of Lake Victoria

Abstract

The sharp rise in the level of Lake Victoria between 1961 and 1964 has been found difficult to explain in terms of the components of the water balance. After reviewing lake inflows and the method of calculating lake rainfall from lakeside gauges, the historic lake water balance has been reproduced. The rise in lake level can be explained through rainfall and resulting tributary inflows, which would allow projections of possible future levels to be made by analysis of rainfall series. Bilan hydrologique du Lac Victoria RESUME L'élévation sensible du niveau du Lac Victoria enregistrée entre 1961 et 1964 était restée difficile à expliquer en terme de bilan hydrologique. Après révision des débits arrivant dans le lac et de la méthode de calcul des précipitations sur l'étendue du lac, un modèle historique a pu être reproduit. L'élévation du niveau du lac peut alors être expliquée par la combinaison des précipitations et de l'effet résultant des débits des tributaires. Les évolutions futures du niveau du lac pourraient être prévues par l'analyse des séries de précipitations.

Water balance of Lake Victoria: update to 2000 and climate change modelling to 2100 / Bilan hydrologique du Lac Victoria: mise à jour jusqu’en 2000 et modélisation des impacts du changement climatique jusqu’en 2100

“…In general, the studies show that direct rainfall over Lake Victoria is the key factor controlling its outflow; that this outflow is of major importance for the whole length of the Nile; and that climate change could have a marked impact on the sensitive link between lake rainfall, inflow and evaporation which controls this complicated water balance. Piper et al (1986) were able to explain a significant rise in lake level between 1961 and 1964 through rainfall and resulting tributary inflows. They used rainfall records from gauges around the lake shore and completed a tributary inflow record using ratios to other gauged records, then extended the series back to 1925 using a net rainfall-soil moisture model.…”

“…The lake rainfall series is much longer than any of the individual tributary flow records, which in any case represent only part of the surface runoff for most of the period (Piper et al, 1986). Through deriving inflows from lake rainfall, a longer inflow series can be produced which is representative of the whole inflow to the lake, rather than just from individual tributaries.…”

“…Hence the runoff is more variable than the rainfall itself. Although average runoff into the lake is small in total volume compared with direct rainfall on the lake surface, its variability from year to year is greater and its impact on the water balance is significant (Piper et al, 1986).…”

“…2 Map of Lake Victoria and its basin (Sutcliffe & Parks, 1999). Although tributary inflow is small compared with direct rainfall on the lake surface, its greater variability is important in the fluctuations of the basin supply (Piper et al, 1986). Since lake rainfall is almost balanced by the lake evaporation, which varies little from year to year, the net basin supply is unstable.…”

“…Conway & Hulme (1996) investigated the potential impacts of climate change on Nile discharge. They used a Lake Victoria water balance model similar to the Piper et al (1986) version, which was driven with climate change scenarios from three global climate models and produced changes in Lake Victoria outflows ranging from -9.2% to +11.8%. Sensitivity analysis showed that a 10% increase in Lake Victoria rainfall causes a 31% increase in runoff, and a 4% increase in potential evaporation causes an 11% decrease in runoff.…”