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2. Components and processes of land degradation and recovery
2.1 Components of degradation
2.2 Hazards and extent of present-day degradation of drylands
2.3 Time scales, cycles and reversibility of land degradation
2.4 Climate change as cause and result of degradation of dryland areas
There are different forms of human-induced land degradation, related to the various vertical components of the land units: atmospheric, vegetation, soil, geology and hydrology.
2.1.1 The degradation may be in the sense of a deterioration of atmospheric climatic conditions, i.e. human-induced adverse global climatic change. Most of the currently available, rough General Circulation Models do not predict a change-to-the worse for the current arid and semi-arid regions such as the Sahel, which would be irreversible in a "business-as-usual". scenario, but do so for the Mediterranean area [slide of the GCAR model in change in "soil moisture" conditions].
2.1.2 The degradation of the above-ground vegetational and animal populations (denseness, diversity) is prevalent in many areas, through direct human influence and aggravated by droughts of a more or less cyclic nature (Sahel, Southeastern Africa, Northeastern Brazil). By and large this biotic degradation is proving to be reversible in a few years time after return of the rains and the "resting". of the land from excess human or animal occupation The reversibility may, however, not extend to the full range of biodiversity.
2.1.3 The degradation of soil conditions is more serious in the sense that it is not easily reversible. This is because soil formation and regeneration processes are predominantly slow. Along GLASOD lines (Oldeman et al. 1991) one can distinguish the following types:
Loss of topsoil through water erosion is the most common type of human-induced soil degradation. It is generally known as surface wash or sheet erosion. It occurs in almost every country, under a great variety of climatic and physical conditions and land use. As the topsoil is normally rich in nutrients, a relatively large amount of nutrients is lost together with the topsoil. Loss of topsoil itself is often preceded by compaction and/or crusting, causing a decrease in infiltration capacity of the soil, and leading to accelerated run-off and soil erosion.
Terrain deformation/mass movement through water erosion The most common phenomena of this degradation type are rill and gully formation. Rapid incision of gullies, eating away valuable soil is well known and dramatic in many countries. Other phenomena of this degradation type are riverbank destruction and mass movement (landslides), and off-site deposition of the eroded material in a negative way (choking of river beds; smothering of riverside crops).
Loss of topsoil through wind erosion as the uniform displacement of topsoil and selective removal of fine particles by wind action. It is widespread, particularly in arid and semi-arid climates.
Terrain deformation by wind erosion is much less widespread than loss of topsoil. It is defined as the uneven displacement of soil material by wind action and leads to deflation hollows and dunes.
Overblowing, which is defined as the coverage of the land surface by wind-carried particles, is an off-site effect of the wind erosion types mentioned above. Overblowing may occur in the same mapped unit or in adjacent units. It may affect structures like roads, buildings and waterways, but it can also cause damage to agricultural land.
Loss of nutrients is a form of chemical soil degradation which occurs if agriculture is practiced on poor or moderately fertile soils, without sufficient application of manure or fertilizer: "nutrient mining.. It causes a general depletion of the soil fertility and leads to decreased productivity. Loss of organic matter by clearing the natural vegetation is also included in this type, of chemical soil degradation, although it often has a negative influence on the soil physical properties as well. The loss of nutrients by erosion of fertile topsoil is considered to be a side-effect of wind or water erosion, and not distinguished separately.
(...) salinization ("secondary salinization") occurs where human (...) ead to an increase in evapo(transpi)ration of soil moisture in soils on salt (...) parent material or with saline ground water; it is often the result of poor (...)of irrigation schemes.
Soil a (...) on is caused by over-application of some forms of mineral fertilizer. It may lead to reduced agricultural potential, especially in the dry sub-humid areas.
Many types of soil pollution can be recognized, such as industrial or urban waste accumulation, the excessive use of pesticides, acidification by airborne pollutants, excessive manuring, oil spills, etc.
Compaction, sealing and crusting are important forms of physical soil degradation in dryland areas. Compaction is usually caused by the use of heavy machinery on soils with a low structure stability. Sealing and crusting of the topsoil occur, in particular, if the soil cover does not provide sufficient protection to the impact of raindrops. Soils low in organic matter content with poorly-sorted sand fractions and appreciable amounts of silt are particularly vulnerable. Both compaction and crusting can be caused by cattle trampling. Compaction and crusting will make tillage more costly, impede or delay seedling emergence, and lead to a decrease in water infiltration capacity, causing, in its turn, a higher surface run-off, which may lead to significant water erosion.
2.1.4 The degradation of the surface hydrological conditions is closely related to degradation of the vegetation- from shrub to herbaceous cover or even to bare soil - and soil surface crusting and sealing. The hydrological regime is affected by this modification of the ground surface conditions and results in:
- a less regular run-off with heavy flash floods followed by low-flow or zero-flow periods,
- an increasing soil erosion and sediment transport resulting in serious sedimentation problems in surface reservoirs
- a decreasing infiltration in the soil affecting the soil moisture dependent vegetation and also resulting in a much lower aquifer recharge. As a consequence, the groundwater resources and especially those of the shallow unconfined aquifers are seriously affected.
Land degradation is therefore a self-accelerating phenomenon since it produces itself the conditions for a further soil and water resources degradation.
2.1.5 Geohydrological degradation refers to the reduction in the amount and the quality of freshwater storage in deeper layers, without contact with the surface immediately above: confined aquifers. This reduction can be because of mining of fossil aquifers for large-scale irrigation or town water supply (lowering of the surface of the aquifer, sideways or upward intrusion of saline waters). Overexploitation of active aquifers in relation to their rate of recharge from far-away sources, often several hundred km, is another form of geohydrological degradation. These sources may still provide a sustained inflow, or themselves be subject to one or another form of land degradation reducing inflow. Studies over whole geologic basins are required to define the long-term degradation of the geohydrological conditions of the land.
2.1.6 The degradation of human settlements and infrastructures is still another form of land degradation in its holistic context, often as the result of the other forms, but also on its own, for instance because of rising living requirements of the local population.
2.2 Hazards and extent of present-day degradation of drylands
2.2.1 Hazards and vulnerability
An early global overview of the hazards of "desertification" (i.e. land degradation of drylands in present day terminology) was given in a joint FAO-Unesco-UNEP-WMO project, in preparation for the UN Conference on Desertification in 1977 in Nairobi (FAO/Unesco 1977). The map at scale 1:25 M indicates
• three qualitative degrees of desertification hazards in respective hyper-arid, arid, semi-arid and sub-humid areas (see Unesco classification of Chapter 1.1) viz. "very high", "high" and "moderate";
• four types of vulnerability of the land to desertification processes, viz. "surfaces subject to sand movement", "stony or rocky surfaces subject to areal stripping by deflation or sheet wash", alluvial or residual surfaces subject to stripping of topsoil and accelerated run-off", and "surfaces subject to salinization or alkalinization";
• two types of pressure on the land, viz., "human pressure, including mechanization., and "animal pressure".
As a follow-up on this activity, FAO in cooperation with UNEP and Unesco prepared a methodology for soil degradation assessment at a larger scale, accompanied by 1:5 M provisional maps for the Northern half of Africa and the Middle East on the "present degradation rate and present state of soil" and on "soil degradation risks", respectively.
2.2.2 Extent and severity of actual degradation
There is one current global assessment of the present-day status of land degradation, and this relates to soil degradation only: the Global Assessment of Soil Degradation (GLASOD) project of ISRIC-UNEP, in cooperation with FAO and other institutions, including about 300 regional and national resource persons. This undertaking has resulted in a world map of the status of human-induced forms of soil degradation at an average scale of 1:10 M [map fragment and legend slides]; for Africa there is also unpublished map material at 1:5 M scale.
UNEP/GEMS/GRID subsequently processed the GLASOD information for a series of computer printouts on individual components for the African continent with a delineation of the dryland areas [slide].
As a follow-up of the GLASOD study, at the request of ECOSOC, FAO in cooperation with UNEP and UNDP provided a more detailed report on land degradation in the South Asia region 1 with special attention to socio-economic impacts.
1 Afghanistan, Bangladesh, Bhutan, India, Iran, Nepal, Pakistan and Sri Lanka.
2.2.3 Action-oriented data collection and processing
More quantitative information on the current extent and on the hazards or risks of land degradation in all its forms, awaits better, fully georeferenced data bases on natural resources (climate, topography and land form, vegetation and land use, hydrology) and current socio-economic conditions on a more meaningful scale, say 1:1 M, in the drylands of the world, preferably in GlS-supported digital form and with remote sensing as a tool (see also Ocana 1991).
Action-oriented data collection and appraisal, however, needs more than GIS and remote sensing. These are useful tools, but cannot replace district- or village-level detailed ground surveys, in close contact with the local land users, and subsequent people's participation in the design and implementation of desertification control and land rehabilitation programmes. Depending on the severity of the degradation problem and the number of people involved, the investments can then be small or may still have to be very substantial.
2.3 Time scales, cycles and reversibility of land degradation
2.3.1 Past land degradation
All the above information relates to the human-induced soil degradation over the period after the Second World War only. Mapping of human-induced soil degradation that had taken place during early civilizations up to 250 years ago, or during the period of European expansion in the Americas, Asia and Africa, 250 to 50 years ago, proved to be unfeasible in the time frame of the GLASOD project because of insufficient georeferenced information. However, much valuable information on dryland use and degradation in the past is contained in the classic Unesco publication "A History of Land Use in Arid Regions" (Dudley Stamp 1961) and a large number of subsequent case studies. This information can be collated without undue effort, preferably in combination with georeferenced information on all dryland areas that have actually been improved by mankind in the course of history from their original natural status, or rehabilitated after a period of degradation (terracing, bunding, drainage and salinity control, reforestation, soil organic matter and fertility improvement, etc.).
2.3.2 Cycles of land degradation
Land degradation can be a gradual process, but more often it occurs in cycles, of different time spans, alternating with land improvement or recovery.
Large time spans, in the order of centuries, have been associated with rise and fall of civilisations and empires. In the early days of development of a civilisation, the available land is often being reclaimed and improved, providing the basis for rapid urban and cultural growth. Already at the zenith of a civilisation, the exigencies of the wealthy and powerful segments of the society, and the requirements of defence of the empire tend to go beyond the carrying capacity of the land, leading to overexploitation. In the declining phase of a civilisation, care of the land becomes neglected, in some cases hastened by outbreaks of infectious diseases and military incursions from upcoming neighbouring powers. Abandonment of the land - the real meaning of "desertification." is the result, followed by a very gradual recovery through nature' build-up of new soil and associated new land cover and hydrological qualities.
Short time spans, in the order of decades, are often associated with cyclic climate variations, such as the El Niņo Southern Oscillation and the Sahelian drought recurrences. On return of the rains, the land cover (vegetation) re-establishes itself quickly, at least in those places where soil and hydrological degradation has not been very severe.
Intermediate time spans, often rather irregular in length, are involved where socioeconomic conditions are subject to changing external influences, inducing strong population growth, increased aspirations for the well-being of rural populations, and changing international markets for agricultural products. In the modem context, these external influences extend to successes and failures of worldwide agricultural research, unfair terms of trade, variations in the flow of funds from rich nations to rural development programmes, structural adjustment programmes, increased access to equipment for modern warfare, etc.
2.3.3 Sustainability and resilience
The reversibility of land degradation has different time spans as well. In the framework of UNCED and INCD, one objective is reversibility in decades, or rather: the complete avoidance of any decline of the productive capacity of the land, i.e. the sustainability of land use. This sustainability can be equated with stability or even, purposely scaling down the intensity of present-day land use in industrialised countries. But this cannot hold good for most developing countries with their strong population growth; there, stability means stagnation at a very low level of intensity and inputs. In such cases, one should aim at a sustained growth of agricultural productivity of the land without negative environmental effects, apparent or hidden, that would cause an ultimate collapse of the land use, on-site or off-site.
Related to sustainability is the concept of resilience of the land, which can be defined as the capacity of an agro-ecosystem on a given unit of land to return to its original equilibrium after a major natural or man-made disturbance. Again, in the case of developing countries' needs, it would be the capacity of an agro-ecosystem to return to a sustained growth of productivity (Fig. 2).
When, exactly, an (agro-)ecosystem will break irreversibly through its resilience level is not only a matter of technical assessment of the threshold values of individual elements of soil and water resources, nowadays eagerly discussed at international scientific gatherings. It is also a matter of temporal perspective and financial implications of recuperation, which, on its turn, depends on the local socio-economic conditions, such as for the number of people to be catered for.
The possibilities for economic development and recovery from emergency situations depend on site-specific combinations of prevailing socio-economic and biophysical conditions. If the major disturbance (a social or physical "accident") takes place in a situation of gradual biophysical degradation, then the appropriate type of external intervention is different from the situation where the normal trend is one of gradual biophysical improvement or development (Fig. 3). In the first case, the external assistance has to be much more substantial, of a more structural nature and of longer duration.
2.4 Climate change as cause and result of degradation of dryland areas.
A distinction should be made between local climate variability and human-induced global climatic change, as reported in the recent Inter Governmental Meeting on the World Climate Programme, April 1993 in Geneva.
Sahelian droughts are a recurrent feature, although the most recent one was particularly strong and recuperation to normal rainfall conditions took many years - which in itself may be a foreboding of global climatic change. El Niņo is another example of a natural climatic anomaly of cyclic character. Concentrated studies in the framework of the World Climate Programme have now revealed that the phenomenon has its repercussions through charges in the atmospheric jet streams, on weather anomalies elsewhere in the world, such as drought, in northeastern Brazil and southeastern Africa. 1
1 Moura, A.D., L. Bengtson, J. Buizer et al 1992. International Research Institute for Climate Prediction; a proposal. IRICP Task Group, Silver Spring, Maryland, USA.
As stated before, human-induced global climate change may, or may not negatively affect the climatic conditions of drylands, and hence their degradation. The current Global Circulation Models are yet too rough to come to unequivocal conclusions for regions or local areas. The implications of a coupling between terrestrial and ocean models, and of the direct effect of increased atmospheric CO2 on plant growth are not yet known. In contrast to the situation with methane and nitrous oxide, the increase in the c concentration of atmospheric carbon dioxide has a direct positive effect on plant growth through the so-called "CO2-fertilization" and the "CO2-antitranspiration" phenomena. Also, a slight increase of the surface temperature of open waters because of any global warming would result in a strong intensification of the global hydrological cycle. This implies more rainfall in many parts, and thus more transpiration-cum-growth of plants, or more run-off to be used on-site or downstream for extra irrigation if stored adequately. FAO well these potentially positive aspects of human-induced climate change in the field of agriculture and rural development, while also concentrating on how to deal with the potential negative consequences. Among the latter are the potential increase in the frequency and severity of extreme weather events (droughts, floods, hurricanes) because of an intensification of the hydrological cycle.