Adaptation to climate change in international river basins in Africa

Even though the paper "Adaptation to climate change in internationalriver basins in Africa: a review" from 2009 by Marisa Goulden et al. mostly addresses to the issue of climate change in relation with water management she points out some interesting facts regarding climate change and freshwater resources.

About 90% of African river basins form borders of several countries. Varying river discharges can therefore lead to further conflicts if the precipitation variability and geographical distribution lowers further. Freshwater is indispensable to life of people providing them with work, food, energy and industrial supplies. Accompanied with a population growth and urbanization the already high demand of water rises even higher.  

The African continent is separated into 3 climate regimes: wet (>1000mm/year), intermediate (400-1000mm/year) and dry (<400mm/year) - greatly illustrated in Maarten de Wit´s paper - including seasonal-arid tropical, sub-tropical Mediterranean and humid climate near the equator. Central Africa receives almost 40% of the annual precipitation even though it contains only 20% of the total continental area whereas northern Africa with about the same area receives less than 3% precipitation. 

The intermediate regime spreads around 25% of the country area including some of the densest populated regions like the South Africa Orange river basin, the east African upper Nile basin and the east-west-belt from Sudan to Senegal including Lake Chad, Niger etc. This regime is feared for the most due to already highly varying seasonal rainfall.

Green water (stored as soil moisture) is also an important contribution to the water circle. It evaporates from plants, at a certain threshold contributes to groundwater recharge and is essential for any kind of agricultural use of soil.  

Local climate is influenced by the Atlantic and Indian Ocean dynamics. Changes within those patterns will lead to changes in Climate in the African continent as well as floods and droughts in coastal areas. Globally thinking a sea level rise could have a major impact on Africa.

Therefore, it is reasonable to say that some of the densest populated areas are located in the intermediate regime and local river basins and lakes suffer from a severe water stress caused by the demand of millions of people connected with a lowering river discharge.

Data supporting theories of scarcity only refer to "blue water" from lakes and rivers not taking account of local aquifers or green water due to bad monitoring. Climate Models predict temperature rises of 3-4°C within the next 60 years, drier climate in southern Africa and Sahara, wetter climates in West and East Africa.

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.