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1. Definitions

1.1 What are arid, semi-arid and dry sub-humid areas?
1.2 What is land?
1.3 What is land degradation?

In the context of assessment, desertification is defined as:

"Land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities" (Chapter 12 of UNCED's Agenda '21).

The various elements of this definition need to be quantified before one can enter upon a discussion of causes, general extent and physical consequences of the process.

1.1 What are arid, semi-arid and dry sub-humid areas?

An early classification and delineation is given in Unesco's MAB Technical Note 7 of 1977, with an accompanying map at 1:25 million scale of the world distribution of arid regions, prepared as part of Unesco's erstwhile "Arid Zone Research" programme. Combinations of two climatic variables, bioclimatic aridity and temperature regime, define the various regions. Degree of bioclimatic aridity was established on the basis of the ratio P/Etp (average total annual rainfall/potential evapotranspiration).

The ratio between mean annual rainfall and potential evapotranspiration provides only a crude measure of aridity or humidity of climate, and does not have a close relation with agricultural or grazing potential. The potential productivity of arable crops and grazing land, or crop choices or farming or grazing management options, depend more on the length of the period in the year when moisture supply, from rainfall and soil storage, is sufficient for the growth of crops or vegetation. A Length of Growing Period (LOP) concept was developed and used in FAO's Agro-Ecological Zones studies.

A Reference Growing Period starts once rainfall exceeds half of the potential evapotranspiration (Etp); may be interrupted by a period with low temperature during which growth is not permitted; and ends after the date when rainfall falls below half of Etp, plus the period required to evapotranspire 100 mm water assumed stored in the soil from excess rainfall (or less if less excess rainfall during the Growing Period).

Areas with an LGP than 1 day are hyperarid (true deserts); less than 75 days arid, 75 to less than 120 days (dry) semiarid, 120 to less than 180 days (moist) semiarid. These areas together correspond closely to the areas denominated as Drylands (INCD Secretariat, April 1993). Fig. 1 shows the different dryland areas for the developing world. The overall similarity with the earlier map of annual P/Etp ratios is clear; differences can be seen in southern Africa, for example, where the LGP map more clearly identifies the large extent of arid conditions.

1.2 What is land?

Land, in the sense used by FAO's Interdepartmental Working Group on Land Use Planning, is a delineable area of the earth's solid surface, the characteristics of which embrace all attributes of the biosphere vertically above or below this surface, including those of the lower atmosphere, the soil and the underlying geology, the hydrology (including lakes, rivers, marshes and swamps), the plant and animal populations, the human settlement pattern and the physical results of past and present human activity (terracing, water storage or drainage structures, roads, etc.).

In this holistic approach, a unit of land has both a vertical component - from atmospheric climate down to confined aquifers in the deeper substratum, and a horizontal element - an identifiable repetitive sequence of soil, terrain, hydrological and land use elements ("landscape", "land unit" or "terroir" units).

1.3 What is land degradation?

Degradation of the land involves the reduction of the renewable resource potential by one or a combination of processes acting upon the land. The resource potential relates to agricultural suitability (rainfed or irrigated arable cropping, animal husbandry, forestry, inland fishery), primary productivity level, and natural biotic functions.

Such a reduction, leading to an abandonment, or "deserting," of the land (e.g. of parts of the Sahara, populated until some 6000 years ago) can be because of natural processes: such as a natural acidification of atmospheric climate, natural processes of erosion, some processes of soil formation such as primary salinization and hardpan formation, a natural change in the base level of river catchments, or natural invasion by noxious plants or animals.

The reduction in resource potential can also be human-induced, either directly on the terrain or indirectly through man-induced adverse climatic change.

Some aspects related to climate change are discussed in Section 2.5. Human activities that can cause drylands degradation include:

- the cultivation of soils that are fragile, or credible by wind or water;

- net export of plant nutrients leading to loss of soil fertility, such as cash cropping without adequate nutrient replenishment;

- the reduction in fallow periods and lack of organic or mineral fertilizers;

- overgrazing - often selectively - of shrubs, herbs and grasses;

- the overexploitation of woody resources, and in particular for fuelwood;

- uncontrolled use of fire, e.g. for regenerating pasture, or agricultural clearing;

- unsuitable agricultural practices or machinery that destroy soil structure.

All these activities derive from two root causes: from poverty and underdevelopment, or from 'modern' development which disregards the impact of the technologies on the land.

Factors typical of poverty and underdevelopment include:

- undernutrition or malnutrition, leading to physical weakness and vulnerability to disease;

- lack of access to credit, thus preventing any chance of investment in tools, seeds or fertilizers;

- limited access to basic schooling and technical training;

- short-term survival strategies (e.g. annual or seasonal migration);

- lack of an effective framework to support rural communities with technical advice, infrastructure, energy, training, organization of barter exchanges or market access;

- a lack of basic security.

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