Part I

Desertification, Drought, and the Global Environment

Interrelationships between the global climate system, drought and desertification, including the impact of desertification on climate change and global warming.
Presented by : Professor Robert Balling, Arizona State University, USA

Desertification and desiccation as a threat to the conservation and utilization of biodiverty.
Presented by : Professor A.M. Imbevbore, Obaferni Awolowo University, Nigeria

Effects of desertification and dessication on ground and surface hydrological systems, water availability and water quality.
Presented by : Mr. Habib Zebidi, Director, Hydrology Division, United Nations Educational, Scientific and Cultural Organization (UNESCO)

Modern techniques for assessing the global environment, including satellite imagery, remote sensing, and geographic information systems.
Presented by : Dr. Norberto Fernandez, United Nations Environment Programme (UNEP)

Interrelationships between the global climate system, drought and desertification, including the impact of desertification on climate change and global warming

Presented by : Professor Robert C. Balling, Arizona State University, USA

Introduction

Over the past several decades, desertification has emerged as one of the major research themes in climatology. This focus on desertification is driven, in part, by : (a) the ongoing drought and associated human tragedy in the Sahelian region, (b) an intense interest by atmospheric scientists in the interactions between surface processes and climate, and (c) the many important questions regarding how the current greenhouse gas build up may impact global and regional climates. The scientific literature is full of articles dealing with the interaction between desertification and climate (see bibliography section); in this short review paper, some of the key concepts and themes from this emerging literature are discussed.

Climate variability in drylands

Many researchers have noted that dryland climates are prone to unusually high levels of variability through both time and space (for excellent reviews, see Druyan, 1989; Nicholson and Palao, 1993). Recognizing the need to characterize this variability (particularly the high level of variability in precipitation levels), scientists have applied a battery of descriptive statistics to climate data from dryland areas. They generally have concluded that precipitation data (and closely related drought data) show : (a) little persistence from year-to-year or season-to-season and (b) little spatial coherence in the observed patterns of precipitation. The statistical tools vary from study to study, but the results reconfirm the fact that climates in drylands, and particularly precipitation-related parameters, are extremely variable in both time and space.

Recognizing the variability of climate through time in these dryland areas, many scientists have attempted to identify statistically significant cycles in local and regional precipitation data (e.g., Adejuwon et al., 1990; Anyadike, 1992; Nicholson and Palao, 1993). In many studies, there is a proposed connection between dryland precipitation levels and sunspot activity. Spectral analytical techniques have not been able to identify prominent cycles in all or most dryland areas - the results vary tremendously from one study to the next. In some areas during some time periods, certain statistically significant cycles are found in the data. However, the results are rarely found to be the same in datasets from nearby locations or from different time periods.

While the variability of climate data from dryland areas has received attention, trends in the data have been widely discussed throughout the entire literature. For example, as shown in Figure 1, the most prominent feature in Sahelian rainfall data is the downward trend of the last 30-40 years. This pattern is seen consistently in precipitation and drought records from throughout the Sahelian area, but the pattern has not been noted in all dryland areas of the world. In fact, in other regions, the opposite pattern has been observed (Diaz et al. 1989).

Explaining and predicting dryland climate patterns

Obviously, identification of the time and space variations in dryland climates is an important scientific step, however, identification of the patterns raises the next question regarding the causes of the variations. Once the underlying causal mechanisms are understood, then the possibility exists from actually predicting dryland climate patterns months or seasons in advance. Here lies a major focus in the modern literature - inspection of the many titles in the bibliography section will illustrate the importance of this research focus.

Many of the researchers have chosen an empirical approach to explaining temporal variability in dryland climates; particular emphases has been placed on explaining variance in precipitation patterns (see reviews by Lamb, 1979; Druyan, 1989; Nicholson, 1989). Statistical techniques are almost always multivariate with combinations of principal components and regression analyses remaining popular. The precipitation patterns through time act as dependent variables, and various combinations of the following serve as independent, predictor variables.

i. Global and regional sea-surface temperatures show a strong correlation with precipitation patterns several months and seasons in advance (e.g., Lough, 1986; Bah, 1987; Nicholson and Entekhabi, 1987; Semazzi et al., 1988; Fontaine and Bigot, 1993). The strength of these correlations and the month-to-month persistence of the sea-surface temperature patterns combine to allow precipitation predictions to be made, with limited accuracy, several months in advance. Linkages to El Niño/Southern Oscillation events have been popular in this research direction.

ii. Many investigators have quantitatively or qualitatively described the linkage between selected atmospheric general circulation patterns and observed patterns in dryland precipitation. In the case of the Sahel, the synoptic patterns associated with dry or wet conditions have been described in great detail (e.g., Lamb, 1978; Nicholson, 1981; Lockwood, 1986; Wolter, 1989).

iii. Climate variations on a global scale are known to be forced, in part, by volcanic eruptions and output from the sun. Recognizing the global-scale linkages, some scientists have attempted to show the linkage between regional dryland climatic variations and volcanic eruptions and/or solar activity (e.g., Wood and Lovett, 1974; Landsberg, 1975). The correlations are not particularly strong and the results have varied considerably from place to place.

iv. Several scientists have determined that teleconnections exist between precipitation in drylands and unusual meteorological conditions in other parts of the world (Gray, 1990; Landsea and Gray, 1992). One study by Landsea and Gray (1992) has received considerable attention; their results suggest a strong linkage between precipitation in the Sahel and the appearance of strong hurricanes striking the eastern coast of North America.

v. Antecedent rainfall levels have been shown to have a reasonably high correlation with concurrent precipitation totals in dryland areas (e.g., Winstanley, 1974). Findings from these studies reinforce the notion that surface conditions in drylands have an important role in determining observed precipitation patterns.

vi. Many investigators have shown that human activities in drylands can be likened statistically to trends in temperature and precipitation (e.g., Charney, 1975; Berkofsky, 1976; Otterman, 1981; Balling, 1988; 1991). Their results (discussed in the next section) strongly suggest that humans and their activities have a recognizable, statistically significant impact on local and regional climate conditions.

All of the research discussed thus far has been based on an empirical approach that is based heavily on statistical analysis. However, another line of research is far more theoretical - researchers in this line rely on numerical climate models for their work. In many of their studies, the statistical linkages that appear in the climate records are explored via the existing global climate models. The literature contains many articles in which the theoretically-based models produce patterns that are consistent with the statistical associations (e.g., Anthes, 1984; Sud and Fennesy, 1984; Cunnington and Rowntree, 1986; Novak, 1990; Druyan and Hastenrath, 1992; Franchito and Rao, 1992). These modelling studies allow scientists to better understand the physical processes that underlie the statistical relationships determined by the empiricists. Ongoing developments in these models may serve to increase the predictability of climate patterns on a monthly or seasonal basis.

Human impacts on dryland climates

Human activities have impacted the surface and the atmosphere in drylands by : (a) reducing vegetation cover (by overgrazing, cultivation, deforestation), (b) increasing the surface albedo, (c) decreasing the roughness thereby increasing wind speeds, (d) altering soil moisture patters, and (e) burning vegetation and dislodging dust at the surface. With these, and many other, changes occurring at the surface in dryland areas, generalizations about the overall climate impact of human activities are difficult to generate.

In fact, a significant debate exists regarding some of the most fundamental questions regarding the human impact on the climate system (for a review, see Lockwood, 1986; Balling, 1988). For example, many scientists believe that human activities increase surface albedo, this increase in albedo cools the surface, the atmosphere becomes more stable, and rainfall via convective processes is reduced. These scientists have theoretical models and empirical data to support their beliefs. However, other scientists argue that human activities may impact albedo slightly, but these same activities lead to a decrease in vegetation and soil moisture. These changes cause a warming at the surface that may de-stabilize the atmosphere and enhance the changes for convective rainfall. Any increase in rainfall would be made ineffective due to an increase in evapotranspiration rates. Like the other group, these scientists have theoretical models and empirical datasets to support their viewpoint. And there is yet another group of scientists concluding that human activities have minimally impacted the climate in the dryland areas.

While the arguments continue as to whether human activities warm, cool, and leave climate unaffected, there is yet another debatable issue regarding the scale of the climate impact. Many scientists have been able to show evidence for local and regional climate impacts of human activities, but climate changes at the hemispheric or global scales are much more difficult to detect with any degree of confidence.

Theoretical and empirical arguments on the climate impact of human activities in drylands are likely to continue for years to come. One generalization can be made despite the debate - any human-induced climate change in drylands likely have exacerbated desertification processes. Some argue that dryland precipitation has been reduced due to human activities, others argue that evapotranspiration increases have overwhelmed any small rise in precipitation levels; both outcomes contribute to desertification processes.

Greenhouse Effect and Desertification

Scientists have shown convinsingly that human activities around the world are increasing the atmospheric concentration of many greenhouse gases (e.g., carbon dioxide, methane, nitrous oxides, chlorofluorocarbons). Due to turbulent mixing in the atmosphere, concentrations in these gases have increased in all parts of the world, including even the most remote drylands. So while dryland areas are not major sources or sinks of greenhouse gases, their atmospheric concentrations of the gases have increased right along with the global concentration levels.

Numerical models from laboratories around the world have been used to simulate the climate impact of the build up of the greenhouse gases. The model outputs vary considerably even at the global scale - regional scale predictions for future concentrations of the various greenhouse gases must be treated with some caution. Nonetheless, the following generalizations can be reached regarding predicted future climate changes in many dryland areas (rind et al., 1990; Le Houérou, 1992):

i. Dryland temperatures are projected to increase by 2oC to 5oC for the doubling of greenhouse gas concentrations expected to occur near the middle of the next century.

ii. Predictions for future precipitation levels are extremely varied from model to model and from region to region.

iii. Increases in evapotranspiration rates (associated with the increase in temperature) will overwhelm any changes in rainfall. Soil moisture levels will decrease and droughts will become more frequent according to many of the models.

iv. Changes in temperature, precipitation, and soil moisture are likely to deteriorate surface conditions, particularly in dryland areas that are prone to desertification processes.

One must keep in mind that these are theoretical predictions made by developing numerical climate models, and a debate regarding the validity of these greenhouse predictions is raging throughout the scientific community. The basic problem is that we have already gone about half way to doubling the concentration of the anthropo-generated greenhouse gases and many climate signals related to the greenhouse effect should be evident. Many of these signals are clearly missing, others are difficult to find; this has led to the debate between the theoretical and empirical climate scientists. Nonetheless, it is noteworthy that empirical research shows desertified areas to be warming much faster than non-desertified areas over the past century.

We may be debating the climate impact of increasing greenhouse gases for decades to come, particularly if the issue is a climate change predicted for a specific region of the earth. Many argue that we cannot delay policy actions until the scientific debate is resolved - they argue that many of the mitigation strategies aimed at combating desertification produce a win-win situation. They argue that any increase in vegetation in drylands can uptake atmospheric carbon dioxide, thereby helping to alleviate the buildup of the major anthropo-generated greenhouse gas. The same increase in vegetation will likely increase soil moisture levels, cool any rising temperatures, and possibly increase rainfall totals. These scientists are able to cite considerable theoretical and empirical support for their arguments.

Conclusions

Based on my own reading of the professional literature, the following generalization can be made (with reasonable confidence) about the interactions between desertification and climate:

i. Dryland climates have always been prone to extreme variability, particularly when dealing with precipitation levels. This extreme variability makes it difficult to clearly identify the human impacts on the local and regional climatic patterns.

ii. Despite the high levels of variability, monthly and seasonal prediction of dryland climate patterns are becoming increasingly accurate. Empirical research linking precipitation levels to sea- surface conditions and associated atmospheric circulation patterns is very promising; the continued development of global numerical climate models will also lead to an increase in the understanding and predictability of dryland climate variations.

iii. Human activities are impacting the surface in drylands and in turn, they are influencing the local and regional climates. Most human-induced changes in climate have exacerbated the desertification processes.

iv. Numerical models of climate predict significant warming in dryland areas for a doubling of the greenhouse gases; this predicted warming is predicted to increase evapotranspiration rates and cause a decline in soil moisture. Indeed, empirical evidence shows warming in drylands that is grater than warming for the globe as a whole.

v. Strategies aimed at combating desertification could be useful in combatting climate changes associated with the greenhouse effect.

See Figure 1

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DESERTIFICATION AND DESSICATION AS A THREAT TO THE CONSERVATION AND UTILIZATION OF BIODIVERSITY

Presented by: Professor A.M. Imevbore, Institute of Ecology, Obaferni Awolowo University, Nigeria

INTRODUCTION

Drylands (areas in which average annual evapo-transpiration exceeds rainfall and in which productivity is limited by available moisture) occur in all continents of the world. The term dryland is used here to include lands which are hyper-arid, arid and semi-arid, as well as the tropical sub- humid zones prone to the same degradation processes of widespread over-use and inappropriate management that occurs in arid lands. Appendix 1 describes some of the general features of dryland areas.

These dryland areas are found in about 100 countries, cover about 43 per cent of the Earth's land area and are the habitat and source of livelihood of about 900 million people or 20 per cent of the world's population. Of the 6.45 billion hectares of dryland area, only 0.9 billion is hyperarid (1).

Dryland degradation - desertification - involves progressive destruction of biological and physical resources of the land which results in reduced usefulness to humanity. The processes of desertification vary but always result from the combination of human mismanagement of fragile ecosystems with natural causes such as periodic drought. The annual rate of land degradation globally has been calculated to amount to 5.25 million hectares. Of this amount, the degradation of rangelands accounts for up to 3.2 million hectares, that of rainfed croplands about 2.5 million hectares and that of irrigated lands about 0.125 million hectares.

For more than twenty years, desertification has been affecting more people in developing countries than any other global environmental problem. As many as 10 million refugees are thought to have been dislodged. In Africa alone, drylands, including hyper-arid deserts, cover 1959 million hectares in 25 countries, or 65 per cent of the continent and about one third of the world's drylands. In several of these African countries the pressure on land and resources is particularly great. For example in Egypt, where human population and activities occupy only 4 per cent of the entire land area of the country, the rest is vast desert in need of development.

Dryland degradation is thus the cause of major global loss of productive land resources which threatens the health and economic well being of affected populations and prevents the achievement of sustainable development.

As a process, dryland degradation is related to four principle factors:

i. natural fragility of the ecosystems in arid lands and adjoining sub-humid territories;

ii. population pressures often leading to the over-exploitation of resources and inappropriate environmental actions;

iii. economic considerations that hinder the establishment of appropriate land use on a long- term basis and;

iv. political unrest that is not conducive to long term sustainable actions.

Its most obvious symptoms have been shown to be the following:

i. reduction of yield, or crop failure, in irrigated or rainfed farmlands;

ii. reduction of perennial plant cover and biomass produced by rangeland and the consequent depletion of food available to livestock;

iii. reduction of available woody biomass and the consequent extension of distances to sources of fuelwood or building material;

iv. reduction of available water due to decreases in river flow or groundwater resources;

v. encroachment of sand that may overwhelm productive land, settlements or transport and communications systems;

vi. increased flooding and sedimentation of water bodies; water and air pollution;

vii. societal disruption due to the deterioration of life support systems that calls for outside help or leads to people leaving degraded areas as environmental refugees (1).

DRYLAND ECOSYSTEMS

Although the ecosystems in dryland vary greatly and the level and location of several species are less well known than those in other major ecosystems of the world, their principle characteristics may be summarized as follows:

i. Dryland ecosystems are generally characterised by tree-shrub savanna woodlands and sparse forests. With increasing aridity, the diversity of trees decreases and thorny xerophites and dry resistant species become more important. Most of such trees are slow growing and difficult to regenerate.

ii. Species diversity and regeneration ability are generally lower than in humid ecosystems; biological productivity and moisture supply are highly interdependent and only with irrigation can these features be changed.

iii. Primary productivity tends to be patchy in response to the spottiness of rainfall and the run- off characteristics of the land. In consequence, a small portion of the land may provide a large proportion of the available biomass.

iv. Native plants tend to have efficient mechanisms for resilience both as individuals and populations. In the case of grasses, the predominance of perennial species, with strong development of underground organs and a root-shoot ratio generally above one is well related to their tolerance of such stresses as drought, fire and herbivory.

v. The co-existence of trees and grasses and the competition between them for resource acquisition particularly water, are reduced by differences in root distribution in the soil. The establishment of tree seedlings is therefore best limited to wet years in order to allow their roots to cross the soil layer dominated by grass roots.

vi. Complementarity between grasses and dicotyledonous plants also allows grasses in general to be more dependent on nitrogen supply while legumes and other dicots are more limited by phosphorus supply.

vii. The fact that dryland vegetation is nutrient limited emphasizes the significance of symbiosis for nutrient acquisition. Legumes-rhizobium mycorrhizal symbiosis and rhizospheric associations with free living nitrogen-fixing bacteria are widespread. The activity of root symbiosis together with the presence of blue green algae provide a good basis for establishing nutrient budgets, particularly the recovery of nitrogen loss induced by extreme burning.

With regard to animals, these are known to cope with dryness through various physiological and behavioral adaptations such as highly irregular spatial distribution, grazing and browsing food habits, and specific diets exhibiting complementary preferences as well as high mobility and migration patterns.

The overriding feature of dryland ecosystems is their nutrient variability due to lack of available moisture. This means that the biological processes in these regions are highly dynamic, sometimes on a time scale of a few weeks and months. A year or two may pass in which precipitation is well below average and then followed suddenly by years of sufficient rainfall. Intensification of traditional farming systems to meet growing demands often has undesirable or ecological effects such as erosion, salinity, waterlogging and the contamination of groundwater because fundamental ecological principles are ignored. This means that the use of which dryland ecosystems can be put must be as dynamic as the environment itself.

LAND USE PRACTICES AND USES OF BIODIVERSITY IN DRYLANDS

Drylands cover a wide range of land types which are put to different uses, such as grazing, cropping, forestry, tourism, settlements and mining. The intensity of land use also varies widely from small scale subsistence agriculture and animal husbandry to extensive commercial cropping and gazing. Social and cultural traditions often impose particular constraints on land use rights, ranging from open access grazing by nomads on common lands to communal farming and pastoralism on privately owned lands. Natural resources which were once community property are increasingly becoming open access resources. Whereas open access resources are almost always inevitably degraded or destroyed as individuals maximise exploitation for short term gains, community resources tend to be well managed (2 and 3).

However a fundamental development problem of forests in drylands is that they cannot be easily differentiated into single purpose uses. This difficulty stems from two main factors. The first is that precipitation is precarious and erratic. it is thus difficult to distinguish between forest, bush or grazing lands since trees, shrubs, herbs and forbs are all closely intermixed and ecologically interdependent. Under drier conditions, the vegetation period of herbaceous plants becomes steadily shorter as aridity increases and then plants are found mainly under the shelter of trees and bushes. Secondly, under such circumstances, natural vegetation becomes open ranging from woodlots to scattered isolated trees and bushes. Management of trees for timber or other purposes is uneconomic. The best option then is using plants for forage for both domestic livestock and wildlife or as fuelwood (3).

This creates some competition between farmers and pastoralists for scarce land, which results in the extension of cultivated lands in marginal areas. For farmers this means shorter fallow periods leading to depletion of soil nutrients and deforestation which makes the cultivated land more vulnerable to wind and other erosion.

Of the 350,000 plant species that have been described in drylands, only about 3000 are reported to be sources of useful material for people. Less than 100 of these plants are cultivated on a large scale and none are xerophytic. However, the search for xerophytic plants of economic value has been intensified in recent years. Another striking feature of the use of biodiversity in drylands is the use of plants which might have multiple values. For example, the plants that supply fuel are generally those which adapt well to different sites, establish easily, require little care, can protect steep hillslopes in many areas, generally have low nutrient requirements and are not consumed by goats and wildlife (3).

Nevertheless, different land use practices generate different priorities and make different demands on the available biological resources. As a result, the intensity of biodiversity management tends to be inevitably related to land use types.

The main uses to which the biodiversity of drylands are put may be summarised as follows:

i. supply food for man and animals, which must survive a dry season lasting 5-11 months annually, or even longer in cases of abnormal droughts;

ii. produce firewood and charcoal, which represents over 90 per cent of energy consumed by most dryland communities, especially in Africa;

iii. supply wood or timber for building and fencing;

iv. produce wood and fibre for making tools and utensils;

v. produce fibre for clothing, rope and other uses;

vi. produce medicines, dyes, tannins, gums and other materials;

vii. provide shade and act as a micro-climate for plants, for animals and soils to prevent erosion;

viii. provide a means of maintaining soil fertility and productivity, especially in highly sensitive ecosystems;

ix. utilization of wildlife as important resources for providing protein, skins, animal trophies, local crafts and rural industries and support tourism.

ORIGIN AND EVALUATION OF DRYLAND CROPS

The drylands of the world have been the origin of some of the world's most important food crops, particularly cereals and pulses. Many of these crops are to-day grown far away from their centres of origin, and for some it is uncertain where their centres once were. Domesticated dryland plants with probable origins in Africa or the Middle East include wheat, barley, sorghum and millet. Cotton and a variety of pulses also have African origins.

Centres of origin normally contain both individual plants that collectively display large amounts of genetic variation and the wild relatives from which crops may have developed. There are therefore these additional genetic pools with clear genetic relevance for future breeding work. Also, other plant species, insects and micro-organisms may exist around centres of origin - organisms that may be of current or future importance for the cultivation of crop plants.

The genetic variation displayed by crop plants results from traditional selection methods (mass selection) practised by traditional farmers. It reflects the efforts many generations of farmers working with an even larger number of generation of domesticated plants. This informal crop improvement process resulted in the land races of our important crop plants. During the process, the adaptation of land races to local environment was enhanced, and (occasional) wild crossing with wild relatives incorporated.

Traditional varieties of our common dryland crops represent living gene pools on which modern plant breeding, and increasingly, biotechnological methods, may draw. These gene pools have been dynamic; farmers have selected the seed that they thought would give safe and good yields for the next seasons. As weather conditions changed and traditional agricultural technologies developed, so the gene pool changed. Farmers implicitly became part of an enhanced evolutionary process, selecting in a variable domesticated gene pool, yet surrounded by a wider wild gene pool.

Farmers in the drylands of Africa have also adopted important crops from other continents: e.g. maize, cassava and ground nuts. Through traditional selection methods local varieties have been developed even at village level in dryland Africa, representing an implicit local optimalization of risks and returns in traditional agriculture. Although the genetic material is probably still present in centres of origin e.g. in Latin America, particular combinations may have been developed locally.

The drylands of Africa while representing marginal areas for many crops still harbour active plant evolutionary processes exemplified by the existence of land races of many of the food crops that provide not only for dryland people but for people of the whole world whether they are growers or consumers. Edible fruit bearing and food producing species include Phoenix, Borassus, Hyphaene, Ficus carica, Opuntia, Ceratonia and Olea europea (fruit) Pistachio, Prunus amygdalinus Pinus pinea, P. cembroides P. edulis (nuts). Many of these plants also play multiple roles in dryland agroforestry systems, providing soil cover, wind protection and fuelwood as well as food.

MAJOR DRYLAND SPECIES AND THEIR USES

Fuelwood Species of Drylands

Drylands face more difficult fuelwood problems than either the humid tropics or the tropical highlands. As a result, a fairly large number of plant species are grown for fuelwood. Many of them have nitrogen fixing ability, can grow rapidly, can produce wood of high calorific value that burns without sparks or toxic smoke and can withstand a wide range of environments, including different soil types, rainfall regions and amounts of sunlight and terrain. The modern approach of multiple purpose management has replaced the traditional forestry approach, resulting in integrated land use. For example, intercropping of trees with food crops now takes place to achieve higher production than when crops and trees grow separately. Another advantage of multiple management is multiple products. Instead of producing a single end product, trees are grown to meet multiple end uses. Appendix 2 provides a list of fuelwood crops used in different parts of the world.

Forage and Browse Plants

Perennial forage grasses are numerous throughout the dry region of the world. Some are easy to grow such as Agropyron festuca, Bromus, and thus are used in seeding programmes. Others are difficult because of poor germination e.g. Hyparrhenia. Presence of physical impediment to mechanical handling of the seeds (e.g. the long and branched awns in Stipa, Aristida, and Stipagrostics spp) harvest problems due to uneven ripening., or seed shedding of germination inhibitors (4). Forage trees and shrubs essentially play more important roles as browse plants than as grasses. For example, from the nutritional and economic standpoint, browse represents up to 20 - 50 per cent of animal intake during the dry season and 5 per cent in the rainy season in the Sahelian and Sudanian zones of Africa. In North Africa, they constitute 60 per cent to 70 per cent of rangeland production and represent 40 per cent of the total animal fodder available in the region. They are richer also in minerals and protein. The intake of free water by drinking also becomes relatively minimal for livestock and wild vugulates where abundant green browse is available. Thus browse trees and shrubs help to ensure livestock maintenance in drylands (6,7).

In most Sahelian annual species seed dormancy is restricted to the first half of the following dry season. In the following rainy season due to high germination capacity most of the seeds germinate. Thus, soil seed stock is transient and varies from year to year and interannual variations of seed per square meter are greater than those of the biomass. Consequently, the resulting differences in seedling densities further add to the heterogenity of the herbaceous layer.

Annual grass species that dominate the various land units in the Sahel (sand dunes, hard- surfaced plains, heavy soil depressions) have an outstanding ability to produce large stocks of seeds under many constraining circumstances.

Several traits contribute to this ability:

i. flowering is governed by day-length and therefore occurs all at once for most of the species in early September. Growths cycles adapt to the rainfall distribution pattern allowing at least some seed production even when rains are poorly distributed;

ii. plants develop protective devices against grazing when flowering and setting seed; they also possess efficient seed dispersal mechanisms;

iii. most grass species produce large numbers of flowering tillers depending on plant density, available soil moisture and nutrients;

iv. defoliation tends to enhance tiller numbers although few tillers reach the flowering stage;

v. under extreme continuous grazing pressure shorter-cycle lower-yielding annuals (such as Tragus bertheronianus, Zoornia glochidiata, Tribulus terrestris) replace longer cycle species, they are palatable but very resistant to grazing due to their short growing period, as little as 2 weeks from germination to flowering (5).

Medicinal Plants

At the present, a list of over 400 species in medicinal plants has been prepared from 64 flowering plants families found in drylands(3). These plants contain a wide range of chemical substances and are very varied in their effects and uses. For example, Agave Sisalana provides diosgenin for wounds and stomach ailments, Balanites Aegyptica provides Steriodal Saponins and sapogenins useful as anthehelminthes and purgatives; Calotropis Procera provides Gloycosides and Calotropin with strong Cardiotonic action, Artemisia Absinthium provides Alkaloid Artemitin, and Commiphora Nukul provides Astringent Carminative Resins, white antiseptic and analgesic stachydrine. Other compounds come from Capparis Decidua; for example Ephedrine, a Bronchodilator from Ephedra sinica, astrogalin, rutin and cardiotonic Glycosides from Nerium Oleander, Hyoscyamine providing atropine for Ophthalmology from Duboisia Leichardtii and components that show promise for the treatment of cancer from Caesalpinii Gilliesii and Byrsera microphylla.

The collection of drug plants from drylands has long been a gainful occupation for many people who prepare them as powder, pills, lotions, decoctions and liquid extracts. Analyses of useful plant derived chemicals is now on-going in many research centres in drylands.

Oils and Extracts

Plants producing essential oils are very common of dryland areas and some have produced essences e.g. rosemary, (Rosemarinus officianalis), thyme, (Thymus vulgaris) and lavender (Lavandula angustifolia) which are extracted by steam for perfumery. Tunisia, for example, provides over 120 tonnes of distillates per annum from natural aromatic plants. Eucalyptus oil is also produced for medicinal purposes (inhalants, embrocations, soaps gargles sprays and lozenges), industrial uses (disinfectants solvents, synthetic thymol and menthol) and perfumes (eudesmil, geranyl acetate citronella1). Some dry zone species (e.g. E.astringents, E. leucoxylon, E. melliodora, E. occidentalis and E. polulnea) produce oils in commercial quantities. Other plants which produce oil include Salvador, oleoides, Veteveria, zizanoides, Cymbogon martini, Rosa demascena as well as Simmondsia Chinensis which produces jojoba oil, Jatropha Curcas, which produces lubricating oil, Pilocarpus Jaborandi Tabebuia toxofora, which produces a white wax and Parthenium argentatim which produces a latex similar to rubber. (3)

Fibres

Dryland woody species also offer considerable scope for the extraction and use of fibres for cordage, ropes and handicrafts. Borassus aethiopium, and Hyphaene Thebaica provide fibres from leaves while Phoenix dactylifera is a source of fibre for paper. In India, fibres are produced from Bauhinia Vahilli, the bark of Calotropis gigantea, C. procera and Leptadenia pyrotechnica. Esparto grass or alpha grass (Lygeum Spartum and Stipa tenacissima) provide important sources of fibre in North America (3).

Resins

Resin bearing plants which are common in drylands include the following:

i. In USA, Hrindelia camporum. Pinus cembroides Larrea tridentata,

ii. In North Africa, Pinus brutia. P halepensis Tetraclinis articulatea,

iii. In Latin America Schinus terebinthifoluius Juniperus california,

iv. In the Sahel Commi phora africana Boswellia dalziella,

v. In India Boswellia serrata Commiphora urghtii and in the near East, Juniperus macropodia and Boswellia sacra.

Resins are generally used in adhesives, paper sizing, surfacing fixtures for perfumes medicines and for the manufacture of synthetic polymers (3).

Gums

About 85 per cent of the world supply of gum is produced in the Sudan. This gum is mainly obtained from the Acacia senegal and the related species A laeta. A polyacantha and A mellifera. The production is now about 40,000 tons per annum. Other gums come from Sterculia urens, S villosa, S setigera, Ceratonia siliqua Proposis latifolia and Urginea indica. Their principal use is in foodstuffs especially conventionary food, flavouring and soft drinks. They are also used in pharmaceutical applications as additives in pill manufacture and as emulsifying agents. Other uses include industrial purposes for adhesives, lithography, paints and inks.

Tannins

Tannin is normally produced from the fruit bark, roots and leaves of arid zone shrubs and trees. It is used with feric salts in compounding inks of greenish-bark to blush-black colours. The arid zone plants which yield tannin include Acacia nilotica, A Cyanophylla, Eucalyptus astrigens, Parkia biglobosa, from the bark; Calotropis procera, A farnesiana from the fruit; A polyacantha, Schinopsis lorentzii from the wood and Punica granatum, Zizyphus sphina - christe from the root. Tannins are also useful for the treatment of inflammation, skin eruptions and bowel disorders.

Plants used for Land Restoration and Revegetation

Land restoration and revegetation are essential activities in drylands. For example, sand dunes are stablised by planting of short shrubs such as Euphorbia spp perpendicular to the direction of the wind. Commonly called palisades, this method requires a great deal of labour to keep the plants closely spaced and to diminish the effects of lateral sand transport. Whatever the species of plants chosen, a programme of protection and management must be established to ensure the anti- erosion success of projects. Plants such as Haloxylon persicum, H. ammodendron and Caligonum spp are used in the Near East. Other plants used for their multipurpose value in conservation and rehabilitation of soils include Opuntia spp for their fruits, fleshy pods and joints in North Africa; mulberry Morus alba for its fruits and wine making, feed for livestock and wildlife and silworm forage; cashews Anacardium occidentale, European filberts Corylus avellana and Pistacia vera for fruits, Eucalyptus spp and Prosopis spp for honey Agave spp for fibres and Acacia spp for tannins.

Traditionally, trees and shrubs are also used as shelter belts to reduce wind speed. Shelter belts are to reduce temperature and potential evapo-transpiration which in turn reduces the natural demand of crops and animals. Legumes such as Acacia, Prosopis, Albizia Gliricidia and Leucaena are also used to store soil fertility. Salt tolerant plants such as Tamarix asphylla, T.Stricta, Eucalyptus Occidentalis, Lagunaria patersonii, Phoenix dactylifera, Atriplex halimus, A canescens, A. nummularia myoporum serratum, Festica elatior and Hadysarum carrosum have been used for decades in the revegetation of salt affected lands in the mediterranean region (8). A recent report has also shown the effectiveness of restoring magnesite mining sites in the Indian Himalayas with such trees as Quercus leucotrichophora, Acacia nilotica, Pinus roxburghii, Populas ciliata, Melia azedavach, Grevia optiva and Ficus auriculata as well as the gravus Erogrostis curyula, Puraria hirsuta Pinnisetum cladasitnum and Rumexhastatus. The plants were found to have been successful in reducing the environmental degradation caused by mining (9).

Plants for Construction

Dryland shrubs and small trees are used considerably by people in rural areas for houses, shelters, animal pens and fences. Their woody parts are also incorporated in mud plastered wall of many structures. Many useful implements and household tools are also made from sticks, stems, bark and fibres of dryland plants. There are also timber trees in drylands which are grown in plantations for commercial purposes.

In summary, there is abundant evidence of the utilization of dryland plant species for a variety of purposes. Indeed there is ample variety within the plant species native to drylands, of suitability to the stressful conditions characteristic of drylands.

Utilization of Wildlife

Wildlife is a most valuable resource to any country. The great benefit of wildlife utilization lies in tapping its lucrative recreational market. It is a most important source of meat for pastoralists and marginal cultivations particularly in times of drought or livestock disease. Skins, skulls, trophy horns and various crafts are also made for sale. However the greatest benefit of wildlife comes from maintaining wildlife resources for tourism. Kenya, for example, is by far the leader in Africa in mobilizing wildlife in semi-arid rangelands for tourism. Aabout 90 per cent of total external tourist revenues comes from wildlife in semi-arid rangelands.

In Zimbabwe, commercial wildlife is also replacing livestock husbandry and providing significant advantages in semi-arid rangelands in terms of both earning capacity and sustainability.

The conservation of wildlife is particularly helpful for preserving land and biodiversity in areas where overcultivation and overgrazing have degraded ecosystems. For example large areas of former agricultural land in South Africa are being converted to wildlife for recreational purposes suggesting this form of land use to be the most valuable in this regions (10).

THREATS TO BIODIVERSITY FROM DROUGHT AND DESERTIFICATION

Drought and desertification impose total constraints on biodiversity management because in drylands life is usually pushed against its physical limits. The ecosystems therein are finely balanced and adjusted to exchanging energy and materials with low total biomass. With intermittent and limited water supply dryland ecosystems are particularly susceptible to human misuse and the vegetation cover to the physical and organic stability of the soil.

The enormity of the problem is compounded by the fact that expansion in the numbers of people increases the demand for cropland while simultaneously expending the need to take land out to accommodate other requirements. Further expanded production in response to rising demand increases the pressures on marginal lands that are vital to sustaining production often with serious environmental consequences.

In most cases, keeping pace with rising demand has meant that plant and animals will have to be produced in ways that progressively approach the optimum potential of the environment implying major changes to those production systems that have resulted from expediency, economic opportunism or sheer necessity. In extremely dry deserts for example rainfall less than 100mm dynamic successions are far less clear and respond to a large extent to edaphic,topographic conditions rather than to climate. Regeneration of plants occur very rarely and under specific climatic conditions especially determined by rainfall.

The rapid increases of livestock and human population which occurred during the late 1950's until the late 1960's in the Sahel were due to:

i. a prolonged series of above average rainfall years;

ii. intensification of medical care for humans, and veterinary health and disease eradication programmes for livestock;

iii. improved techniques for water extraction leading to intensified exploration and exploitation of water resources which opened up vast areas for year-round grazing that were hitherto used only in the rainy season;

iv. after independence, centralisation of control of land and water resources with the state claiming to hold all communal land in trust for its "people";

v. discouraged mobility promoted greater sedentarization to new nation-states to better control people, livestock and land.

vi. focus on infrastructural development (roads, health, water, markets, education, communications) which further enforced this settlement process.

For pastoralists in the Sahel, these trends have several adverse consequences:

Traditional and newly introduced land tenure codes favoured sedentary (and irrigated) cropping, alienating communal land from pastoral pursuits. This process was further accelerated by two serious droughts (1971-73; 1983-84) during and after which pastoralists were further impoverished due to severe livestock losses and to adverse terms of trade between foodstuffs, particularly grain, and livestock. As a result many stockless pastoralists and their dependents became part of the jobless poor, and concentrated in administrative centres, many of them becoming permanently dependent on food-aid. Also, lack of employment opportunities forced young pastoralist to migrate to southern regions. This reduced the pastoral labour force and weakened the interactive transfer of skills in livestock and resource management between older and younger generations (5).

It is thus clear that although most of the factors currently threatening species are anthropogenic in nature i.e. induced or influenced by man, they are greatly accelerated by drought. These factors have been identified to be the following:

i. Habitat loss: Owing to high population density in most drylands, especially in Africa, some habitats have already been destroyed and those that survive are often small and fragmented. As a result, there is considerable risk of extinction of certain species because the remaining habitats are not large enough to support viable populations. Some habitats are also in danger of being compromised as they contain human settlements. In the drylands of Australia for example, rebuilding cities cannot proceed without also reforming agriculture and all other aspects of ecosystem management and land use.

In other drylands areas, agricultural crops have expanded over rangelands at approximately the same rate as demographic growth. This means that less and less suitable lands are cultivated. The result is lower yields which in turn necessitates larger and larger acreages under cultivation. Habitat loss or fragmentation is heightened in drylands by such practices as pastoral development, bush clearance, shortening or total disappearance of fallows, overcultivation, and use of fire.

In Nigeria, the severe droughts that hit the Sahel in the 1960's and those that ravaged many arid and semi-arid lands in the early 1980's forced many pastoralist to remain in fragile ecological zones longer than previously, thus increasing conflicts between farmers and grazer (11).

ii. Loss of vital topsoil: There is no single resource more important to plant life than soil which contains essential materials, stores water for plant growth and provides nutrients in which plant grows. Soil loss is already serious in dryland ecosystems and has increased to the point where losses far exceed formation of new soils through weathering. In the drylands, soil is being regularly mined converting renewable resource to non renewable. In Ethiopia, losses between one and three billion tonnes of fertile soil takes place every year. This loss translates to 1.5 million tonnes of grain a year equal to all the food relief shipped to Ethiopia in 1985 (12)

iii. Loss of soil nutrients: The loss of soil nutrients is often heightened by the cultivation of unsuitable terrain and soils that are sandy, too stony, too shallow, too saline or on steep slopes. The result is soil degradation due to decreased organic matter content, which diminishes the cation exchange capacity, lessening water retention and accelerating soil nutrient leaching and loss. In Lesotho, average soil depth has been reduced from 38cm to 28cm during this century. The overall consequence is low yields (12).

iv. Over exploitation of suitable sites: Drylands are known to be overexploited for commercial purposes including livestock maintenance, and collection of animals for trade and plants for human use. Severe overgrazing has resulted in serious degradation in many parts of the Sahel. Excessive grazing by sheep, goats and fire have led to severe degradation of ecosystems in most IGADD and SADCC countries. In the Sahel, the problem of overgrazing has been severely compounded by prolonged drought. The result is a downward trend in the grazing value of the vegetation. The main mechanisms are the following:

i. reduction and elimination of palatable species;

ii. reduction and elimination of browse species;

iii. trampling down and sealing of the soil surface, reduction of water intake, and consequently of primary production, and increase in run off and erosion;

iv. denudation of soil and wind erosion.

Soil erosion has also been responsible for huge areas being devoid of perennial vegetation.

v. Species loss: The overall effects of all these negative trends is loss of biological diversity. Accidental introduction of exotic species, which compete with, prey on or hybridise with nature species could also decrease both endemic and exotic species abundance and diversity. The fauna and flora of the Sudanian and Sahelian regions of West Africa have been relatively impoverished with few endemics. Of the 1200 or so plant species in the region fewer than 3 per cent are believed to be strictly endemic and are now on the verge of extinction. Among the ungulates, the addax and oxyn have been seriously reduced and are now on the verge of extinction.

Diseases present in arid areas also threaten wildlife in drylands. For example rinderpest is endemic in Ethiopia, Kenya and Chad and threatens all ungulates. There is also a possible threat of screw worm infection spreading southwards from Libya.

vi. Decline in crop yields : All these factors lead to persistent declines of crop yields and recurring crop failure, lower income generation, as well as promote breakdown of traditional, socially and economically accepted farming systems. This point is illustrated by the severity of the drought in most of Africa. Within eleven years of drought (1970-80) per capita food production fell by more than 25 per cent in Botswana, Gambia, Mauritania, Mozambique and Somalia and by 10- 25 in Angola, Ethiopia, Kenya, Mali, Niger, Sudan, Tanzania, Uganda, Zambia and Zimbabwe. The 1984 drought in Kenya reduced the maize harvest by 34 per cent, wheat by 395 per cent beans by 73 per cent and milk sales by 25 per cent. Sorghum and millet fell by 50 per cent.

vii. Loss of gene pools: Periods of severe land degradation, possibly leading to abandonment of large geographical areas, and loss of traditional seed sources, pose serious threats to the gene pools of dryland crops. Breakdown of the farming systems in which these genepools are contained may also affect other species which co-exist with the crops, including various forms of symbiosis, also useful in pest control. It is likely (but not quantitatively proven) that the severe droughts, with associated famines, in the 1970s and 1980s, caused significant loss of germplasm of sorghum and millets in parts of Africa. This loss was caused by successive failures of crops to germinate or mature, with a subsequent depletion of seed stocks, and possible conversion of seed into food grain as a last desperate measure to avoid starvation. As larger areas became unsuitable for cultivation through land degradation, the risk of loosing local land races increased.

During periods of severe environmental stress, farmers tend to overutilize valuable crops, bushes and trees, whether domesticated or not. This directly causes erosion of the most valuable genetic material. The threats to Acacia Senegal (gum arabic) systems in Eastern Africa during the last severe drought exemplify the dangers of genetic erosion.

Land degradation may also affect the gene pool of cultivated plants in indirect ways. Death of livestock may deprive farmers of draught animals and of manure, both important to maintain established farming systems in which the germplasm exists. Decreased human fitness through undernutrition, malnutrition and associated diseases will also lower the human activity level at critical periods in the production process.

This being said, there are also well-documented cases (e.g. from Ethiopia) of farmers forced to flee their degraded farming areas who as a last act have buried seed in durable pots in the ground, in the hope of being able to return to again cultivate their old land with their well-proven seed.

Periods of severe weather conditions, coupled with severe degradation through weather or human mismanagement of land, will likely reduce the genepool of some of the world's important food plants, further decreasing the food security of dryland people unless supportive measures are undertaken.

viii. Damage from Pests and Parasites : Drylands are also under severe constraints from pests and diseases such as locusts, crickets, birds and crop parasites such as striga. Pests tend to be common because of the unstable nature of predator-prey relations. Pests do great damage in drylands. For example, whole villages have been rendered unhabitable by the parasite striga in the Sudan. Locusts have also been reported to devastate crops in the Sahel, while tsetse fly has been a main constraint to livestock development in many areas in Africa.

Other Threats to Crop Biodiversity in the Drylands

During recent droughts and famines, the distribution of seed of important varieties of sorghum, millets and barley by well-meaning relief agencies has posed additional threats to the gene pool of the African drylands. However, there have also been some opportunities for crop production improvements.

The replacement of local varieties with imported varieties entails a number of threats to biodiversity. Easy, often gratis, availability of imported seed may cause a changeover to the exotic variety. Seed treatment (e.g. seed dressings) may make such seed even more attractive for farmers. Imported seed, normally improved varieties showing low genetic variability compared to land races, may over some years be shown not to be well adapted to local conditions by which time the original land races may have been lost. Relief agencies, often supported by misguided extension officers, may tend to push the imported seed as part of relief campaigns, and directly or indirectly discourage the maintenance of the traditional varieties.

At times of distress many relief agencies will not observe national or international quarantine rules causing possible introduction of weeds, pests and disease, which again may be detrimental to the original gene pools.

It must be recalled, however, that such distress periods may also provide opportunities for the introduction of very useful crop varieties, that are higher yielding and offer improved yield security compared to original varieties. The recent drought in dryland Southern Africa demonstrated that improved varieties of sorghum and millets released through the international research institute ICRISAT and through national agricultural research systems, were superior to most local varieties, and especially to maize, in securing some yield even at adverse times. In particular, the success of improved sorghum and millets as compared to maize clearly demonstrated the need to maintain sorghum and millet production systems even in areas where both domestic and foreign agencies have been advocating maize growing. Much caution, particularly in relation to the maintenance of original gene pools, is therefore warranted.

The contribution which each of these major land use types makes to dryland degradation has been assessed in all regions of the world. The magnitude of each of them has been provided as percentages of the overall phenomenon of dryland degradation. (13 and Appendix 3.

EX SITU AND IN SITU CONSERVATION OF DRYLAND CROP BIODIVERSITY

Gene banks normally represent high-technology solutions to the preservation of genetic variability in crop plants. The international agricultural research centres in the CGIAR-system are the major custodians of dryland crop biodiversity (wheat and maize at CIMMYT in Mexico, sorghum and millets and important pulses at ICRISAT in Hyderaban in India and barley and other dryland pulses at ICARDA in Syria). The Ethiopian Genetic Resources Centre is an example of a strong African national seed bank for many dryland crops of importance there, including barley, and there are also regional efforts e.g. the SADCC gene bank in Zambia. Gene banks elsewhere also play major roles in ex situ conservation of dryland genetic material, e.g. USDA and the Vavilov Institute in Russia. Both the international centre IBPGR and the FAO provide strong leadership on the preservation of plant genetic material. The ex situ sites rely on refrigeration and desiccation techniques whose ultimate permanence and sustainability remains unproven. Recent events in the former Soviet Union have demonstrated the vulnerability of gene banks, and the maintenance of the integrity of the Ethiopian gene bank under extreme physical pressure during political change generated similar concerns. Although no major calamities have befallen larger gene banks, a recent report has questioned the sustainability of some important collections.

It would therefore be wise to consider ex situ gene banks as important supplements to other modes of preserving crop biodiversity in the drylands of Africa and elsewhere. Agenda 21 advises that such gene banks be placed locally rather than in industrialized countries.

Considerable thought needs to be given to in situ conservation of the biodiversity of not only the important crop plants of the drylands, but also of the habitats in which this biodiversity exists, not the least in Africa. Encouragement given to the maintenance of traditional practices, and the considered integration of traditional practices with modern agricultural technologies, offer scope for maintaining biodiversity of crop plants, wild relatives and associated organisms. In this context nature reserves form but one element-equally important is to ensure that the rationale that has led African farmers to maintain a high level of genetic variability in their dryland crops is preserved in modern approaches to agricultural development.

In conclusion it can be said that the drylands of the world especially in Africa are marginal areas of crop production. The poor peoples of the drylands have little possibility of controlling the highly variable environment through the use e.g. of irrigation and agro-chemicals. High genetic variability in crop varieties has traditionally been an important tool for poor dryland farmers to improve harvest and thus food security. By doing this the poor farmers of Africa and elsewhere have therefore maintained and developed a high level of biodiversity in crop plants of importance for them and for the rich parts of the world. Both in view of sustainable development in lands threatened by degradation and for improving raw materials for modern bio-technological developments, it is therefore of considerable interest both to the dryland countries and industrialized countries to maintain a high level of biodiversity in dryland crops. Long term economic returns to investment in controlling dryland degradation are high if sufficient attention is devoted to their biodiversity.

CONCLUSIONS

Although low productivity of the resource base in the drylands, coupled with fluctuations in yields due to erratic precipitation, has tended to discourage investment and the development of scientific inputs to conserve and develop biodiversity in dryland areas, the Convention to combat desertification and drought must guarantee the maintenance of the widest possible biodiversity in drylands. It should be realised that efforts to improve the socio-economic situation of the human communities in drylands will only happen if they are translated into concrete benefits for the biodiversity as their habitats of locally adapted crops. The biodiversity combined with a knowledge of potential uses represent enormous and invaluable assets for the conservation and management of biodiversity in drylands.

Development activities to date have not placed sufficient emphasis on biodiversity management in drylands. Indeed they have seriously neglected those areas most threatened by desertification i.e. rainfed farming, livestock and range management and integrated rural development. Forestry which has received much attention must now increasingly give room to agroforestry.

What the dryland communities require to improve their well being are the various skills and capabilities which they do not have as well as the investments which they cannot make to improve biodiversity management. Developing the right policies for biodiversity management can positively affect all major aspects of the problems of dryland people because biomass protects soils and hence its productive potential. It also provides many of the needs of dryland peoples.

Desertification control should therefore encourage biodiversity management as a major vehicle for achieving sustainable development in drylands. Their biodiversity needs to be further defined and explored. Inventories and research into ecosystem linkages must be given high priority. Taxonomic skills will also need further strengthening. The wetlands which play exceptionally important roles in drylands will also need special attention. The various research needs for the conservation of biodiversity in drylands have been proposed by several authors as follows:

i. need to focus on drought preparedness, early warning systems, insurance schemes and other measures of contingency planning for recurrent drought;

ii. studies of the variability within species and populations for drought tolerance, salt tolerance, leaf/stem ratios, palatability, reaction to various management treatments and potential productivity;

iii. studies on easy and cheap methods of establishment of plants in different areas;

iv. investigations of variations in feed value in relation to management treatments and ecological conditions;

v. efforts in the integration in viable exploitation systems and the search for greater perenniality, and

vi. studies of ecological ability to regenerate naturally and to reach high productivity levels under various conditions;

vii. studies in the optimum integration of fodder shrubs in animal systems and in mixed farming systems (14);

viii. consideration of how best to integrate wildlife management in multiple landuse systems;

ix. capacity building to strengthen institutional structures for the management of wildlife fauna and flora;

x. efforts to ensure adequate conservation of aridland genetic resources and representative samples of dryland bio-geographical zones.

Lessons learned from development activities which have failed because they have not placed sufficient emphasis on biodiversity management must be applied. Most attempts have so far been based on single technical measures thought to be a solution to the problem and have been planned and implemented on a high decision making level remote from the farms and villages directly affected by desertification. The need for holistic designs and a participatory bottom-up approach is now recognised.

Actions to maintain biodiversity must also be focused at three levels: ecosystem diversity, species diversity and genetic diversity. All three should be pursued to provide a much higher level of protection and conservation to redress current degradation trends. Much also remains to be done in habitat or ecosystem classification and in protecting rare and threatened species. Clearly a network of protected areas will need to be set up on the basis of representative samples of major ecosystem types.

Four major management options apply broadly to drylands. They are:

i. rainfall and groundwater management;

ii. efficient irrigation systems;

iii. use of alternative fuels;

iv. and the integrated system of natural resource use and methods of management. In all of these efforts, biodiversity conservation must be incorporated to:

(a) maintain still productive lands that is, to stop or prevent degradation;

(b) rehabilitate degraded lands; and

(c) ensure effective management of dryland ecosystems to provide sustainable livelihoods for dryland communities and to redress global environmental threats.

[Appendix 1: Characteristics of dryland zones]

[Appendix 2: Some fuelwood Crops: plant species used for Energy Production ]

Appendix 3

Extent of Dryland Degradation in different Regions of the World

The magnitude of the phenomenon of dryland degradation has been assessed in different regions of the world and the estimated causes of desertification in percentage of land affected have been shown to be the following:- (2)

In North West China: the desert is expanding by 1000 km2 annually (0.6 per cent) and is caused by:

i. land clearing (crop expansion) 45 per cent;

ii. firewood collection 18 per cent;

iii. salinization 1.5 per cent;

iv. overstocking 18 per cent;

v. urbanization roads 3 per cent;

vi. sand seas expansion 5.5 per cent.

In North Africa and the Near and Middle East: with an arid zone area of 3 million km2, the process is progressing at a pace of at least 0.5 per cent (more this 15,000 km2) per annum; and the relative importance of causes has been estimated as:

i. clearing of steppe shrubland and rainfed high risks cropping 50 per cent;

ii. urbanization and erosion 1 per cent;

iii. overgrazing 26 per cent;

iv. fuelwood collection 21 per cent;

v. salinization 2 per cent.

In the Sahel and East African Zone over 3.1 million Km2 desert expansion takes place at a rate of about 0.5 per cent i.e. 15,500 Km2. The relative importance of various causes being estimated as:

i. clearing and cultivation 25 per cent;

ii. wood collection and bushfence building 10 per cent;

iii. salinization negligible;

iv. overgrazing and overbrowsing 65 per cent;

v. urban tourism negligible.

In the arid zones of Middle Asia, over 1.07 million Km2 is desertified and the various causes were found as follows:

i. overgrazing 62 per cent;

ii. water development 10 per cent;

iii. technological development 10 per cent;

iv. water erosion 1 per cent;

v. undergrazing 0.4 per cent;

vi. wind erosion 5 per cent;

vii. salinization 9 per cent.

In the USA where deserts are 5.6 million km2, and the causes are as follows:

i. range depletion is responsible 73 per cent;

ii. wind erosion on farm land 16 per cent;

iii. sheet and roll erosion on cropland 6 per cent;

iv. salinization 5 per cent.

In Australia where arid lands represent 70 per cent of the surface area of the continent overstocking with livestock and or wildlife is responsible for 75-80 per cent of the damage. (Source: LE HOUEROU, 1993)

REFERENCES

1. UNEP, (1991); Status of Desertification and Implementation of the United Nations Plan of Action to Combat Desertification.

Nairobi, Kenya 88pp

2. Dixon; J.A: D.E. James and P.B. Sherman (1989) The Economics of Dryland Management Earthscan Publications London 302pp

3. FAO (1989) Proceedings of the FAO Expert consultation on the Role of Forestry in Combatting Desertification, held in Saltillo, Mexico, 24-28 June 1985, Rome 333pp

4. Le Houerou, H.N. (1985) Forage and Fuel Plants in the Arid Zone of North Africa, the Near East and Middle East Plants for Arid Lands: 117-141

5. Hiernaux P, de Leeuw and L. Diarra (1993) The Pastoral Crises in the Sahel:; 1993 Annual Programme Review, ILCA, Addis Ababa

6. Le Houerou, H.N. (1980) Browse in Northern Africa in pages 55-82 in Browse in Africa ed H.N. Le Houerou, Proc. of International Symposium on Browse in Africa, Addis Ababa, April 8-12 1980. 491pp

7. Le Houerou, H.N. (1980) The Role of Browse in the Sahelian and Sudanian Zones p83- 102 in Browse in Africa: ed H.N. Le Houerou Proc of International Symposium on Browse in Africa, Addis Ababa, April 8-12, 1980 491pp

8. Le Houerou H.N. (1984), Salt Tolerant plants of economic value in the Mediterranean basin (ecology, productivity management and development potential Seminar on Forage and Fuel Production from Salt Affected Waste Lands. Western Australia 17-29th May 1984

9. Ahmad, A. (1992) Environmental Degradation and Possible Solutions for

Restoring the land: A Case Study of Magnesite Mining in

the Indian Central Himalayas: Desertification Control bulletin 21: 15-23

10. World Bank (1990) Living with Wildlife: Wildlife Resource Management with Local Participation in Africa:Washington DC. 217pp

11. Gefu, J.O. and J.L. Gilles (1990) Pastoralists, Ranchers and the State in Nigeria and North America: A Comparative Analyses: Nomadic Peoples 25-27: 34-48

12. Ayoub, A.T.; 1988) Food for Land security in Africa. Desertification Control Bulletin 17, 27-29 13. Le Houerou H.N. (1993) Deserts: in Illustrated Library of the Earth: Published by Weldon-Owens publishers, Sydney, 1993

14. Le Houerou H.N and C.H. Hoste (1977) Rangeland Production and Annual Rainfall Relations in the Mediterranean Basin and in the Sahelo-Sudanian Zone. J. Range Manage 30: 181-189

EFFECTS OF DESERTIFICATION AND DESSICATION ON GROUND AND SURFACE HYDROLOGICAL SYSTEMS, WATER AVAILABILITY AND WATER QUALITY

Presented by : Mr. Habib Zebidi, Director, Hydrology Division, United Nations Educational, Scientific and Cultural Organization (UNESCO)

INTRODUCTION

Desertification, Aridity and Drought

The United Nations Conference of Desertification (Nairobi, 1977) defined desertification as:

"the diminution or destruction of the biological potential of the land, (which) can lead to desert-like conditions".

There are several other definitions of desertification, most of which emphasize the role played by people in the deterioration of an ecosystem which has often already been weakened by natural climatic conditions, particularly drought:

"desertification is the impoverishment of arid, semi-arid and sub-humid ecosystems through the combined effect of human activities and drought" (H.E. Dregne, 1977)(6).

What are the actual effects of such changes on, in particular, rivers and groundwater reservoirs defined as hydrological systems? W. Meckelein (quoted by N.G. Kharin and M.P. Petrov, 1977 (9) describes desertification as consisting of the following elements:

i imate: increasing aridity;

ii. hydrological processes: runoff becoming more irregular;

iii. morphodynamic processes: accelerated soil erosion by wind and water;

iv. soil dynamics: desiccation of soils and accumulation of salt;

v. vegetation dynamics: decline of vegetation.

In this brief article we shall confine our attention to the effects of deteriorating hydrological conditions on surface water and groundwater, particularly in arid and semi-arid regions which are regarded as the most vulnerable, and, consequently on water quality and availability.

HYDROLOGICAL CHARACTERISTICS OF THE REGIONS CONCERNED

Aridity is a function of the relationship between the amount of rainfall and potential evapo- transpiration. It is exacerbated when rainfall decreases and evapo-transpiration increases.

It is this relationship between rainfall and potential evapo-transpiration (R/PET) which was used by UNESCO in preparing the Map of World Distribution of Arid Regions (UNESCO 1977); additional information on temperature will make it possible to define the subdivisions within the different arid regions.

A new world map of arid lands was prepared in 1991 by the UNEP GEMS/GRID Programme Activity Centre on the basis of climatic data covering the period 1951-1980. This map, reproduced in figure 1, shows that over 6,100 million hectares, i.e. more than 40 per cent of the planet's land surface is arid land, providing the habitat and resources of about one fifth of the world's population (12).

FIGURE 1

Africa, the Arabian Peninsula and Australia are the most affected, with very large areas of arid land (80 per cent of Australia) around deserts such as the Sahara in the northern part of Africa. Aridity is also a problem in vast areas of the Indian subcontinent, as well as the west and southwest of North America and South America. In Central Asia, particularly in Turkmenistan, Uzbekistan and Kazakhstan and in China and Mongolia, the arid regions have cold winters, while the summers are hot as in all the other regions.

Rainfall

Whether the rainy season is in winter or in summer, or spread over the two, rainfall in the regions is limited (less than 100 mm/year to 300 mm/year) and, above all, extremely irregular, particularly from one year to another. Figure 2 clearly shows this irregularity, which is characteristic of arid and semi-arid regions, expressed as a percentage of the average.

FIGURE 2

Rainfall may take the form of scattered showers or short violent storms which may account for much or even most of the annual rainfall; in two days (21-22 January 1990) 307 mm of rainfall was recorded at the Bir el Hafay station (central Tunisia), which was more than the annual average of 269 mm recorded over the last 51 years (1).

Rainfall records over long periods show a succession of rainy and dry periods (annual rainfall higher or lower than average), without revealing any clear pattern for the distribution of these periods, which vary in length. However, the term "drought" is used when rainfall is lower than average for two or three years in succession; such is the case in the diagram in figure 3 for the period beginning in 1970 in Niger.

FIGURE 3

Surface Runoff

The irregularity of rainfall means that water courses in arid and semi-arid regions are dry most of the time, but swollen by floods, which are often sudden and short, during the rainy periods (e.g. Oued Zeroud in central Tunisia, figure 4).

FIGURE 4

This same Oued Zeroud, the main water course in central Tunisia, with a drainage basin of 9,100 sq. kms., carried 2,500 million cubic metres of water in the space of two months when rainfall was particularly heavy (September-October 1969), with a flood peak of 17,000 cubic metres per second, although its annual average flow is estimated at 95 million cubic metres per year (17).

Some water courses gradually seep away in their lower courses as a result of water infiltration in the proximity of desert areas, while others flow into internal depressions where their water evaporates completely, leaving behind large quantities of salt (such as the wadis of North Africa, which flow into depressions or "sebkhats"). It is mainly when a river rises in mountainous areas where rainfall is higher or outside the arid zone that it manages to maintain an adequate flow until it reaches the sea, even it if has to cross quite extensive arid zones, as do the Nile and Niger in Africa or the Indus and the Amu Darya and Syr-Darya in Asia. Only then is there a reasonably regular low flow.

Water quality is very dependent on river flow regime: the wadis (rivers) of North Africa carry fresh water in periods of flood (less than 1 g/l), but become much saltier when the water level is low (3-5 g/l).

Finally, sediment transport is one of the main features of surface flows: heavy rainfall and a sudden rise in water levels, particularly in river basins with a sedimentary soil type and sparse vegetation (accentuated by desertification), always result in rivers transporting enormous amounts of sediment. The Oued Zeroud (Tunisia) transports 5 million tonnes of sediment per year for an annual average flow of 95 million cubic metres, although a flood of 191 million cubic metres in October 1964 carried with it 12.5 million tonnes, which was equal to the total amount of sediment transported over the four preceding years (17).

Groundwater

It is in arid and semi-arid regions that groundwater is most likely to be found, when the geology and land structure allow water to be stored.

Some aquifers are located fairly close to the surface and can be tapped by so-called "surface" wells, whereas others lie in geological strata as much as 1,000 to 2,000 metres below the surface.

In arid and semi-arid zones both these types of aquifer are replenished mainly by infiltration form rivers during the rainy season and from large rivers that have a substantial low flow. Some aquifers are no more than the groundwater underflow of a river within its bed.

Another feature of these regions is that they contain large sedimentary basins consisting of layers of permeable rock from a few hundred metres up to 1,000 to 2,000 metres thick that are saturated with fresh water from periods in the distant past when they enjoyed more favourable climatic conditions. Thus, study of the deep aquifers of the northern Sahara (Algeria-Tunisia) has shown that these fresh water reserves accumulated during the early Quaternary period (10,000 years) when climatic conditions in the area were much more favourable (as witness the rock paintings of Tassili)(15).

Present climatic conditions only allow very limited replenishment of these aquifers which are consequently known as fossil aquifers like oil or coal reservoirs.

The following table shows the main features of some of the major African aquifers.

Some major aquifers providing non-renewable resources in the Sahara and the Sahel

(click here to view table)

However in vast areas of the African Sahel the situation is different. There, the underlying bed of crystalline rock, considered to be relatively impermeable, contains no very extensive reserve of groundwater and their discovery is often a matter of chance. However, considerable progress has been achieved in recent years, with the result that drilling is now mich better planned and the success rate higher.

SURFACE WATER AND GROUNDWATER CATCHMENT SYSTEMS

The availability of water in these arid and semi-arid regions depends on the type of catchment system used, which will depend itself on the nature of the resource.

The following brief summary, based on experience in the Maghreb, illustrates the various types of water resource catchment and mobilization, which can be classified on the basis of climate (8).

Rainfall-Surface Water

Where there is a reasonable amount of surface runoff (in semi-arid to sub-humid regions) the preferred course of action will naturally be to block the water course, either by means of large dams, or by creating small impoundments or by spreading flood water directly on to crops.

Flood waters are spread by means of traditional techniques that formed the economic basis for a flourishing civilization during the Aghlabid period (seventh century) in the Maghreb (8). Small earth dikes are built across the wadi beds in order to divert the water flow and lead it by channels in the ground towards the fields to be irrigated. This also helps to replenish the aquifers.

In small drainage basins it is more common to create small impoundments using traditional or modern construction techniques. This enables several thousand cubic metres of water to be stored in the rainy season for human consumption, irrigation or watering cattle. Thus, there are about 540 small earth dams in Burkina Faso and hundreds of other such dams in Mauritania and Niger.

The largest kind of structure for collecting runoff water is clearly the traditional kind of large dam, whenever surface runoff justifies it. However, large amounts of water can be lost by evaporation in arid and semi-arid climates and this in turn may significantly increase salinity.

In very arid regions with no large rivers a whole range of traditional techniques is often used to harvest surface water, including the construction of small impluvia for the benefit of cultivated land situated further downstream (meskats in Tunisia)(8) and of dry-stone walls in the ravines of small water courses, behind which the sediment resulting from erosion and runoff will collect (jessours in Tunisia)(8). This creates a microzone in which fruit trees and cereals can be grown without the need for irrigation. These techniques have been updated and the reception basin can now be sealed with special products or fabrics which make it watertight.

Under these same difficult conditions, drinking water in the Maghreb used to be supplied from underground cisterns which collected rain-water from the roofs of houses. In addition to these individual arrangements, larger underground cisterns were built with impluvia at ground level, often faced with stone to increase the runoff. The island of Jerba (Tunisia) had almost 200 public cisterns, each of which could store about 50 cubic metres per ;year and whose impluvia covered an average area of 250 square metres (8).

Cisterns are also constructed along routes for the purposes of watering cattle.

FIGURE 5

Groundwater

Wells began to be dug and bore holes drilled where no surface water was available or where rainfall was very low and these techniques are now being used in all the arid and semi-arid regions, often to complement surface water collection.

The catchment of springs, the natural outlets of aquifers, is clearly the least expensive course of action especially when the flow is continuous and of reasonable volume.

However, wells are still the most common way of tapping water from aquifers up to depths of about 50 metres. This technique is relatively easy to apply and quick to learn and it has thus spread throughout most of the arid and semi-arid regions where aquifers are available.

Wells are tending to replace more traditional techniques such as "foggaras" (irrigation tunnels), which under different names, are widespread in the arid zones of North Africa, Yemen and Iran. They consist of an underground tunnel linking several wells so that the water table is lowered in higher areas and the groundwater flows downhill by gravitational force to irrigate the lower-lying land. Even today, the city of Marrakesh (Morocco) receives part of its supplies of drinking water from a "foggaras" producing about 80 l/s.

Finally, drilling, a modern technique introduced at the end of the nineteenth century, but which has now become widespread, gives access to deeper aquifers, particularly the so-called "fossil" aquifers, which provide the basis for the development of irrigated areas which are often like oases.

FIGURE 6

EFFECTS OF DESERTIFICATION AND DESICCATION ON SURFACE WATER AND GROUNDWATER SYSTEMS

Basically, a succession of dry years means a considerable reduction in rainfall and, consequently, in surface runoff. Evaporation assumes considerable proportions and this, together with the increasing destruction of plant cover as desertification advances, leads to the desiccation of the soil which very soon presents the distressing spectacle shown in figure 7, taken in the Sahelian zone of Senegal.

FIGURE 7

This will lead to a considerable reduction in the soil's capacity to retain water and thus a substantial increase of the runoff. Floods become more violent and more destructive, carrying an even greater quantity of sediment, which reduces infiltration and the replenishment of aquifers.

Clearly, it is the catchment systems for rainwater and surface waters that are the first to be affected by deteriorations in climatic conditions and by desertification, with the result that it becomes more difficult to provide water for human consumption and, obviously, to develop stock-raising and farming.

Small catchment systems are always the first to suffer.

The large natural lakes whose drainage basins lie entirely within arid or semi-arid zones are likely to suffer considerably:

i. their yields will be drastically reduced;

ii. evaporation and evapotranspiration will be at their most damaging, considerable volumes of water being lost in this way;

iii. the quality of the water will deteriorate as salt from evaporated water accumulates, possibly making the water unfit for human consumption and sometimes even unusable for agriculture.

This was how Lake Chad shrank to a third of its previous size during the drought of 1968- 1973, making the pumping stations installed in Nigeria unusable and forcing the country to find an alternative solution, which involved digging wells to obtain groundwater for irrigation.

Reservoirs behind large dams will be less affected by drought and will have stores of water that can be drawn on for a much longer period, even when there is very high evaporation. Also important are the size and shape of the reservoirs, those that are deep rather than wide and shallow being the more drought-resistant. The flow of rivers rising outside the arid zone is not affected by the climatic conditions prevailing at the reservoir itself.

Even when upstream areas are suffering from the same deterioration in the climate large dams still have a major role to play. For example when the nine-year drought in the horn of Africa (1979-1988) drastically reduced the flow of the Nile which fell below the level of water consumption in Egypt, the very large quantity of water stored behind the Aswan Dam saved the country from disaster (2).

The impact on groundwater supplies will take longer to make itself felt. Reserves of water accumulated in the subsoil help to offset the effects of climatic changes.

Groundwater supplies are thus seen in the first place as a stand-by reserve to be drawn on when climatic conditions take a turn for the worse. Thus in order to cope with the most recent drought to affect West Africa during the 1970s and 1980s international organizations introduced vast programmes to dig wells to provide drinking water for rural populations and livestock, until such time as conditions improved.

However, if aquifers are not replenished from surface water, they will gradually become exhausted and water levels in wells fall lower and lower until they run dry.

Aquifers located in the crystalline stratum are particularly sensitive to drought as their reserves are generally limited.

A drop in the level of aquifers located in coastal zones or bordering salt lakes may in the long term - if the aquifer continues to be depleted by pumping - result in an intrusion of sea water and a deterioration in the quality of the groundwater.

Only aquifers in large sedimentary basins will escape the effects of drought and desertification since the size of their reserves means that they are little affected by the rate of recharge. These large bodies of groundwater, which are widespread in northern Africa (Fig. 8), will continue to provide a constant flow from their springs and will also provide substantial amounts of water from artesian boreholes or by pumping. However even if these water reserves are very large they are not inexhaustible, and their exploitation will therefore need to be planned with great care.

FIGURE 8

Conflicts Over Water Use in Time of Drought

Drought gives rise to conflicts over the use of surface water and groundwater, as well as over water quality, particularly for human consumption (4). It is for the national water authorities to draw up contingency plans for the distribution of available water resources in accordance with national priorities and, depending on circumstances, for more or less stringent restrictions on the use of water for domestic use and for irrigation. All those concerned should be associated in this task under the authority of a senior national official endowed with appropriate powers.

Once the urgent problems have been solved and the crisis is over, these authorities could usefully draw up a programme for the development of water resources (construction of new dams, or drilling of aquifers) in order to avoid further crises, or at least to alleviate them.

One example of contingency planning is the plan prepared by the French authorities to deal with conflicts over water use in the Adour-Garonne basin (115,000 square kilometres) during the drought of 1989-1990 (7). This plan established measures for the gradual restriction of the use of water for domestic and agricultural purposes. It entailed the establishment of networks to monitor the various indicators of the quantity and quality of the water available and the dissemination of information to over 100 different agencies involved in the programme. The establishment of such a system made possible continuous negotiations with all those involved in order to make the best possible use of the resources available.

CONCLUSION

In this paper we have only examined the hydrological aspects of desertification.

This is seen as an increase in aridity, involving in the first place much lower and more irregular rainfall. As a result, there will be an overall decrease in surface flows, but the runoff coefficients will increase because of the soil's much lower retention capacity, which will cause more violent and destructive flash floods.

The intensity of these flash floods will reduce infiltration into aquifers, which will gradually be depleted, except for the so-called "fossil" aquifers in large sedimentary basins.

As regards water availability, catchment systems drawing on rainwater and surface water will be the first to be affected in terms of both quantity and quality, while wells and bore-holes tapping on groundwater will continue to function much longer.

This shows how important it is for countries in arid and semi-arid regions to be as fully informed as possible about their water resources - both surface water and groundwater - with a view to introducing complementary measures to combat desertification. In this connection, the role of the large so-called "fossil" aquifers deserves particular attention.

BIBLIOGRAPHY

R. Abdallah Crues exceptionnelles du 21,22 et 23 janvier 1990 dans la Tunisie centrale - in Ressources en Eau de Tunisie. No 12, 1991.

M. Abu Zeid, S. Abdel-Dayem The Nile, the Aswan High Dam and the 1979-1988 drought. International Commission on Irrigation and Drainage. Fourteenth Congress. Rio de Janeiro, 1990.

E.S. Ayensu Desertification in Africa. Biology International - Special issue - 1 - 1983.

A.G. Babaev, I.S. Zonn, N.S. Orlovsky, V.V. Vladimirov Desertification problems and strategies to solve them. Centre of International projects on the USSR State Committee for Science and Technology.

Moscow 1985.

M. Besbes L'estimation des apports aux nappes souterraines - un modèle régional d'infiltration efficace. Thèse - 1978.

Conférence des Nations Unies sur la désertification 29 août-9 septembre 1977. Situation de la désertification dans les régions arides chaudes. Carte des index de l'aridité du climat. Schéma expérimental à l'échelle mondiale de l'aridité et de la probabilité des sécheresses. A/CONF:74/31.

G. Couzy La gestion des conflits d'usage de l'eau en période de sécheresse: les plans de crise du bassin Adour-Garonne (France) in Actes du VII Congrès mondial des ressources en eau. 1991.

S. El Amami Les aménagements hydrauliques traditionnels en Tunisie. Centre de Recherche du Génie Rural - Tunisie 1983.

M.H. Glants, N. Orlovsky Désertification: A review of the concept in Desertification Control. A bulletin of world events in the control of deserts restoration of degraded lands and reforestation. Number 9. December 1993.

J. Margat Les gisements d'eau souterraine. La Recherche No 221 mai 1990.

J. Margat Quelles ressources en eau les grands réservoirs offrent-ils? Evaluation et stratégie d'exploitation. Observatoire du Sahara et du Sahél. Atelier du Caire. Novembre 1992.

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A. Saidi Les systèmes de captage traditionnels dans les oasis sahariennes. Institut National des Ressources Hydrauliques. Alger, 1983.

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MODERN TECHNIQUES FOR ASSESSING THE GLOBAL ENVIRONMENT INCLUDING SATELLITE IMAGERY, REMOTE SENSING AND GEOGRAPHIC INFORMATION SYSTEMS

Presented by : Dr. Norberto Fernandez, United Nations Environment Programme

ENVIRONMENTAL DATA AND INFORMATION FOR DECISION MAKING

Sound management of the environment, from the sub national to the global levels, requires good assessment of situations. But in order to produce good assessments it is necessary to have timely and reliable data and information. The kind of data and information needed for a particular decision will depend upon the decision itself. Therefore, it is imperative to have an appropriate mechanism in place which allows an efficient flow of data and information from the data generation to the decision-making level.

BRIDGING THE GAP

The transfer of data and information from the sectorial sources to the decision-making level may be viewed as a continuing, on-going process. The success of such a process will depend upon several factors, among the, proper technology, adequate professional capacity, users and needs, adequate resources and open exchange of data and information. Lack of accessible data can seriously affect the capacity of making informed decisions for sound environmental management. GRID, the Global Resource Information Database, in partnership with many agencies and institutions, help to bridge the gap between the scientific understanding of earth processes and sound management of the environment; so that a continuous flow of data into information, information into understanding, and understanding into action, is kept.

MODERN TECHNIQUES

A geographic information system, GIS, is a computer-based system that provides capabilities to input, store, retrieve, analyse and output georeferenced data.

Remote sensing is the technique of collecting data from a distance. Data collected electronically are referred as digital data. Digital image processing involves the manipulation and interpretation of digital, remotely sensed data with the aid of a computer.

A database is a collection of information about things and their relationships to each other. A database management system (DBMS) is the system used to manage the database. Current DBMSs allow to handle very large amount of data, such as the ones in national, regional and global databases.

A network is a system for interconnecting people or things. for example, a network can be used to exchanging data and information from and amongst different databases through telecommunication systems.

These tools used in combination bring flexibility into the data analysis. Using GISs, data from different sources and scales can be integrated within a georeferenced environment providing different and new views of the problem which may impact assumptions; data can be easily updated, thus better information becomes available.

DECISION SUPPORT: DESERTIFICATION - THE BIG PICTURE

Problem: With a rapidly-expanding global population, the loss of productive land through land degradation is a crucial issue. Reliable identification of vulnerable locations and situations in which land degradation occurs is essential if viable remedies are to be devised. Effective and co-ordinated programmes to address the problem of desertification need reliable information, readily available.

Action: Production of the UNEP's World Atlas of Desertification was supported by GRID centres in Kenya and Norway under the guidance of the UNEP Desertification Control Programme Activity Centre. Data on climate, soil degradation and vegetation were combined and analysed using geographic information systems. Different countries employ different methods to measure and categorise desertification, necessitating the adoption of the Global Assessment of Human-induced Soil Degradation (GLASOD) as a common basis for comparison. Various case studies at scales ranging from global to sub-national were undertaken to understand better an highlight the diverse causes and impacts of desertification. The results of these analyses are presented in the Atlas as annotated maps and statistical tables.

Result: The World Atlas of Desertification is an important contribution to the understanding of a complex environmental issue. It helps to focus international attention on the issue of desertification to providing information in a clear, concise, geographically-referenced format. The Atlas also helps to reveal interactions between socio-economic factors and the relative extent of desertification risk. The information provided helps governments and policy-makers to assess the scope and immediacy of the problem and to examine the courses of action open to the, such as implementation of land use policies which encourage sustainable management of land and water resources. The Atlas is expected to provide a valuable reference point for future research to understand the progress of desertification in the world's dry lands, as well as insights into the study of land degradation processes. 76