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).

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.

Temporal and Spatial Patterns of Rainfall across Africa

The broad spatial and temporal distributions in rainfall over Africa are controlled to a large extent by global patterns of atmospheric circulation in the tropics.

Atmospheric circulation occurs as a result of pressure gradients that develop from unequal heating of the Earth’s surface (Taylor, 2004). Near the Equator, moisture-laden heated air expands, becomes less dense and rises at the inter-tropical convergence zone (ITCZ). When the air rises, it cools, and as cooler air is not able to hold as much moisture, forms condensate and rains. Thus this convergence zone is associated with heavy rainfall. As this air then flows poleward on either side of the Equator, it cools, becomes denser and eventually sinks towards the Earth’s surface at latitudes of approximately 30 °N and 30 °S (Taylor, 2004). The air descending after shedding precipitation is comparatively dry, and thus doesn’t deliver very much rainfall to these areas. The subsiding air then diverges so a portion returns to the low-pressure belt at the Equator and completes a cycle, known as the Hadley Cell (Taylor, 2004). These trade winds are dry at source but collect moisture as they blow towards the Equator before converging at the ITCZ (Taylor, 2004).

Figure 1. Diagram displaying Hadley Cell circulation and the formation of the ITCZ at the Equator. (Source) 

Temporal Patterns of Rainfall

As the world’s second largest continent, covering approximately 30.2 million km² (20.4% of Earth’s total land area) (Sayre, 1999) and extending from 37°N to 34°S (Lewin, 1924), Africa has a significant amount of landmass both above and below the Equator. Annual rainfall over tropical Africa is characterised by a strong seasonality with summer monsoonal rainfall (Ziegler et al., 2013) determined by migration of the ITCZ north and south over the continent as a result of latitudinal variations in solar radiation (Taylor, 2004). After July, the ITCZ moves southwards bringing heavy rainfall to progressively more southern latitudes until reaching its southernmost latitude in late December when solar radiation is at its peak in the Southern Hemisphere. The ITCZ then moves northward bringing heavy rainfall to progressively more northern latitudes until it reaches its northernmost latitude in July, before returning south (Taylor, 2004). This movement of the ITCZ across Africa is illustrated in Figure 2, which shows the southerly regions of Africa receiving more rainfall in austral summer, and more northerly latitudes receiving more rainfall in austral winter.

Figure 2. Rainfall variability over Africa in (a) January and (b) August. The colour bar indicates days per month with measurable rainfall. (Source)

This annual cycle delivers one distinct influx of moisture (a unimodal rainfall distribution) to latitudes at the southern (e.g. Sahel-Sudanian region) and northern (e.g. areas such as Zambia, Zimbabwe) limits of the ITCZ’s latitudinal course, which experience pronounced and often extreme wet and dry seasons (Taylor, 2004; UNEP, 2012). In contrast, lower latitudes experience two rainy seasons (a bimodal rainfall distribution) as a result of both northward and southward movements of the ITCZ between each solstice (Figure 3). So looking back at Figure 2 – essentially between those two red lines is humid Africa which receives rainfall throughout much of the year – and above and below those lines is semi-arid to arid Africa.

Figure 3. Diagram displaying the relationship between the annual migration of the ITCZ and the amount of rainfall received at different latitudes throughout the year. (Source)

Spatial Patterns of Rainfall

Latitudinal migration of the ITCZ determined not only seasonality in rainfall across tropical Africa, but also the broad spatial distribution. Figure 3 shows that environments at lower latitudes (between 10 °N – 10 °S) receive a greater amount of rainfall as they are influenced by heavy rainfall associated with the ITCZ for a greater proportion of the year due to its double passing (Taylor, 2004). This is further exemplified by Figure 2 showing countries such as the latitudinally central Democratic Republic of Congo, Uganda and Kenya substantial measurable rainfall in both austral summer (January) and winter (August). This rainfall pattern is broadly reflected in Figure 4, which shows the annual rainfall surplus (rainfall minus evapotranspiration) decreasing with increasing distance north and south from the Equator. Areas between the two red lines of Figure 2 are largely covered in blue, and areas outside are indicated in grey and yellow by a rainfall deficit. There is a latitudinal symmetry, with the deserts of the Kalahari in Southern African occurring where the pole-ward side of the Hadley Cells are coming down and delivering little moisture. The surplus is however constrained by evapotranspiration, which means 70-90% of rainfall across most parts of Africa is returned to the atmosphere. Approximately 66% of Africa is classified as arid/semi-arid, with extreme variability in rainfall (UNEP,2012).

Figure 4. Annual water balance: an estimate of available runoff after accounting for evapotranspiration. Yellow indicates areas of runoff deficit; blue indicates areas of runoff surplus. (Source)

An Introduction!

Greeting my curious reader!

Today I shall introduce you to the wondrous world of climate change and water in Africa, the start of a journey that shall take us through many problems, solutions, case studies and more! Changing water supply in Africa with regards to environmental change is a hugely complex and varied issue, of paramount importance to the daily lives of many African citizens. Africa as a continent already experiences greater water variability in space and time than any other place on the planet, and this variability has already caused widespread human suffering and economic damage (Conway et al., 2009). In many cases, climate and land use change will be imposed onto already water-stressed catchments, hindering economic development objectives.

Why environmental and land use change?

My particular fascination of this topic stems from its position on the interface of two worlds (figuratively speaking!) – one being the physical, scientific analysis of future projections of climate change over Africa, and its impact on factors such as temperature, precipitation and ecosystem change. The other ‘world’ refers to the translation of these changes into the human impact, through factors such as food security, domestic water availability and perhaps even water-based conflict. All in all, this makes the task of writing a weekly blog on the matter fairly daunting, but also exciting!

Figure 1. A woman pumps groundwater supplies. Source: Wikimedia Commons.

Why is this a topic worth blogging about?

The mean annual temperature rise over Africa is likely to reach 2°C by the end of the 21^st century, in relation to the late 20^th century (Niang et al., 2014). To give a brief idea of the effects climate change would have on African water supplies, we can turn to Wit and Stankiewicz (2006). The non-linear response of surface water supply to rainfall is of particular importance when considering the effects of climate change on precipitation variability across Africa, especially seeing as a large proportion of the population rely on local rivers for water supply. 75% of African countries currently fall into an “unstable” climatic regime, whereby a small change in precipitation caused by climate change could cause considerable changes in the surface water supply.

Figure 2. The yellow areas refer to those currently in the “unstable” climatic regime, which will experience the most change in surface water drainage density with changes in regional annual rainfall. The regime makes up 25% of the total area. Source: Wit and Stankiewicz, 2006. 

A 10% decrease in precipitation over sub-Saharan Africa by 2050 would mean that those regions on the upper boundary of this climatic regime (i.e. receiving 1000 mm/year) would experience reduced surface water drainage of 17% - but the fate is even worse for those towards the lower end – regions receiving 500 mm/year would drop in drainage by half (Wit and Stankiewicz, 2006). Much of southern Africa already falls into this unstable regime, and much is projected to experience considerable losses of the surface water drainage it does possess, with IPCC projections (IPCC SRES, 2000). Imagine the impact this would have on a rural village that depends on local river water – their only water supply would drop to half its previous volume – that’s half the amount of water left for cooking, drinking, sanitation and agriculture!

Of course this is an extremely simplified overview of likely water changes over Africa, but I hope to explore these and other impacts in much more detail over the course of this blog. It’s important to remember that Africa is not one homogeneous unit; it possesses a host of communities both rural and urban, and the continent’s different regions will be impacted and respond to climate change in different ways. As well as future projections of physical climate parameters, I shall assess the human problems they will pose on both rural and urban communities, such as impacts on food security, spread of disease and potential for conflict. And, in a more positive light, I shall explore the strengths and weaknesses of the various strategies for adaptation and mitigation of these climate changes and impacts.