Why Water Resources Assessment? (3) For the world is complex

Last post covered the economic justification for WRA. The water resources in Africa is highly unequally distributed. Factors such as the movement of the Inter Tropical Convergence Zone (ITCZ), huge variation in surface relief, and intensification of climate change give rise to a very complex distribution of water resources, not to mention how this could be further complicated by the human landscape. In this post, we turn our attention to the physical climate and conditions of the continent Africa. 

Logically, when considering the input of water into a system, we think of the precipitation. In Africa, it is governed broadly by the movement of the ITCZ. This is supported by figure1 below which shows a rough latitudinal symmetry in the distribution of annual rainfall across Africa. The mechanism behind this pattern is as follows:

• The unequal heating of the Earth by the Sun causes atmospheric pressure gradient.
• This creates movement of air that flow from high to low pressure, leading to the formation of atmospheric cells. (See an animation here for representation)
• Where warm air meets and rises, the moisture carried by the air condenses once it becomes saturated in higher elevation, leading to rainfall. 
• The band of rainfall migrates up to the Tropic of Cancer in summer and down to Tropic of Capricorn. (See Figure2 and an animation here) 

This means that areas that are constantly within the zone of ITCZ will have higher rainfall and those that are covered by ITCZ once a year receive less rainfall.

Figure 1. The average annual precipitation in Africa (mm).

Figure 2. The movement of ITCZ.

The topography of the Africa continent further complicates the distribution of water. As Figure 1 shows, the centre of the continent receives the highest annual rainfall whereas in the east it is much lower. This is due to the presence of the East Africa Rift System (also known as the High Africa) that led to the formation of orographic rainfall and rain shadows. Figure 3 shows the elevation level across Africa. 

Figure 3. The elevation level in Africa (m).

The topography not only affects water resources availability in terms of the formation of rainfall, it also influences how much water will be taken out of the system via evaporation. It is known that generally the lower the elevation, the higher the temperature, and therefore the higher the potential evapotranspiration (PET). Coupled with climate change which influences the amount of rainfall and evapotranspiration, this could have a huge impact on the availability of water resources in southern Africa where its discharge has one of the highest coefficient of variation in the world (i.e. very variable.). With a 10% drop in prepcipitation, it reduces the river discharge by 17 to 50% (de Wit and Stanklewlcz 2006). Li et al (2015)'s climate projections predict an increase in both runoff and ET in eastern part of Southern Africa and a decrease in runoff in its  driest region e.g. Kalahari Desert, Namibia, southwest of South Africa and Angola. With such spatial variability in both rainfall  (the input) and PET (the output), the consequent river discharge also becomes more uncertain.

In short, after taking into account of factors such as the seasonality of ITCZ, topography and climate change within Africa, we may see the water available is much complicated and uncertain than we imagined. To be able to manage this uncertainty and risk of drought or flood, it is best to carry out a comprehensive WRA.