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NIRE Annual Report
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 1999

Classification of Arid and Semiarid Regions by Aridity Index and Vegitation Index

Inter-Spheric Environment Division
Environmental Assessment Department
Objectives
Dryland occupies 30-40% of the Earth's land surface. Dryland is devided into severe deserts, semiarid regions including savannas and dry steppes. Desertification is mainly the degradation of productive semiarid regions. Desertification continues at a quick pace. It is necessary to find proper methods to estimate degree of dryness and to identify desertification areas. Nowadays, spatial vegetation patterns and their changes through time are identified real timely by remote sensing from satellites. We can look at vegetation of global scale from an uniform point of view.
Results
Aridity zones can be defined by climatological approaches and vegitation types. Aridity index which shows degree of dryness and vegetation index which indicates a degree of abundance of vegetation are both comparatively vague parameters. Here, rough classification of arid regions by aridity index estimated by Thornthwaite's methods and yearly maximum normalized difference vegetation index NDVIymax is tried. The histogram of areas for NDVIymax distribution has two peaks in small NDVIymax (severe desert) and large NDVIymax (regions where vegetation is rich), and between two peaks there exist semiarid region(Gamo,1999). There is a different linear relationships between annual precipitation and NDVIymax in the sub-tropical arid regions and mid-latitude arid regions, respectively. While, there is a relatively similar relationship between aridity index and vegetation index for both arid regions mentioned above(Gamo,1999).
Arid regions of the world are divided into seven categoriesas shown in Fig. 1, including regions where vegetation is relatively abundant in spite of severe dryness, and regions where vegetation is poor in spite of relatively humid conditions. The former category (type I) coincides with irrigated regions existing near big rivers or oases. While, regions which belong to the latter category (type S) seem to be areas where soil degradation is occurring. Distributions of Type S coinside relatively well with soil degradation areas determined by more than 250 soil sientists (GLASOD project, UNEP(1997)).
Fig. 1 Classification of arid and semi- arid regions by aridity index(AI) and NDVIymax.,
I : irrigated area and oases; 
S : areas where there is the possibility of occuring soil degradation,
A : severe desert, B - E, region where AI relates linearly with NDVIymax,
H : humid and vegetation affruent region.
a : NDVIymax=AI+0.15,
b : NDVIymax=0.7AI

Fig. 2. Distribution of aridity classes defined by Fig.1.Simbols are the same with Fig.1.M : highland over 3km above the sea.

Selected Publications
1) Gamo M.,1999: Classification of arid regions by climate and vegitation. Journal of Arid Land Studies,9-1,1999.
2) UNEP(1997) World atlas of desertification.United Nations Environmental Programme,1-182.

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