Physical Characteristics of Sub-Saharan Africa

Hello all!

Previously, I spoke about Sub-Saharan Africa and how it has been met with ecosystem degradation due to an undervaluation of ecosystem services. In this post, I will touch on the general physical characteristics of Sub-Saharan Africa, which will subsequently help us to understand the distribution of water as an ecosystem function within the region.

Physical Characteristics

Sub-Saharan Africa, as a whole, encompasses a massive land area of about 24 million km² (Hammond and Antwi, 2010); the SPUs including lakes, wetlands, rivers and reservoirs span 6% of the continental area of Sub-Saharan Africa with an overall area of 1,448,771 km² (Rebelo et al. 2010). This area is marked by large spatial heterogeneity: 66% of the land area is characterised by arid and desert conditions, and only 26.9% of the total land area is viable arable land for livestock cultivation and food production (Cotula et al. 2009). The disparity in natural conditions is largely explained by the large variability in climatic conditions, which is controlled by global patterns of atmospheric circulation (Taylor, 2004).

Quintessentially, the Hadley cell controls most of Africa's atmospheric circulation. Around the world, a low pressure belt forms where insolation is concentrated at the equator, rises and flows towards the polar regions. As this air flows poleward, it cools and descends at latitudes of approximately 30°N and 30°S forming a high-pressure belt with stable air conditions and little moisture. Subsequently, the subsiding air at these latitudes either head poleward or equatorward to complete the Hadley cell. Due to the Earth's rotation, the Coriolis force comes into play and these equatorward air movements are deflected clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere to form the northeasterly and southeasterly trade winds respectively. They converge to form the Inter-tropical Convergence Zone (ITCZ), where the convective and convergent uplift bring a large amount of rainfall to the region within the ITCZ as seen in Figure 2.

Figure 2. a) January and b) August. The migration of the ITCZ (red band) over the course of the year creates great seasonality over the African continent, bringing most rainfall to Central Africa in August and most rainfall to South Africa in January. Picture from Ziegler et al. (2013)

Due to Africa spanning across equatorial and subtropical latitudes and the movement of the ITCZ, the mean annual rainfall is characterised by marked spatial variability, ranging between 100mm at the Sahel regions and 1500mm at the coast of West Africa (Eltahir and Gong, 1996). Concomitantly, such diverse climatic conditions deliver different distributions of Ecosystem Service Providing Units (SPU) throughout Sub-Saharan Africa, and within countries their dependency on types of ecosystem services varies as physical conditions change (Egoh et al. 2012). In the wetter regions of western and central Africa, agricultural produce of food and benefits from raw materials are more important ecosystem services; in the arid and semi-arid countries of southern and northern Africa, tourism, grazing and water are of greater priorities.

The important role of water resources delivering ecosystem services within Sub-Saharan Africa will be elaborated more in the next post, where I will be looking at the water bodies in greater specificity.

See you next week!