Rainfall Variation

Maps showing 'The True Size of Africa'. Source

The sheer size of Africa is not something to be underestimated. The map above clearly illustrates the size of Africa in comparison to other countries or continents often perceived to be larger in size. Another common perception of Africa is that it is uniform in climate, however the large variability in Africa’s climate is one of the most important things that should be taken away from reading this post, and is one of the things that makes understanding the future of Africa’s climate so complex.

In order to understand the relationship between precipitation and water resources, it is first important to understand the ITCZ. The ITCZ is perhaps the most important control over the spatial and temporal variability in rainfall. An unequal heating of the earth’s surface leads to an atmospheric circulation mechanism in which hot air at the equator expands, loses density, rises, and then flows towards the polar regions. As this hot air flows towards the poles, it cools, increases in density and ultimately sinks towards the earth’s surface around the latitudes of 30°N and 30°S. As the air sinks it loses its moisture content and brings precipitation to the tropics. This dense air then flows towards areas of low pressure at the equator and the process begins again. This circular motion is known as the Hadley Cell.

Map showing the precipitation variation across Africa. Source

The ITCZ moves north and south throughout the year in response to solar radiation. As the ITCZ moves south in the southern hemisphere summer, there is an increase in precipitation in the southern latitudes where the Hadley Cell is now located. The ITCZ then moves northward after December (when the ITCZ reaches its southernmost latitude) and brings rain to northern latitudes until it reaches its northernmost point in July. This cycle means that the northernmost and southernmost latitudes only experience one annual wet season, whereas areas lying between these latitudes receive two due to the northward and southward movements of the ITCZ. This notion can clearly be seen on the above map where the darkest areas indicate a greater incidence of precipitation around the lower latitudes as a result of multiple wet seasons, whereas deserts such as the Sahara and the Namib occur on the periphery of the ITCZ in the lighter shaded areas. This should explain how the ITCZ is crucial in understanding the spatial and temporal variation in precipitation throughout Africa.

However, it is not only global climate circulation that affects the spatial and temporal variation in rainfall. One other significant factor is elevation (Taylor 2004). As elevation increases, air rich in moisture is forced upwards where the air then cools and reduces in its ability to hold moisture, thus inducing precipitation. Reduced precipitation can occur when there is a depletion of air containing moisture downwind from a mountainous area, generating a rain shadow. An example of this is the desert areas in the Horn of Africa that experience low precipitation rates as a result of the rain shadow produced by the Ethiopian Highlands.

To summarise, there is a great temporal and spatial variability of Africa’s climate, and an understanding of this is essential if we are to make sense of the variability in water resources in Africa, but also to comprehend how climate change will affect these water resources in the future.

Introduction...

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This blog will cover the effects that environmental change will have on hydrological systems throughout Africa. Africa is an incredibly diverse continent and the climate systems operating throughout differ considerably. You just need to look at the differences in landscape between Egypt and the Demographic Republic of the Congo to understand how considerable the differences in climate are. These differences are the result of the ITCZ moving poleward north and south of the equator, bringing extreme rainfall with it. The heterogeneous nature of Africa means that environmental change will affect different regions in different ways.

Environmental change can come in a number of forms, but the ones that will be discussed on this blog are climate change and land use change. Both climate change and land-use change can cause an increase or decrease in river runoff. deWit and Stankiewicz (2006) carried out a study where they assessed the changes that a 10% reduction in precipitation would have on drainage. In regions that experienced precipitation of over 1000 mm/yr, it was estimated that drainage would reduce by 17%, however in regions receiving 500 mm/yr of rainfall, drainage was expected to reduce by 50%, showing the disparity in the relationship between environmental change and water across Africa. In a study on land-use change in West Africa, it was found that deforestation and overgrazing had a significant effect on increasing runoff, but concluded by saying that any positive effects from an increase in runoff would be outweighed by the ecological damages that decreased vegetation coverage would consist of.

These two examples should give you a brief insight towards the complexity surrounding environmental change and water in Africa. As complex as this relationship may be, an understanding of it is vital if to aid government and administrative bodies in making quantitative based management decisions for the future provision of water resources.