Changes in Surface Water Supply Across Africa

The paper "Changes in Surface Water Supply Across Africa with Predicted Climate Change" by Maarten de Wit and Jacek Stankiewicz published in Science 2006 relates to the issue of changing perennial drainage density in African countries due to changes in Climate and precipitation. 

African population relies to great parts on overland flow water subtracted from rivers. Changes in that supply caused by climate change will therefore greatly influence most of Africa´s population and rise poverty to even higher standards. 
Drainage is influenced by biological (vegetation), geological (bedrock) and atmospheric (precipitation) parameters. A climate change would therefore directly the effect surface water supply. The authors subdivided the northern African continent into 37 equally large squares of 1000 km² and the southern part into 24 squares of 500 km² to calculate the mean annual precipitation and the perennial drainage density of each block (Fig. 1).

Fig.1: 37 Africawide blocks (black lines), Red block marks southern Africa, which was subdivided into 24 block

Those values were plotted and divided into 3 climate regimes depending on two threshold values of precipitation (>400mm/year and >1000mm/year) in order to analyse the African river systems. 


This shows that areas receiving less rainfall than 400 mm/year have no perennial drainage. Above that an "unstable" regime occurs characterized by increasing drainage densities with rising precipitation. Areas with annual rainfall hitting the second threshold are influenced by biological parameters so that, from that value on, the density decreases with increasing rainfall (Fig. 2).

Perennial drainage density as a function of mean annual rainfall, measured for individual
blocks for the whole of Africa (A) and southern Africa (B). Dashed vertical lines show thresholds of
400 and 1000 mm/year. Black lines show best linear fits of the combined data sets inside the
intermediate and high rainfall regimes

                             
Global estimated shifts in precipitation would therefore affect 75% of African countries rainfall pattern leading them into the "unstable" rainfall regime. Those three regimes are distributed as shown in Fig. 3 and follow therefore the ITCZ movement. Even though the red (dry) regime is the largest one the biggest changes in drainage supply would occur in the yellow ("unstable") regime.
Mayor concerns relate to South Africa which is in most party "unstable", densely populated and highly dependent on the Orange River (mean annual discharge of 11 000 km³).

Fig. 3: Present rainfall regimes in Africa; Dry areas (red) make up 44% of the continent’s total area, intermediate regime (yellow) cover 25% of the total area, high rainfall areas (green) cover 31%

Anthony Nyong, nowadays Manager of the African Development Bank, predicts by 2050 a 10% drop in rainfall in sub-Saharan Africa resulting in large-scale water shortages. For the yellow regions this would mean a 17% reduced surface drainage, for regions with fewer rainfall eg. 600mm/year the drainage would reduce by 30-50%.
Those losses in southeast Africa would also effect the upper part of the Orange river and therefore the entire downstream area that is - except for this river - very arid and a water shortage would lead to very low water levels on Namibian border.
The northern yellow bond (Fig. 3) represents an area that separates the Sahara (dry) from Central Africa (wet) and contains many of Africa´s major water bodies. Shifts in this area will change the desert boundary´s: they would move northward in Niger but expand father west to Mali and spread to the south).
This leads to another big issue of Africa´s water scarcity: river channels and basins often form the country boundaries to neighbouring nations (Fig. 4). An unstable water supply would therefore lead to international conflicts over these water resources.

Fig. 4: Country borders in Africa: inland boundaries formed by rivers 33% (blue) and watersheds 6% (red). These river-related borders apply to 39 of the 48 countries in Africa.

Action has to be taken place in order to secure a minimum water supply for every region in Africa - not only the ones that rarely fear of water scarcity due to favourable climate settings. Otherwise increasing population and rising riots will damage the country as much as ´mother nature´ does.