Semidesert Zones of Tropical Belts

(desert savannas), natural zones of the tropics where semidesert landscapes predominate. They usually lie along the southern margins of tropical deserts in the northern hemisphere and along the northern margins of tropical deserts in the southern hemisphere. These zones occur in interior continental and western oceanic areas, as well as in the more moist peripheral parts of desert basins. In Africa they are found south of the Sahara, in the moist parts of the Namib Desert, and northeast of the Kalahari Desert. Semidesert zones occur on the. Arabian Peninsula and on the broad outer band of the Thar Plain in India and Pakistan. In South America they occupy the northern part of the Atacama Desert and the northeastern part of the Brazilian Highlands, where they are locally called caatinga. They are also found in the eastern part of the Central Lowlands in Australia. In addition, semidesert zones often constitute an elevation zone in the lower parts of mountains in the tropical belts, for example, in the Ethiopian and Mexican highlands.

Semidesert zones in tropical belts have a hot and dry climate. During the summer months temperatures average 30°C, with maximums of up to 50°C; in the winter mean temperatures drop to 12°–20°C. The annual solar radiation ranges from 6.7 to 8.4 megajoules per sq m, or 160–200 kilocalories per sq cm. The annual precipitation usually totals 200–250 mm (up to 400–500 mm in the mountains). Surface runoff is negligible, and there are few permanent rivers. The soils are primarily red-brown, and physical weathering is intensive. Vegetation consists chiefly of xerophilous densely matted grasses, shrubs, and, in some areas, sparse low trees. South of the Sahara and in India the plant cover is dominated by low acacias, with tall perennial grasses and a small number of succulents. In the Kalahari the vegetation often includes saltworts and in Australia, shrub eucalyptuses. The most common animals are desert and savanna species. There is nomadic herding, and farming is possible only on irrigated land.