Africa's Hydrological Variability

Greetings curious readers!

Before we delve into the impacts of climate change on African water resources, it’s important to first assess the intrinsic hydrological variability that already exists in Africa. This is because the high levels of spatial and temporal variability in water resources dictate the highly complex relationship between water and people across the continent. Furthermore, it is possible that shifts in river flows and variability will occur with anthropogenic climate change - particularly important considering the population of Africa is strongly concentrated in the regions experiencing high degrees of interannual rainfall and runoff variability (Conway, 2009). 

In this blog post, I shall discuss the drivers of water resources variability in Africa. A major driver is rainfall. Rainfall in Africa is controlled to a large extent by global atmospheric circulation (Taylor, 2004). Atmospheric circulation occurs due to pressure gradients developing from uneven heating of the earth’s surface. The area around the Equator where moisture-rich, North-east and South-east trade winds converge is called the Inter-Tropical Convergence Zone (ITCZ).

The Importance of the ITCZ to hydrological variability in Africa – Taylor,2004

Unlike the geographical equator, the ITCZ moves north and south throughout the year due to latitudinal variations in solar radiation. During the Southern hemisphere summer, there is an increase in rainfall due to its migration to southern latitudes – i.e. the rainy season. After December, the ITCZ migrates northward brining rainfall to increasingly northerly latitudes, and then returns southwards bringing heavy rainfall to increasingly southerly latitudes until January. As such, movement of the ITCZ determines the annual seasonality of rainfall across tropical Africa. This annual cycle brings one rainy season to latitudes at the southern and northern extremes of the ITCZ course, and two rainy seasons to those at lower latitudes (i.e. a bimodal rainfall distribution).

Movement of the ITCZ dictates the spatial as well as temporal distribution of rainfall across Africa. Lower latitudes (i.e. 10° N - 10° S) receive greater volumes of rainfall than higher latitudes, as they are influenced by the ITCZ for a greater proportion of the year.

Other drivers of hydrological variability

It’s important to note that the atmospheric flows responsible for rainfall and it’s variability at specific locations and times are more complex than these broad patterns described. For example, above average or extreme variations in rainfall are often associated with Indian Ocean dipole events and their complex interactions with the El Niño Southern Oscillation (ENSO) (Conway et al., 2009). Furthermore, the physical setting is also important in determining rainfall variability, and local features can exert a significant effect on rainfall patterns. For example, a rise in elevation from a horst, volcano or rift shoulder can generate orographic rainfall, and a subsequent rain shadow effect (Taylor, 2004). The East African Rift System (EARS) is particularly significant in this respect.

Water at the landsurface

River flow is strongly influenced by these latitudinal variations in rainfall associated with movement of the ITCZ. At low latitudes, the year-round influence of the ITCZ on rainfall gives rise to fairly consistent and high total discharges for rivers. In contrast, lower river discharges and more extreme seasonal and monthly variations in flow are recorded at higher African latitudes. The seasonal variations at higher latitudes can be huge; for example over 80% of the River Nile’s annual flow occurs between July – October as a result of the heavy unimodal rainfall pulse compelled by both the Northward movement of the ITCZ and highland areas in Europe (Taylor, 2004). To further explore this relationship between rainfall and river flow in Africa, I have drawn upon the UNESCO Global River Discharge Database (RivDIS v1.1), and played around with their data on African river discharges. For all the following graphs, I’ve used data for the year 1976, as it seemed to have the most data across the continent.

Figure 1 shows an increase in maximum river discharge values at increasingly lower latitudes, attesting to the year-round influence of the ITCZ at these locations. At higher latitudes, the river discharge generally remains low, as these sites receive only one pulse of rainfall a year.

Figure 1. Scatter graph showing the 1976 data for Maximum Discharge for 49 African rivers, plotted against Latitude. Data obtained from: Vörösmarty et al., (1993) River DischargeDatabase, Version 1.1 (RivDIS v1.1). The discharge values for each river have been divided by the Upstream Area of the river, in order to normalise the data.

Figure 2 shows the annual changes in river discharge in 5 rivers at varying latitudes in 1976. This figure illustrates the course of the ITCZ across Africa on an annual basis; as the ITCZ passes over a river at a particular latitude, the heavy rain associated with it causes a peak in the discharge of that river. 

Figure 2. Montage of time-series graphs showing annual changes in river discharge across 5 rivers at varying latitudes in 1976. Data obtained from: Vörösmarty et al., (1993) RiverDischarge Database, Version 1.1 (RivDIS v1.1).

As such, the Nile for example receives much of its rainfall between June and October, seen by the peak in river discharge on the graph. In contrast, those rivers at lower latitudes receive their peak rainfall in the southern hemisphere summer rainy season beginning in December. Further, notice how the discharge values in the Oubangui River, which sits at lower latitudes, is much higher than the rivers at higher latitudes such as the Vaal River. Again, this attests to the higher levels of rainfall experienced at lower latitudes.