The Water Budget and Potential Water Reserves of the East Africa Source Region of the Nile

Abstract

An energy budget model has been used, with daily synoptic surface and upper air data from four stations in East Africa, to calculate all terms of the surface energy balance once for each day and each night of 1973. The calculated evaporation has been combined with precipitation values to give estimates of water surplus or deficit for a variety of natural regions. The surface included is the main source region of the White Nile, the study comprising the drainage area down to Nimule on the border of Sudan. The average outflow there is about 22 ? 109 m3 year?1, and the investigation attempts to estimate the effects on this outflow, and the amount of water potentially available for irrigation, by changing certain surface parameters in the energy budget model. These surface parameters describe the vegetation density, the soil types as characterized by water holding capacity, and surface slope. Lake Victoria is sufficiently large to generate its own air circulation and a precipitation maximum. Nevertheless, the lake does not enhance the regional outflow because of the high evaporation rate. The smaller lakes have substantial negative water budgets, as would be the case for all man-made reservoirs. The mountain regions contribute more than 60% of the total annual runoff from the land area. By a number of changes in the surface parameters in the model, a series of estimated values have been obtained of the water potentially available for irrigation and the resulting altered outflow in the Nile at Nimule.