Distribution in Surface Waters

Africa’s water is held in large rivers and empoundments, widespread aquifers, lake and wetlands as well as in atmospheric water vapour and soil moisture (UNEP, 2012). For this introduction, I’ll be focusing on rivers. Rain fall is the main driver of variability in river flows, particularly at the large river-basin scale (Conway et al., 2009). The surface water flows across Africa also broadly mirror the spatial and temporal patterns in rainfall discussed in my last post, with Central Africa holding 50.66% of the continent’s total internal water and Northern Africa only 2.99% (UNEP, 2012). River flow is strongly influenced by latitudinal variations in rainfall (Taylor, 2004), and this pattern is further reflected in records of maximum and minimum river flows. Figure 1 compares the maximum and minimum river discharge values for the River Congo near the Equator (Kinshasha station; 4°30’S) and the River Nile at higher latitudes (Dongola station; 19°11’N), which have similar gauged station areas. For the Congo, the year-round influence of the ITCZ at low latitudes gives rise to comparatively higher overall monthly discharges, reflecting the spatial patterns in the amount of rainfall received. The peak mean monthly flow of the Nile constitutes just 14% of the Congo’s. Furthermore, the River Congo experiences lower seasonality, seen through the relatively smaller difference between its maximum and minimum monthly discharges. For the Congo, mean monthly low flow is 55% of the mean monthly peak flow, compared to just 10% for the Nile. In fact, over 80% of the Nile’s annual flow occurs between July-October compelled by northward movement of the ITCZ.

Figure 1. Graph displaying maximum and minimum mean monthly discharges for the River Congo and River Nile. Created using data cited in Taylor (2004); data originally from UNESCO (1995).

An important point is that beyond these broad global circulation patterns, more regional and local features can have a pronounced effect on rainfall (Taylor, 2004). Relief (topography) also strongly influences the distribution in received rainfall and river discharge. A rise in elevation such as the presence of a horst, volcano or rift shoulder originating from tectonic activity can induce orographic rainfall by forcing moisture-laden air upwards which subsequently cools and sheds its moisture as rain or snow (Taylor, 2004). Evidence for this process arises from increased rainfall observed in mountainous areas such as Kisozi in Burundi, compared to lower lying areas like Bujumbura in Burundi (Taylor, 2004).

Topography and Surface Drainage

The development of relief through tectonic activity not only compels local rainfall but also defines, to a large extent, patterns of surface drainage (Taylor and Howard, 1998), as rivers tend to flow along faults in the Earth’s crust. Furthermore, long thin lakes in East Africa result from the collection of river flow in grabens and other trough-like depressions of the land surface, for example lakes Turkana, Malawi and Albert (Taylor, 2004). Figure 2 shows several of Africa’s “water towers”. These are forested uplands in several African watersheds, generated by identifying areas of relatively high elevation (generally 200-800 m above surrounding area), precipitation over 750 mm, and runoff over 250 mm (UNEP, 2012).

Figure 2. Map of Africa's "water towers". (Source)

These high-elevation water towers store water and contribute disproportionately to the total streamflow of Africa’s major rivers. Their strong influence on the distribution of water resources can be seen through comparison with the map of water surplus (see my last post!) in Figure 3. Notice how all the water towers correspond to regions of especially great rainfall surplus, especially the Ethiopian Highlands, Jos Plateau, Fauta Djallon and the Central High Plateau in Madagascar as circled. They allow for high water surpluses in otherwise arid regions, such as the Middle Atlas Range in the North and the Lesotho Highlands in the South.  Rivers such as the Nile, the Niger, the Senegal and the Orange flow from relatively rain-abundant areas corresponding with these water towers to places that would otherwise be too arid to support much life (UNEP, 2012). For example, the main flows of the Niger River, which discharges into the Niger delta in Nigeria, originate from the Fauta Djallon highlands in Guinea, which generate orographic rainfall which flows downstream. An important repercussion of orographic rainfall is the generation of a rain shadow – this is the area downwind which experiences low rainfall due to the depletion of moisture from air passing over high ground. For example, reduced rainfall in eastern Kenya and the horn of Africa (Figure 3b) is believed to stem in part from the stripping of moisture from air currents passing over the Central High Plateau of Madagascar (Kendrew, 1961) (Figure 3a).

Figure 3. a) Map of Africa's water towers; b) Map of Africa's annual water balance. Red circles highlight where water towers correspond to high rainfall surpluses. Created using maps from UNEP (2012).

Note: Atmospheric flows responsible for rainfall experienced at specific time and locations across Africa are far more complex than the broad patterns presented in these last two posts. For example, above average or extreme variations in rainfall have been associated with regional-scale Indian Ocean dipole events and their complex interactions with the El Nino Southern Oscillation (Conway et al., 2009).