Lake Victoria water balance model and irrigation impact analysis
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Date
2000
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Publisher
University of Dar es Salaam
Abstract
A monthly water balance model has been developed for Lake Victoria for the period1925 to 1998. A model developed in a previous study for the period 1925 to 1990 has been adopted and modified and the simulation period was extended to 1998. Evaporation from the lake is computed from the Morton's Model. A methodology to estimate the rainfall over the lake for the period 1925-1998, by integrating a GIS and real-time satellite data with statistical analysis is explained. Linear relationships between the cold cloud duration (CCD) and the rainfall estimates from the CCD (RFE) over the lake and lakeshore have been established. This relationship has then been used to estimate the rainfall over the lake from the rainfall along the lakeshore for the period 1991 to 1998. Further the developed monthly water balance model has been used to analyse the impact that basin irrigation would have on the lake levels, inflows and outflows. Considering four hypothetical irrigation development scenarios and using the monthly water balance model, irrigation impact analysis was carried out. From the study it was found that the monthly rainfall over the lake is 21% higher than that along the lakeshore using data derived from satellite images of CCD. It was also found that the rainfall estimates from the satellite data underestimated the observed lakeshore rainfall by 5%. The water balance model developed for the lake showed good agreement between the observed and predicted levels. The calculations suggested that, the mean annual rainfall over the lake for the period 1925 to 1998 is 1730 mm, evaporation is 1567 mm, inflow is 312 mm and outflow is 455 mm, when expressed as depth over the lake surface area. From the irrigation impact analysis it was found that irrigating 5%, 10%, 15% and 20% of the total lake basin area would require on average 5, 10, 15 and 20 billion cubic metres of water per annum respectively. The corresponding lake level reduction would be 0.06%, 0.1%, 0.13% and 0.17% respectively. Percentage reduction in total inflow to the lake is 24%, 48%, 71% and 95% respectively whereas the percentage reduction in the outflow from the lake is 0.36%, 0.70%, 1.03% and 1.39% respectively. It is concluded that rainfall over Lake Victoria is much higher than that along its shores, as has been suggested by previous studies. The water balance model showed that the variations in the levels of Lake Victoria are related primarily to variations in rainfall over the lake with little influence from tributary inflows. From the irrigation impact analysis, it is concluded that the land use changes in the lake basin would have only a small influence on the lake levels and outflows. The main input factor affecting the water balance of Lake Victoria is the rainfall over the water surface and the main output factor is the evaporation from its surface. Transnational crimes will be difficult
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Keywords
Irrigation, Management, Tanzania, Water balance (hydrology), Lake Victoria, Tanzania
Citation
Acheng, I. O. (2000) Lake Victoria water balance model and irrigation impact analysis, Masters’ dissertation, University of Dar es Salaam. Available at (http://41.86.178.3/internetserver3.1.2/detail.aspx)