A GIS Based Water Balance Study of Africa - Physical Land Resources

6SUMMARYNkezeah, A.J., 2010. A GIS Based Water Balance Study of Africa. M.Sc Thesis, VrijeUniversiteit Brussel, Belgium, 55pp.The GIS-based water balance Model WetSpass was applied to the continent of Africa toinvestigate the amount and variability of annual water balance components for an area ofapproximately 30 million Sq. Km. Average annual spatial patterns of surface runoff, actualevapotranspiration and ground water recharge were computed. Gridded 1km spatial resolutionmaps of elevation, soil, landuse/landcover, slope, and climatological variables were used. Theresults show variability in water balance components across the continent. These changescorrelate with local and regional changes of the parameters above.Estimates of Evapotranspiration (ET) go beyond 1000mm/yr around equatorial Africa. The aridregions of the Sahara and the western tip of southern Africa fall below 100mm/yr. In the semiarid subtropics of southern Africa the value ranges between 200 to 700mm/yr. Very little or nosurface runoff occurs around the northern Sahel except for the mountainous areas further northwest with values of 10-50mm/yr. Higher values of 100-400mm/yr occur around the ruggedterrain of east Africa. The groundwater recharge estimates are higher around the humid regionsof the tropics than further north and south. This obviously is related to higher amounts of rainfallthat this region receives annually.Generally there is a good agreement between the calculations of this study and others that havebeen carried out with other approaches/ methods e.g. Nicholson et al. (1997). The average annualsurface runoff for Africa is computed at 57mm/yr, ET 588mm/yr, groundwater recharge34mm/yr. interception 104mm/yr, transpiration 383mm/yr and soil evaporation 72mm/yr.While these values are not exact but only estimates, they do provide useful insights on thevariability of these critically important water balance components across the continent.