Land Resource Stresses and Desertification in Africa
P.F. REICH, S.T. NUMBEM, R.A. ALMARAZ and H. ESWARAN
Published in: P.F. Reich, S.T. Numbem, R.A. Almaraz and H. Eswaran. 2001. Land resource stresses and desertification in Africa. In:Bridges, E.M., I.D. Hannam, L.R. Oldeman, F.W.T. Pening de Vries, S.J. Scherr, and S. Sompatpanit (eds.). Responses to Land Degradation. Proc. 2nd. International Conference on Land Degradation and Desertification, Khon Kaen, Thailand. Oxford Press, New Delhi, India.
Vulnerability to desertification in Africa is assessed using the information on soils, climate, and the previously evaluated land resource stresses. Desertification is, "land degradation in arid, semi-arid, and dry sub-humid areas resulting from various factors, including climatic variations and human activities". Excluded in the definition are areas that have a hyper-arid or a humid climate. The GIS Desertification Vulnerability map was coupled to an interpolated population density map to obtain estimates of the number of persons affected by desertification. Desertification processes affect about 46% of Africa. The significance of this large area becomes evident when one considers that about 43% of the continent is characterized as extreme deserts (the desert margins represent the areas with very high vulnerability). Only about 11% of the land mass is humid and by definition is excluded from desertification processes. There are about 2.5 million km2 of land under low risk, 3.6 million km2 under moderate risk, 4.6 million km2 under high risk, and 2.9 million km2 under very high risk. The region that has the highest propensity is located along the desert margins and occupies about 5% of the land mass. It is estimated that about 22 million people (2.9% of the total population) live in this area. The low, moderate, and high vulnerability classes occupy 14, 16, and 11% respectively and together impact about 485 million people.
The three basic concerns in much of Africa are population growth, agricultural performance, and environmental degradation (Cleaver and Schreiber, 1994). Population growth has increased from an average of 2.5% in the 1960s to more than 3% in the 1990s. Coupled to an increased lifespan from about 40 to about 50 years, the ability to feed and clothe the population is increasingly a challenge. The caloric intake of the average African is 90% of the level required by a normal healthy person. With a low purchasing power, the ability to supplement home-grown farm products is also very low. The productivity of the African farmer is low for many reasons (Eswaran et al., 1997b) including non-adoption of modern technologies, availability of capital for off-farm inputs, and land tenure. As a result, purchase of food by and food aid to many countries has increased dramatically in the last few decades.
It has been demonstrated around the world that low-input agriculture, particularly in the absence of appropriate conservation practices, leads to degradation of the land. As African farming is essentially a low-input low-output system (Badiane and Delgado, 1995), land degradation is rampant and recent studies show a progressive decrease in the performance of the land. The situation is further constrained by the geometric increase in animal population. When populations were low, shifting cultivation and transhumance pastorals were appropriate to circumvent declining productivity. Due to land availability and national regulations, these are no longer options. Per capita land has now declined from about 0.5 ha person-1 to less than 0.3 ha person-1. To survive, the landless are clearing forests, eking out a living on poor quality land or moving to the cities (Barnes, 1990). In addition, forest consumption is estimated at about 3 million ha per annum. Land clearing methods are traditional with slash-and-burn as an integral way to remove vegetation and reduce pests. Release of CO2 into the atmosphere contributes to global warming and there is also an important loss of biodiversity.
Land degradation and desertification have emerged as issues of global concern over the last few decades and have been given special prominence since the United Nations Conference on Environment and Development (UNCED, 1993) which issued Agenda 21—a blueprint to address global environmental problems. The Convention to Combat Desertification (which met in Senegal in December 1998) discussed, developed, and planned to fund a global action plan to combat the processes leading to desertification.
Modern discussions on the ability of the land to support its people probably started in the late 18th century when Thomas Malthus published An Essay on the Principle of Population. The notion of desertification was probably first introduced by Aubreville (1949) who evaluated the alarming degradation of land through erosion and other processes resulting from mismanagement by the resource-poor farmers of Africa.
The concept and emphasis of desertification as a degrading process requiring international collaboration has changed over time. Initially the focus was on desert margins (UNEP, 1986) though there was clear recognition that land degradation was a widespread problem. An accepted definition of the time was that of Dregne (1977): "Desertification is the impoverishment of terrestrial ecosystems under the impact of man. It is a process of deterioration in these ecosystems that can be measured by reduced productivity of desirable plants, undesirable alterations in the biomass and the diversity of the micro and macro flora and fauna, accelerated soil deterioration, and increased hazards for human occupancy". As national and global databases improved, the anthropic role became more evident and the accelerated nature of the process resulted in the call for combating actions.
The formal definition of desertification adopted by the United Nations Intergovernmental Convention to Combat Desertification is "land degradation in arid, semi-arid, and dry sub-humid areas resulting from various factors, including climatic variations and human activities". Excluded in the definition are areas that have a "very cold (boreal), hyper-arid or a humid" climate. The definitions of these climatic zones are related to the agro-ecological zones of FAO (1976) and due to an absence of comprehensive databases, maps depicting the affected regions show significant variations. The World Atlas of Desertification(UNEP, 1993) provides perhaps the most authoritative estimate of the distribution. This report is probably based on climatic analysis (details of the methods are not provided) with minimum use of soil resource information. However, it provides an estimate of the global area affected and is thus useful for global evaluation and action plan considerations.
Land degradation is one of the consequences of mismanagement of land and results frequently from a mismatch between land quality and land use (Beinroth et al., 1994). Land degradation, due to the large area and number of people affected by it, is clearly human induced. The linkage between land degradation and climate change is yet to be established but there is increasing evidence that land degradation is a driver of climate change. The other causes of land degradation include drought, population pressure, failure to implement appropriate technologies, poverty, constraints imposed by recent international trading agreements, and local agricultural and land use policies (Virmani et al., 1994).
In the present assessment, a method was devised to assess vulnerability to desertification based on soil resource conditions and climate. Such an initial assessment would provide a basis for more detailed on-site assessments of those areas where the problem is critical. A second aspect is to evaluate the number of people affected by the process. This presents initial data on risks and is a basis for mitigating technologies and corrective measures.
Reliable databases are currently inadequate to unequivocally address desertification in Africa. However, information on African biophysical resources (Eswaran et al., 1997a,b and reproduced in Table 1), enables an assessment of vulnerability of the land resource base. The actual intensity of the desertification process is a function of socioeconomic and other human factors. A global soil and climate GIS database of Africa developed by Eswaran et al. (1997a,b) was used to provide the biophysical resource characteristics. For the assessment of vulnerability to desertification, soil units were empirically assigned to vulnerability classes after excluding regions that were cold, dry or humid as per UNEP's definition of desertification. The procedure of Eswaran and Reich (1998) employing the following considerations is used in this assessment:
Published reports of degradation (Oldeman et al., 1991) were used to validate specific locations on the map. The final map is at a scale of 1:30,000,000 and areas of the different classes are computed. To evaluate the number of people affected, a map of vulnerability to desertification was superimposed on an interpolated population density map developed by Deichmann (1994).
Land resource conditions
Land quality is the ability of the land to perform its functions in a sustainable manner. For detailed land use assessments, quantitative land quality assessments can be made. As the intent here is to make a qualitative continent-level assessment, only factors that reduce the ability of the land to perform in an optimal manner for agriculture are considered. Constraints to sustained use of land are multiple and include socioeconomic constraints, which are not considered here. Only the biophysical constraints are evaluated with the objective of having some continent-level estimates.
Soil moisture stress, as indicated earlier, is perhaps the overriding constraint in much of Africa. As shown by Eswaran et al. (1997a,b) about 14% of Africa is relatively free of moisture stress. The other major stresses discussed here relate to soil properties and are considered individually, although the same soil may have more than one of these constraints. A more detailed GIS analysis than the one attempted here will provide details of areas with multiple stresses.
Moisture stress is not only a function of low and erratic precipitation but also of the ability of the soil to hold and release moisture. About 10% of the soils have high to very high available water holding capacity (AWHC). These are mainly the Mollisols, Vertisols, and other clayey soils with 2:1 layer lattice clays. The 29% of soils with medium AWHC are mainly the Alfisols and Ultisols and some loamy Inceptisols and Entisols. The low AWHC class soils are the Oxisols and other sandy loam soils. Despite their clayey textures, Oxisols have low AWHC. The very low AWHC class soils are the sandy soils such as Psamments and other sandy and sandy loam soils.
In the arid and semi-arid parts of Africa, salinity and alkalinity are major problems affecting about 24% of the continent. These are included in the soils designated as having a pH >8.5. The extremely acid soils, which are mainly the acid sulphate soils (both potential and actual) occupy a small area around the Niger Delta and occur sporadically along the coastal plains of West Africa. The soils that have high aluminum problems occupy about 15% of the continent and are mainly in the moist parts of the semi-arid zones and the sub-humid areas. Many of the Ultisols and some Alfisols have acid surface and sub-surface horizons which, coupled to the moisture stress conditions, make these soils extremely difficult to manage under low-input conditions. In West Africa, the annual additions of dust from the Sahara brought by the Harmattan winds, raise the pH of the surface horizons and so the problem is less acute, but subsoil acidity remains.
Soil depth is frequently understood as being depth to rock or an impermeable layer. The term ‘effective soil depth' is used here to include chemical barriers, which reduce the volume of the soil for root exploitation. Limited effective soil depth is a problem in more than 50% of the soils and this reduces the potential of the soil for crop production.
Wind and water erosion is extensive in many parts of Africa. Excluding the current deserts, which occupy about 46% of the land mass, about 25% of the land is prone to water erosion and about 22% to wind erosion. High intensities of these erosion forms occur mainly in the semi-arid and sub-humid areas. The soils in Central Africa are largely low activity Oxisols and Ultisols and are less susceptible to water erosion, unless severely mismanaged. These estimates include human-induced erosion, good estimates for which are available in Oldeman et al., (1991).
Assessment of sustainable development potentials
Soil properties, including soil climate, provide some preliminary information to address land quality. Land quality is used here to indicate the ability of the soil to perform its function of sustaining agriculture under the current low-input system of agriculture. There are areas, particularly in South Africa and some other countries, where agriculture is characterized by medium- or high-input systems and a separate assessment can be made for such situations. Low input implies that large-scale irrigation is absent, use of fertilizers, and pest and weed control is minimal, and soil management does not require high-energy mechanized equipment. The soil classification name provides adequate information to make this preliminary assessment on a continental scale. The properties incorporated in the soil name are empirically related to crop performance and a judgment is made on the potential of the soil for sustaining agriculture. A more refined assessment would require a significantly larger database.
Land resource stresses and land quality
Using information from the soil and climate resources of Africa, an assessment was made initially of the land resource stresses in the country. Twenty-five stress classes were defined and prioritized according to the severity of the constraint in terms of the effort required to correct it for agricultural use and the data is presented in Table 2. The units on the soil map of Africa were sequentially tested (commencing from class 25) until the soil condition was matched by one of the classes. Soils that did not match any of the classes 2–25, were considered to have few constraints and the inherent land quality (ILQ) was designated as Class I. Depending on the severity of the constraint, subsequent stress classes are assigned to ILQs. The results of the spatial analysis using geographic information system are presented in Table 2.
Fifty-five percent of the land is unsuitable for any kind of sustainable agriculture apart from nomadic grazing. These lands are largely the deserts that include salt flats, dune, and rock lands, and the steep to very steep lands. The constraints for managed agricultural systems are so great that such lands must be maintained in their natural state. Though some soils in desert areas may have favorable attributes for irrigated agriculture, their quality in this general appraisal is ranked low for two reasons. First, the high initial and continuing investments needed to assure the required performance of the soils, and secondly, the rapid buildup of salinity and/or alkalinity that is a continuing problem in this environment. Though agriculture on these lands is considered unsustainable, about 30% of the population of Africa or about 250 million people are living on or are dependent on this land (Eswaran et al., 1997a,b). A large portion of these people live on the desert margins of the Sahara and Kalahari regions and along the larger rivers that traverse the areas, such as the Nile and the Okavango Delta.
Classes I to IV occupy about 10.6% of the land area (Table 2). About 3.1 million km2 of land with these qualities support about 400 million people. The soils are relatively free of major constraints and rainfall is usually stable and adequate for one major crop. Moisture stress ranges are minimal and when present, rainfall is confined to the short dry season. Zones with adequate rain during the year and generally with a dry season of less than one or two months, have some form of plantation agriculture or are under forests.
The soils with low quality (Classes V and VI) are those that have limitations which are not easily overcome with low-input agriculture. The limitations include high soil acidity, impermeable layers in the soil, frequent waterlogging, propensity to accumulate salts or some attributes, which require major investments to correct and to manage. Such soils occupy about 38% or 11.2 million km2 of land and currently support about 200 million persons (23%). The quality of these soils may be an inherent property of the land or it may be due to human-induced degradation over long periods of time (Oldeman et al., 1991). The present analysis does not make this distinction.
Vulnerability to desertification
The results of the empirical assessment of vulnerability to desertification are presented in Table 3 and a map showing the distribution of these classes in Africa is presented in Figure 1. The region that has high propensity is located along the desert margins and occupies about 5% of the land mass. It is estimated that about 22 million people (2.9% of the total population) live in this area. The low, moderate, and high vulnerability classes occupy 14, 16, and 11% respectively and together impact about 485 million people.
Table 4 presents estimates of vulnerable areas for each country of Africa. Practically every country of Africa is prone to desertification, but the Sahelian countries at the southern fringe of the Sahara are particularly vulnerable. Only about 19% of Niger is non-desert and of this 17% belongs to high and very high vulnerability classes. The Mediterranean countries of North Africa have similar large areas subject to this land degradation process as do countries on the fringe of the Kalahari Desert.
Figure 1. Desertification vulnerability of Africa
Given the distribution of soils with different qualities, the need to attain a semblance of food security by existing populations coupled to the potentials of desertification, an important concern, as posed by Conway and Barbier (1988), is the level of risk faced by the people with respect to sustainability. For this analysis, risk is considered in terms of the quality of the land and the level of inputs. This approach is similar to that of FAO (1982) in their study of the population-carrying capacity of countries. A low quality soil under a traditional low-input system poses a very high risk for the farmer. However, it does not imply that a soil with high quality but under a similar low-input system has a lower risk. Such a land may be initially productive but with mismanagement, productivity may be quickly lost and thus, risk of agricultural sustainability is still high. In this study, risk of sustainability is evaluated in a qualitative way, which can be significantly improved when better and more reliable databases become available.
It is estimated that currently, there is about 22% (6.7 million km2) of land which has a low to very low risk for sustainable use. As inputs increase and management is more refined and responsible, an additional 4 million km2 of land can be brought into this class. At the other extreme, there is about 54% (16.6 million km2) of land that has very high to excessive risk. The lands with very high risk (8.2% or 0.7 million km2) are mostly at the desert margins in the Sahel and along the eastern seaboard of Africa. There is also a large contiguous area in Zaire where the Kalahari sands have penetrated. About 200 million people live on these deserts and desert margins.
Risk of human-induced desertification
Accelerated land degradation takes place with increasing population density and particularly under low-input systems. In some situations, this generalization may not be true but this assumption was made to evaluate risk of human-induced desertification in Africa. Three classes of population density (<10, 11–40, and >41 persons km2) were used and a map of these three classes was superimposed on the vulnerability to desertification map. A matrix (Table 5) was developed to relate vulnerability and population density to risk of human-induced desertification.
As shown in Table 5, a high population density in an area that is highly vulnerable to desertification poses a very high risk for further land degradation. Conversely, a low population density in an area where the vulnerability is also low, poses in principle, a low risk. Figure 2 shows the distribution of the risk classes. The Mediterranean countries of North Africa are very highly prone to desertification. In Morocco, for example, erosion is so extensive that the petrocalcic horizon of some Palexeralfs is exposed at the surface. In the Sahel, there are pockets of very high risk areas. The West African countries, with their dense populations, have a major problem in containing the processes of land degradation. Table 6 provides the area in each of the classes of Table 5.
About 2.5 million km2 of land are low risk, 3.6 million km2 are moderate risk, 4.6 million km2 are high risk, and 2.9 million km2 are very high risk.
Figure 2. Risk of human-induced desertification
The challenge to African agriculture is not only to enhance production to meet the increased food demands of the expanding population, but also the judicious use of soils so that their productivity is sustained in the foreseeable future. The earlier study of Eswaran et al. (1997a,b) showed that 55% of the land area in Africa is unsuitable for agriculture, and 11% has high quality soils that can effectively be managed to sustain more than double its current population. These soils are spread among many countries making it difficult to develop a continent-level strategy to equitably help all countries. Africa has more than 8 million km2 of land with rainfed crop potential, however much of it has not been used for this purpose. This potential land reserve needs to be carefully evaluated so that rational policies can be developed for their exploitation. The present study shows:
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