Progressive Gardening

Groundwater Resources of Sub Saharan Africa

Understanding the general distribution of water resources in SSA is made difficult by the paucity of data. According to the FAO (2003b, p. 51): 'The information available is uneven and very poor for some of the African countries.' In addition to basic data problems, the distribution of water within Africa is not equal and the continent has the greatest spatial, and temporal, supply variability of any region in the world (Walling, 1996), thus making broad overviews difficult. In general, though, rainfall is greatest on the Guinea coast and in the west-central regions, and drops as one moves east and away from the equator. Low rainfall regions also tend to have irregular rainfall, often leading to crop failures. The unequal rainfall distribution is offset to some degree by the prevalence of exotic rivers such as the Niger, Nile and Okovango. The rainfall and surface water patterns, along with underlying geology, determine groundwater availability, accessibility and its utility for agricultural use.

SSA is generally divided into four main hydrogeological provinces: crystalline basement complex, volcanic rock, consolidated sedimentary rock and unconsolidated sediments (Fig. 5.1). Of the four provinces, the basement complex is largest and occupies over 40% of the area including most of West Africa as well as Zambia, Zimbabwe, the northern belt of South Africa and northern

Hydrogeological domains

| | Precambrian 'basement' rocks

I Volcanic rocks

|_| Consolidated sedimentary

' " rocks (post Precambrian)

| | Unconsolidated sediments (mainly quaternary)

1:40,000,00 Robinson projection

Data sources

USGS Open File Report: 97-470A, 1997. UNTCD, Groundwater in North and West Africa, 1988. UNTCD, Groundwater in Eastern, Central and Southern Africa, 1988.

Foster SSD, African groundwater development - the challenges for hydrogeological science, IAHS 144, 1984. Guiraud R, "hydrogeologie de l'Afrique, Journal of African Earth Science, 7, 519-543, 1988.

Fig. 5.1. Hydrogeological provinces of Sub-Saharan Africa. (From BGS, 2000.)

Mozambique. Basement complex aquifers have very little or no primary porosity and the groundwater in them is held in the weathered mantle and in fissure zones. These aquifers are characterized by poor storage and low yields, typically less than 1 l/s (Field and Collier, 1998; UNEP, 2003, p. 17).

The second largest aquifer complex, consolidated sedimentary rock, underlies 32% of the area and can hold substantial groundwater reserves (Walling, 1996). However, mudstone areas, which make up approximately two-thirds of this variety, store little groundwater. Most of South Africa, Botswana, southern Angola, eastern Namibia, eastern Democratic Republic of Congo, north-west Zimbabwe and western Zambia are underlain by this aquifer type with limited groundwater occurrence.

Unconsolidated sediments make up 22% of the region's area and often hold groundwater in unconfined conditions within sands and gravels. These aquifers are often found in river beds, and so their groundwater may be especially important for human use due to potential ease of access (MacDonald and Davies, 2000). Unconsolidated sediments are found along the Limpopo River and several of its tributaries, and also in coastal areas, such as the Cape Flats aquifer in South Africa and the coastal zones of the countries at the horn of Africa (Fig. 5.1). However, Purkey and Vermillion (1995) note that many African river systems are typified by fine to very fine sediments, rather than coarse sand and gravel, thus reducing extraction possibilities.

Volcanic rocks cover only about 6% of SSA. In paleosoils and fractures between lava flows they can produce high groundwater yields and supply springs (MacDonald and Davies, 2000). In Djibouti, where groundwater represents 98% of all water used, volcanic aquifers are an important source of water (Jalludin and Razack, 2004). However, in other volcanic areas, groundwater storage can be highly limited (Walling, 1996).

To exemplify the low-yielding aquifers in many parts of SSA, Table 5.1 shows the typical yields in the main aquifers found in South Africa where groundwater studies have been more rigorous than elsewhere in SSA. In Botswana, yields of up to 27 l /s (Table 5.2) have been reported, but generally yields are less than 5 l /s. Where high yields have been found, these have been unsustainable in the long term as they decline rapidly due to limited storage in lower layers of the aquifers (Water Surveys Botswana, Colombo, 2003, unpublished data). In

aFrom WMA Report to IWMI (2003). bFrom Department of Water Affairs (2000).

aFrom WMA Report to IWMI (2003). bFrom Department of Water Affairs (2000).

the basement complex aquifer in Burkina Faso, yields are typically less than 1 l/s, whereas in the sedimentary aquifers yields reach 27 l /s (Obuobie and Barry, forthcoming). Planning for the use of groundwater in basement complex aquifers is further complicated by large seasonal variation in groundwater levels. These have been observed to range from 1 to 5 m in basement complex aquifers (Chilton and Foster, 1995). Depth to extractable groundwater appears to be another limiting factor for its use in SSA. In the Limpopo basin in South Africa depth to groundwater is highly variable, and borehole depths range from 50 to more than 100 m. In Lesotho, groundwater occurs mostly at depths of more than 50 m; in Zambia most boreholes are drilled to 44 m depth (Wurzel, 2001); in Zimbabwe borehole depths range from 25 to more than 100 m (Interconsult, 1986). In Mozambique, depth to extractable groundwater is up to 35 m in some areas, but can be up to 100 m in others. The high costs of abstraction associated with groundwater use including costs of unsuccessful drilling are seen as a major drawback to the use of groundwater in SSA.

The relationship between population distribution and SSA's groundwater provinces provides some insights into current agricultural groundwater use patterns and potential future development. Around three quarters of the SSA population lives in areas of poor groundwater availability, with 220 million people in low-yielding crystalline basement complex areas and about 110 million in areas of consolidated sediment. In these areas dwell most of the rural population, the socio-economic group often affected by problems of water access and who could potentially benefit from groundwater use. But because of the limiting factors alluded to above, there is a limit to how much groundwater they can use and the extent to which groundwater can impact their livelihood. Another 15% (60 million) of the population lives in areas with unconsolidated sediment, though most are not near areas with easy access to productive alluvial aquifers. The remaining 10% of the population (45 million) lives in volcanic rock zones with high but variable groundwater potential.

The most comprehensive water resource availability and use database for SSA to date is the fao aquastat. Although this database was originally designed with reference to agricultural use, it remains the most complete source of data for SSA. This database shows that for many of the SSA countries, groundwater is a small component of overall renewable water resources, suggesting limited contribution of groundwater to overall water requirements. Only 11 out of the 45 SSA countries listed in the aquastat database have at least 10% of their renewable water resources made up of groundwater (Table 5.3),1 and only 6 of these countries have per capita groundwater availability above 1000 m3. Per capita water availability of surface water in Africa is generally much higher than the groundwater availability indicated here (see Savenije and van der Zaag, 2000),

aDerived from aquastat. bFrom http://www.geohive.com cFrom literature.

aDerived from aquastat. bFrom http://www.geohive.com cFrom literature.

though in regions without surface water, groundwater becomes the only source available. Thus, to the region as a whole, groundwater will only play a relatively small role in agriculture because of the absolute levels of resource availability and the size of the resource relative to surface water. However, such generalizations can be misleading in national or even subnational contexts because of the great spatial and temporal variability in both ground and surface supplies.

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