Classification of drylands

Dryland ecosystems are mainly categorised into four subtypes according to aridity index and annual rainfall levels into hyper arid, arid, semi-arid and dry sub-humid areas as shown in Table 1.

World drylands

Dryland ecosystems occupy over 41 per cent of the earth's land surface. Desertification affects 70 per cent of the world drylands, amounting to 3.6 billion ha or one fourth of worlds land surface (IFAD, 1995).

Asia possesses the largest land area affected by desertification, 71 per cent of which is moderately to severely degraded. In Africa two thirds of which is desert or drylands. 73 per cent of agricultural drylands are moderately to severely degraded (IFAD, 1995). Africa is under greatest desertification threat, with a rate of disappearance of forest cover of 3.5 to 5 million ha per year bearing down on both surface and ground water resources and with half the contents farmland suffering from soil degradation and erosion.

Table 1. Total dryland categories according to FAO (1993), classification and extension (UNEP, 1992)

Subtypes

Aridity

Index

Current area

Dominant

Biome

Current

Population

Mk m2

%global

*1000

% global

Hyper arid

9.78

6.6

Desert

101,615

1.7

Arid

15.66

10.6

Desert

222,204

3.7

Semi arid

22.59

15.3

Grassland

828,341

13.9

Dry sub humid

0.50-0.65

12.87

8.7

Forests

909,273

15.3

[Source : World resource Institute, 2002]

 
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