Laurent Bergonzini

Mbaka lakes isotopic (18O and 2H) and water balances: discussion on the used atmospheric moisture compositions

Abstract The water balances of small crater lakes Masoko, Katubwi, Kyambangunguru, Ilamba, Kingiri, located north of lake Malawi (Tanzania), are established with the Craig–Gordon model of evaporation. In this approach, it is difficult to evaluate the isotopic composition of evaporated vapour (rδ E), which is indirectly estimated from the atmospheric moisture signature (rδ A). Given that ‘precipitation equilibrium’ approach does not lead to realistic solutions in this tropical region, the lake balances are established and discussed from both the sampled rδ A and those derived from the terminal lake Rukwa. The mean local sampled signature ( : rδ18O=−13.7 ‰ and rδ2H=−76 ‰) is inferior to that derived from lake Rukwa ( : rδ18O=−10.4 ‰ and rδ2H=−64 ‰), which may be due to altitudinal and latitudinal effects but both are influenced by recycling continental vapour. Water balances enable us to confirm and quantify high inflows and groundwater outflows that account for typologies of Mbaka lakes. The proposed sampling and measurement method allows us then to access reasonable moisture compositions in far-away regions.

Variation interannuelle du bilan hydrique du lac Tanganyika (1932-1995): changement dans la relation précipitation-excédent lacustre

Abstract The data collected have allowed the evaluation of the variations in the water levels of Lake Tanganyika between 1932 and 1995, the discharge at the outlet, the excess of the lake budget (or “level without discharge”, ΔH*), and the precipitation on the drainage area. The evolution of the annual lake budget is related to the precipitation variability. However a marked increase in the excess after 1961 is shown, which corresponds to a change in the relationship between annual excess and precipitation, which further corresponds to a higher effectiveness of rainfall. This marked change in the lake water budget could partly correspond to wetter conditions at the beginning of the rainy season, but the man-induced changes in land use and in the surface runoff conditions appear partly responsible for the difference in the lake sensitivity to precipitation.

Aerosols, atmospheric transmissivity and hydrological modelling of climatic change over Africa south of the equator

Given the uncertainties associated with GCM modelling of aerosol effects on regional precipitation, a simple model is used to make a preliminary assessment of the influence of aerosols on rainfall in the Lake Tanganyika region of tropical southern Africa. No attempt will be made to consider the effects of changing carbon dioxide and moisture contents of the atmosphere that might have occurred at the same time. Atmospheric transport and recirculation of air and aerosols are considered and shown to occur on a large scale. Size distributions, residence times and concentrations of aerosols are examined and air volume and aerosol mass transports are estimated. South African data from stations on the interior plateau are used to approximate present‒day seasonal and interannual variations in transmissivity due to changing aerosol loading in the atmosphere. A lake‒catchment hydrological model is used to determine rainfall over Lake Tanganyika and its environs and from this a sensitivity analysis of rainfall receipt to aerosol‒induced changes in atmospheric transmissivity is carried out. It is shown that a decrease in transmissivity commensurate with the present‒day interannual range of variability of 10 per cent produces, in the absence of other changes, about a 15 per cent diminution in rainfall. The analysis is extended to show that in the past, during the sixteenth‒century Little Ice Age, the transmissivity induced rainfall decrease could have been about 25 per cent and that at 18 000 years BP it would have been more. Aerosol loading is shown to have important consequences for the occurrence of droughts through its modification of the surface radiation balance.

Computed Mean Monthly Water Balance of a Large Lake: The Case of Lake Tanganyika

Freshwater is, and has always been, a primordial and strategic resource. Consequently, the large lakes of the East African Rift, which represent one of the most important surface fresh water reserves of the world, are a major wealth for the adjoining countries. Moreover, these large lakes constitute the water supply for some of the major African rivers, the Congo River, the Nile River and the Zambezi River. They also are economically important for the nutrition and the development of the lakeside populations. Beyond these functions, they constitute particular hydrological objects, which have a climatic memory making them topics for scientific research unique throughout the world. Furthermore, the climatic dependence of the region of the African lakes from the atmospheric circulation registered above oceans has been recently evidenced. This actualizes the analysis of the lake balances, considering that numerous studies confirm that the African lakes underwent important lake level oscillations during the Quaternary. However, despite their economic and scientific relevance, the hydrological balance of the Lake Tanganyika water masses has been poorly investigated.