Surface Water Distribution in Africa

Overall, “Africa” appears to have sufficient freshwater by the inappropriate metric of the Water Stress Index, however, water is not distributed evenly across the huge continent spatially and temporally, and there are more factors to water availability than freshwater volume (Damkjaer and Taylor, 2017). Surface water distribution is significantly influenced by climatological factors such as precipitation and evapotranspiration, but other factors such as topography, vegetation, and land use also affects the presence of surface water. This post will briefly cover some continent-wide patterns and trends in water availability under the influence of climate change.

Current Distribution of Surface Water

The water balance map below (Figure 1) is a better representation of water resources across the continent instead of a precipitation map since evapotranspiration consumes 70-90% of precipitation. In northern and southern Africa, the water balance is significantly lower than humid, central Africa. Surface water resources are located around the equatorial region where there is a large surplus of rainfall, on the other hand there is a distinct lack in surface water sources in the northern part of the continent where the Sahara Desert is located. There is also an area around Kenya and Somalia that appears to have a larger rainfall deficit. 

Figure 1. Estimated annual water balance across the African continent (UNEP 2012) 

The intertropical convergence zone ITCZ is defined as an “east-west oriented low-pressure region near the equator where the northeast trade winds meet the southeast trade winds”, which forces warm, moist air upwards then rapidly cools and condenses, creating a band of heavy precipitation. Rainfall seasonality in Africa has long been assumed to be controlled by the ITCZ moving between the northern and southern hemisphere and producing the bimodal seasonal cycle of dry and wet seasons in the tropics.

Figure 2. (a) January and (b) August. Modern rainfall variability over Africa (Ziegler et al., 2013)

Another key factor to the distribution of surface water is the topography and geology of the continent, which defines the location, size and pattern of river basins regardless of political boundaries. River networks in the equatorial band appear to be more complex due to flatter topography (potentially created by rain erosion over time), and steeper but less complex in the south of the continent. 

Figure 3. Elevation of the African continent with river basin boundaries (Grid-Arendal 2013)

Potential Changes to the Distribution of Surface Water

As the atmosphere warms, climate models project a “deep-tropics squeeze” of the ITCZ, describing a narrowing and strengthening of the wet, tropical in the core of the ascent region in the 21st century as the dry subtropical region largely widens. This is partly due to the Clausius-Clapeyron relation, in which the amount of water air can hold increases exponentially with temperature. As such, it is expected that precipitation would shift from fewer light precipitation events to more frequent and heavy precipitation events. 

 

Therefore, tropical regions in Africa are affected disproportionately by climate change compared to other countries, having warmed faster than the global average temperature over land and ocean combined. While intensified precipitation may also cause secondary events such as intensified flooding, the heavy rainfalls tend to deplete the air of its moisture, and may lead to more persistent and longer droughts, amplifying variability in river discharge and groundwater recharge. 

Although these general trends were identified, the exact extent of climate change is unknown and models created to predict its effects are riddled with uncertainty due to drawing estimates from other models which also have uncertainties. In IPCC AR5, the models show substantial uncertainty in water resource projections under 2C rise in global mean air temperature, around 40%. And so, climate scientists recognised that the uncertainty would not help management and advised that adapting to existing climate variability would go a long way to adapting to climate change.