Climatic changes and groundwater resources in Africa Open Access

Warming is very likely to be larger than the global annual mean warming throughout the continent and in all seasons, with drier subtropical regions warming more than the moister tropics. Annual rainfall is likely to decrease in much of Mediterranean Africa and the Northern Sahara, with a greater likelihood of decreasing rainfall as the Mediterranean coast is approached. Rainfall in southern Africa is likely to decrease in much of the winter rainfall region and western margins. There is likely to be an increase in annual mean rainfall in East Africa. It is unclear how rainfall in the Sahel, the Guinean Coast and the southern Sahara will evolve according to IPCC (2007).

According to Ashton (2002), van Jaarsveld and van Pul (2005), and UNESCO‐WWAP (2006), water access and water resource management are highly variable across the continent. The 17 countries in West Africa according to Niasse (2007) found that share 25 transboundary rivers have notably high‐water interdependency. Eastern and southern African countries are also characterised by water stress brought about by climate variability and wider governance issues. Significant progress has, however, been recorded in some parts of Africa to improve this situation, with urban populations in the southern African region achieving improved water access over recent years by van Jaarsveld and van Pul (2005). When water is available, it is often of poor quality, thus contributing to a range of health problems including diarrhoea, intestinal worms and trachoma. Much of the suffering from lack of access to safe drinking water and sanitation is borne by the poor, those who live in degraded environments, and overwhelmingly by women and children. The relevance of the problem of water scarcity is evident in North Africa, considering that estimates for the average annual growth of the population are the world's highest: 2.9 per cent for the period 1990‐2002. The water exploitation index 2 is high in several countries in the sub‐region: >50 per cent for Tunisia, Algeria, Morocco, and Sudan, and water exploitation index is the total water abstraction per year as percentage of long‐term freshwater resources: >90 per cent for Egypt and Libya according to Toulmin and Guèye (2005). Until recently, these countries have adopted a supply‐oriented approach to managing their water resources.

Alley (2001) has found that there has been very little research on the impact of climate change on groundwater, including the question of how climate change will affect the relationship between surface waters and aquifers that are hydraulically connected.

Climate projections for Africa suggest that land areas may warm by as much as 1.6°C over the Sahara and semi‐arid parts of southern Africa by 2050 according to Giannini et al. (2007). Robust findings on regional climate change for mean and extreme precipitation, drought. are shown in Figure 1 which shows high correlation between precipitation and mean air temperature for African Sahel. The further inspection of this figure reveals that temperature and precipitation in the African Sahel are negatively correlated – seasonal warming accompanied late twentieth century drying. The regression equation that best fit the relation can be expressed as follows, according to Giannini et al. (2007): Equation 1 Equatorial countries (Cameroon, Uganda, and Kenya) might be about 1.4°C warmer. Sea‐surface temperatures in the open tropical oceans surrounding Africa will rise by less than the global average (i.e. only about 0.6‐0.8°C); the coastal regions of the continent therefore will warm more slowly than the continental interior.In southern Africa and parts of the Horn of Africa, rainfall is projected to decline by about 10 per cent by 2050. All of these changes could increase the drought frequency. Changes in sea level of about 25 centimeters according to IPCC (2001, 2007) might be expected by the year 2050. Climate change impacted on the decrease of runoff by 10 to 30 per cent include the Mediterranean, southern Africa, and western USA/northern Mexico according to Milly and Dunne (2005).These effects could mean locally‐severe, groundwater‐related impacts on water supplies, on property and on ecosystems that depend on groundwater. The impacts of climate change could increase the cost of providing water supplies, already rising as a result of deteriorating groundwater quality. Groundwater, of course, cannot be considered in isolation – impacts of climate change not necessarily related to groundwater, such as changing land use and population density, will have a knock‐on affect on groundwater.

A water level in an aquifer are often observed to respond consistently to precipitation, although the nature of the response can be complex and depends on time of year and prior conditions, etc. Groundwater levels correlate more strongly with precipitation than with temperature, but temperature becomes more important for shallow aquifers and in warm periods.

According to FAO Land and Water Bulletin‐4 (1997), the Niger River Basin (NRB), located in western Africa, (Figure 2) covers 7.5 per cent of the continent and spreads over ten countries. Algeria and Chad together cover about 9 per cent of the total NRB, Guinea cover about 6 per cent but the sources of the Niger River are located in this country, Côte d'Ivoire (1.5 per cent), Mali (26 per cent), Niger (23 per cent), and Nigeria (33 per cent). The most important areas of the Niger Basin are located in Mali, Niger and Nigeria. Mali and Niger are almost entirely dependent on the Niger River for their water resources. In the case of Niger, nearly 90 per cent of its total water resources originates outside its borders (the Niger River and other tributaries from Burkina Faso and Benin), Burkina Faso (4 per cent), Bénin (2 per cent), Cameroun (4 per cent), and Chad (1.0 per cent) but there are almost no renewable water resources in these areas. The quantity of water entering Mali from Guinea (40 cubic kilometers per year) is greater than the quantity of water entering Nigeria from Niger (36 cubic kilometers per year), about 1,800 henry further downstream. This is due among other reasons to the enormous reduction in runoff in the inner delta in Mali through seepage and evaporation combined with almost no runoff from the whole of the left bank in Mali and Niger.

NRB is very vulnerable to future climatic change due to:

• The manifest impact of the changes that have already occurred and exemplified by displacement in the position of rain belts (Figure 3(a)), and the great variability in precipitation patterns and intensities as expressed by scattering around mean value (Figure 3(b)).

• Reduction in the flow of Niger River.

• Lowering in water table of nearby shallow aquifers as shown in Figure 4.

The further inspection of Figure 3(a) shows the shifts of isoheytal contour lines of 200, 500, 700 and 1,000 millimeters for at least 100 kilometers southward in the Sahelian part of NRB.

The shallow aquifers in the nearby locations to Niger River (in Mali) has shown a very strong correlation with both the reduction in precipitation as well as the reduction in the flow of Niger river as a direct consequence to climatic change in West Africa (Figure 5).

The manifest impact of climatic change can be illustrated and confirmed using another data set of piezometers located in Dantiandou and Gogo localities (bounded by Longitude 02°45′30″ Latitude 13°24′40″), where the water level has responded to 1994 reduction in precipitation (Figure 4). The correlation is positive. However, the response is slightly delayed in the aquifer, attenuated with depth, and is more pronounced in unconfined than in semi‐confined aquifers (Sahara and Sahel Observatory – OSS Internal Reports). The same fact remains true for the abnormal rise in the water level of piezometers in Gogo location which corresponds to the heavy rainfall occurred for the years 1996 and 1999 (Figure 4).

The Iullemeden Aquifer System (IAS) is located in the arid and semi‐arid zones of West Africa (Figure 6). IAS extends between latitudes 10°30′ and 19°40′ North and longitudes 0°50′ and 9°20′ East. It is shared by Mali, Niger and Nigeria and covers a total area of 500,000 square kilometers (31,000 square kilometers in Mali, 434,000 square kilometers in Niger and 60,000 square kilometers in Nigeria). IAS is part of the hydrographic NRB.

Precipitations in the IAS area varies greatly from north to south: less than 150 millimeter in the Saharan zone, between 150 and 300 millimeters in the nomadic Sahelian zone, between 300 and 600 millimeters in the sedentary‐Sahelian zone and from 600 to 800 millimeters in the Sahel‐Sudanese zone (OSS Internal Report). IAS is very vulnerable to future climatic change due to:

• the manifest impact of the changes that have already occurred: a 20 to 30 per cent reduction of rainfall since 1968;

• 20 to 50 per cent reduction of runoff; and

• silting and sand dunes establishment.

Since 1968 to 1970, isohyetal lines expressing the same values have shifted southwards about 200 kilometer apart from the original position (Figure 7).

Groundwater resources in Iullemeden Basin are represented by multilayered aquifer system which encompasses two major water bearing formations; the upper Cretaceous (Cenomanian) argillaceous sandstones referred to as “Continental Intercalaire” and the Mio‐Pliocene continental sandy facies known as “Complex Terminal”. Both aquifers constitute the major groundwater resources in Iullemeden Basin.

As a direct consequence to climatic change the water levels in both aquifers were lowered as shown in Figure 8. The further inspection of Figure 8 shown the lowering in the piezometric head as well as the change in hydraulic gradient from steeper gradient and higher groundwater velocity in 1970 to a more gentler gradient and sluggish nature in 2004. The figure also shows a northward shift in piezometric head contour lines towards basin water divide which reflects a less recharge to the aquifer system.

Goddard and Graham (1999) and Yu and Rienecker (1998) have found that in Africa El‐Nino‐Southern Oscillation (ENSO) as well as surface sea temperature (SST) in the Indian Ocean are the dominant sources of climate variability over eastern Africa. Isolated secondary but significant pattern of regional climate variability has been identified and isolated by Schreck and Semazzi (2004). The trend pattern in their analysis is characterised by positive rainfall anomalies over the north‐eastern sector of eastern Africa (Ethiopia, Somalia, Kenya, and northern Uganda) and opposite conditions over the south‐western sector (Tanzania, southern parts of the Democratic Republic of the Congo and south‐western Uganda).

This signal significantly strengthened in recent decades. Warming is associated with an earlier onset of the rainy season over the north‐eastern Africa region and a late start over the southern sector. Examples of rainfall spatial‐ and time‐based variability in Africa are shown hereafter.

It is possible to distinguish between monsoonal and equatorial climates based on seasonality consideration and hence three African sub‐regions can be defined, western (0°N to 20°N, 20°W to 20°E), eastern equatorial (10°S to 10°N, 20°E to 50°E), and southern Africa (25°S to 10°S, 20°E to 40°E), according to Giannini et al. (2007). These regions are broadly consistent with those chosen by Hulme et al. (2001), who also presented the state‐of‐the‐art. The history of annual‐mean (July‐June) rainfall anomalies averaged over these regions is shown in Figure 9. Comparison of the three panels in Figure 9 shows the qualitative difference between the West African time series on one side, and its eastern equatorial and southern African counterparts on the other. West African rainfall is characterized by a high degree of persistence, of anomalously wet (e.g. in the 1930s, 1950s, and 1960s) and dry (e.g. in the 1970s and 1980s) years. Arguably, according to Trenberth et al. (2007), the shift in Sahel rainfall is unparalleled globally, in magnitude, spatial extent and duration. In eastern equatorial and southern Africa interannual variability is more conspicuous. So according to Giannini et al. (2007), when observations of precipitation over Africa are analyzed with a view to their global linkages, two continental‐scale patterns, related to variability in the oceans, appear to dominate African climate variability:

• 1.

  A continental‐scale drying pattern related to enhanced warming of the southern compared to the northern tropics and to a warming of the tropical oceans.

• 2.

  The impact of ENSO on the tropical atmosphere and oceans around Africa.

The response of groundwater systems is often difficult to detect because the magnitude of the response is lower and delayed. However, the effects of climate change on groundwater resources in Africa have included the following facts:

• a long‐term decline in groundwater storage;

• increased frequency and severity of groundwater droughts;

• increased frequency and severity of groundwater‐related floods;

• mobilisation of pollutants due to seasonally high‐water tables; and

• saline intrusion in coastal aquifers, due to sea‐level rise and resource reduction.

In West Africa, shallow aquifers in the nearby locations to Niger River (in Mali) has shown a very strong correlation with both the reduction in precipitation as well as the reduction in the flow of Niger river as a direct consequence to climatic change in West Africa.

In Africa, ENSO as well as SST in the Indian Ocean are the dominant sources of climate variability over eastern Africa.

Rainfall trend over South Africa has shown annual average of rainfall (1922‐1999) that illustrate the frequent persistence of a series of wet or dry years.

In conclusion, and based on the magnitudes and frequencies of rainfall and its variability in Africa, three different patterns of rainfall variability have been recognized; namely the:

• 1.

  northern/southern extremities of Africa, with a Mediterranean climate, and subject to future drying;

• 2.

  margins of monsoons, such as the Sahel, but also possibly a similar region in southern Africa; and

• 3.

  wetter equatorial regions.

Stakeholders as well as regular farmers should be encouraged and prepared for all types of adaptation practices.

Samir Anwar Al‐Gamal is a university Professor in Environmental Hydrology. He has been seconded to the Sahara and Sahel Observatory (OSS) as an advisor in water resources since August 2006. Before joining OSS, he was the Chairman of Siting and Environment Department in Egyptian Atomic Energy Authority and a Professor of Fluid Mechanics and Isotope Hydrology in MUST University and a Visiting Professor of Department of Physical Geography, Stockholm University. He has published more than 40 different articles in recognized journals such Journal of Acta Mineralogica (Belgium), Journal of Theoretical Climatology (Sweeden), Journal of Hydrology (The Netherlands), Journal of Environmental Hydrology (USA), Turkish Journal of Environmental Engineering (Turkey) and Journal of Nuclear Sciences (Egypt) most of which center most widely around the use of Isotope Hydrology Techniques in assessing water resources. Samir Anwar Al‐Gamal is the corresponding author and can be contacted at: samir.algamal@yahoo.com

Youba Sokona is a researcher at the OSS.

Abdel‐Kader Dodo is a researcher at the OSS.