Changes in Surface Water

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Based on previous blog posts, it should now be clear that the planet is warming and this is having a significant effect on precipitation and evapotranspiration rates. Although warming in the near future is guaranteed (see Fifth Assessment Reportof the Intergovernmental Panel on Climate Change), there is uncertainty in how warming an changes in precipitation will affect freshwater availability (Taylor et al 2009).

Africa’s rainfall variability is very, with 95% of Africa’s annual rainfall deviating by 20-40% from the mean (Carter and Parker 2009). This precipitation variation is high both seasonally and interannually, which has a considerable effect on river discharge (Mahé 2009). McMahon et al (2007) found that southern Africa has the greatest median coefficient of variation at 82% out of anywhere in the world, with the global median coefficient of variation paling in comparison at 31%. High rainfall and discharge variability will potentially have devastating implications for Africa’s 1.2 billion people, where lots of the populations water supply is reliant on local streams.

De Wit and Stankiewicz (2006) wrote an article that discussed how changes in surface water supply would occur as a result of climate change. They identified three climate regions in Africa: the dry regime receiving less than 400mm year ^-1 of precipitation; the intermediate regime receiving over 400mm year ^-1 of precipitation and the upper regime receiving over 1000mm year ^-1 of precipitation. It was found that areas in the dry regime had no perennial drainage, in the intermediate regime drainage then increased with increasing precipitation and then decreased once precipitation exceeded the threshold of 1000mm per year. This nonlinear response of drainage to rainfall had the most significant effect on the intermediate regime, where a 10% decrease in precipitation would reduce drainage by 50%. Considering that 75% of African countries fall into the unstable, intermediate regime, it can be said that a reduction in precipitation would significant reduce surface water supply for much of Africa.

One of the regions expected to be most severely threatened is southern Africa. Most of southern Africa lies in the unstable regime, with some areas falling in the dry, arid regime. Southern Africa is expected to experience considerable reductions in its already limited drainage, posing huge problems for the region in terms of drought relief. The densely populated areas around Cape Town are likely to be hit hard, with estimations of a loss of more than half of their perennial supply. This poses a huge threat to poverty eradication in southern Africa as humans are dependent on rivers for food, water and their livelihoods, with water often directly linked to economic development (Postel and Mastel 2005). 

This paper serves as a useful foundation in which African governments can begin to manage surface water resources in response to the threats from climate change. Perhaps the categorisation of Africa into only three regimes is an oversimplification of the issue, but this still provides the opportunity for academics and/or water management agencies to conduct more localised studies on areas of interests to further expand upon the results in this paper.

That Troublesome Thing Called Climate Change

From reading my previous blog post hopefully you now have an understanding of how climate variability, and more specifically precipitation variability, effects water availability across Africa. An example of this precipitation variation can be seen in the comparison of Malawi and Sudan. Malawi has a mean annual rainfall of approximately 1250mm, whereas Sudan’s mean annual rainfall does not even reach 200mm. It is now a generally accepted notion that climate change exists and this is having an impact on the global climate. In this blog post I will explain the effects of climate change on temperature and precipitation, and then briefly discuss the impacts on a change in climate on to the population of Africa.

As we are becoming increasingly concerned with the potential effects of climate change on Africa, more and more climate change models are being developed to try and understand the implications on water resources (see Kingston and Taylor 2010 for an example). Climate change scenarios are projecting an increase in temperature and therefore an increase in evaporation. Warming in Africa (and other Tropic regions) will be approximately 1.5x the global mean. This is because Africa has a considerable amount of land mass around the tropics meaning the continent will warm at a faster rate than other parts of the world (Joshi et al 2011). 

Graph showing the Clausius-Clapeyron relation. 

The Clausius-Clapeyron relation demonstrates that as temperatures increase, the capacity of air to hold moisture also increases which causes an intensification of the hydrological cycle (Owor et al 2009). However this graph shows that the relationship between temperature on the x axis and vapour pressure on the y axis – with vapour pressure increasing more rapidly when temperature has passed a certain threshold. This means that warming in the tropics where temperatures are already high leads to a greater ability of the air to hold moisture and therefore when it rains, it draws from a greater amount of moisture in the air. This explains how an increase in temperature will intensify precipitation, with the occurrence of low and medium intensity precipitation events decreasing but the occurrence of extreme precipitation events increasing (Allan and Soden 2008). This does however mean that rainfall will occur less frequently, which will increase the incidence of droughts. In areas of already low rainfall (see previous blog post) this will have devastating effects on their water resources. In a study by de Wit and Stankiewicz (2006), they found that there was a non-linear response of drainage to rainfall. They ultimately found that a 10% decrease in precipitation in unstable regions of Africa (defined as those receiving less than 500mm of precipitation annually) could experience a 50% decrease in surface drainage. This would then have significant implications upon river discharge and any activities along the river that that rely on sustained levels of discharge such as agriculture.

The IPCC Fifth Assessment Report chapter on Africa summarises that a 2°C mean annual rise in temperature is likely to occur in Africa but some scenarios estimate this increase could reach 6°C by 2100. Any increase in temperature will have a variety of effects on Africa due to the high variability in Africa’s climate. The closest universal rule that we have for understanding how climate change will affect different regions is that wet regions will become wetter and dry regions will become drier (Liu and Allan 2013). But with Africa’s ecosystems ranging from desert to tropical rainforests, there will be significant dissimilarities in the way that climate change will effect Africa.

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.