THE TROPICAL DESERT AND TEMPERATE DESERT

Distribution

Deserts are regions of scanty rainfall which may be hot like the hot deserts of the Saharan type or temperate as the mid-latitude deserts like the Gobi. The aridity of the hot deserts is mainly due to the effects of off-shore Trade Winds, hence they are also called Trade Wind Deserts. The temperate deserts are rainless because of their interior location in the temperate latitudes, well away from the rain-bearing winds.

The major hot deserts of the world are located on the western coasts of continents between latitudes 15° and 30° N and S, as shown in Fig. 20.1.

They include the Sahara Desert, the largest single stretch of desert, which is 5150 km (3200 miles) from east to west and at least 1600 km (1000 miles ) wide. Its total area of 9 million sq. km (3.5 million square miles) is larger than all the 50 states of U.S.A. put together. The next biggest desert is the Great Australian Desert which covers almost half of the continent. The other hot deserts are the Arabian Desert, Iranian Desert, Thar Desert, Kalahari and Namib Deserts. In North America, the desert extends from Mexico into U.S.A and is called by different names at different places e.g. the Mohave, Sonoran, Californian and Mexican Desert. In South America, the Atacama or Peruvian desert is the driest of all deserts with less than 13 mm (0.5 inches) of rainfall annually.

Amongst the mid-latitude deserts, many are found on plateaux and are at a considerable distance from the sea. These are the Gobi, Turkestan and Patagonian Deserts. The Patagonian Desert is due more to its rain-shadow position on the leeward side of the lofty Andes than to continentality.

 

Climate

1) Rainfall: Few deserts whether hot or mid-latitude have an annual precipitation of more than 250 mm (10 inches). For example, William Creek in Australia has 137 mm (5.4 inches), Jacobabad in Pakistan has 102 mm (4 inches, Yuma in Arizona, U.S.A. has 84 mm (3.3 inches), In Salah in the mid-Sahara and Arica in the mid-Atacama have practically no rain at all. In the latter, less than 0.5 mm (0.02 inches) fell within a period of 17 years in three light showers. In another station less than 240 km (150 miles) away at Iquique, not a single drop of rain was recorded for four years, and then a torrential downpour suddenly came one afternoon in which 63 mm (2.5 inches) of rain fell. The aridity of deserts is the most outstanding feature of the desert climate. We shall examine more closely why they are so dry.

The hot deserts lie astride the Horse Latitudes or the Sub-Tropical High Pressure Belts where the air is descending, a condition least favourable for precipitation of any kind to take place. The rain-bearing Trade Winds blow off-shore and the Westerlies that are on-shore blow outside the desert limits. Whatever winds reach the deserts blow from cooler to warmer regions, and their relative humidity is lowered, making condensation almost impossible. There is scarcely any cloud in the continuous blue sky. The relative humidity is extremely low, decreasing from 60 percent in coastal districts to less than 30 percent in the desert interiors.

Under such conditions, every bit of moisture is evaporated and the deserts are thus regions of permanent drought.

Precipitation is both scarce and most unreliable. Coastal stations like Massawa on the Red Sea, as illustrated in Fig. 20.2(a), receive light scattered showers from the on-shore winds, amounting to 150 mm (5.9 inches) for the year.

 

 

On the western coasts, the presence of cold currents (indicated by arrows in Fig. 20.1) gives rise to mists and fogs by chilling the on-coming air . This air is later warmed by contact with the hot land and little rain falls. The desiccating effect of the cold Peruvian Current along the Chilean coast is so pronounced that the mean annual rainfall for the Atacama Desert is not more than 13 mm (0.5 inch). Rain normally occurs as violent thunderstorms of the conventional type. It ‘bursts’ suddenly and pours continuously for a few hours over small areas. Twenty-five millimetres (1 inch) or more of rain may be recorded in one single shower! The thunderstorm is so violent, and comes so suddenly that it has drastic consequences on desert Landforms.

 

2) Temperatures: The deserts are some of the hottest spots on earth and have high temperatures throughout the year. There is no cold season in the hot desert and the average summer temperature is around 30°C (86°F). The highest shade temperature recorded was 58°C (136°F) on the 13th September 1992 at Al Azizia, 40 km (25 miles) south of Tripoli, Libya, in the Sahara. Days are unbearably hot, and in the open barren sands, 77°C (170°F) is often recorded. The reasons for the high temperatures are obvious – a clear, cloudless sky, intense insolation, dry air and a rapid rate of evaporation.

Coastal deserts, by virtue of their maritime influence and the cooling effects of the cold currents, have much lower temperatures e.g. Arica as a mean annual temperature of 19°C (66°F), Iquique 18°C (65°F), Walvis Bay, South West Africa, only 17°C (63°F). The hottest months seldom rise beyond 21°C (70°F) and the annual temperature range is equally small e.g. 5°C (9°F) in Arica, 5.5°C (10°F) Iquique and 5.5°C (10°F) in Walvis Bay.

The desert interiors, however, experience much higher summer temperatures and the winter months are rather cold. For example, Salah, in the Sahara, has a temperature of 37°C (99°F) in the hottest months but only 13°C (55°F) in the coldest month. The annual range is 24°C (44?F). The range for Yuma is 20°C (36°F) and for Jacobabad is 23°C (41°F.) In comparison, the station Massawa, illustrated in Fig. 20.2(a), located near the coast, facing the Red Sea, has only a moderate range of 9°C (17°F). Its hottest month is July, with a mean temperature of 35°C (95°F), and its coldest month is January, with a mean temperature of 26°C (78°F).

The diurnal range of temperature in the deserts is very great. Intense insolation by day in a region of dry air and no clouds causes the temperature to rise with the sun. The barren ground is so intensely heated that, by noon, particularly in summer, a reading of 49°C (120°F) is common. But as soon as the sun sets, the land loses heat very quickly by radiation, and the mercury column in the thermometer drops to well below the mean temperature. A daily temperature range of 17°C to 22°C(30° to 40°F) is common, though in the Death Valley of California, an exceptionally great diurnal range of 41°C (74°F) has been recorded. Frosts may occur at night in winter. These extremes of temperature make desert living most trying. This explains why the desert people wear thick gowns all day long, to protect themselves from the glaring heat by day and chilling frost by night, not to mention the sand grains that are carried by the wind.

 

Climatic Conditions in the Temperate Deserts

The climatic conditions of the mid-latitude deserts are in many ways similar to those of the hot deserts. Aridity is the keynote. These inland basins lie hundreds of kilometres from the sea, and are sheltered by the high mountains all around them. As a result, they are cut off from the rain-bearing winds. Occasionally, depressions may penetrate the Asiatic continental mass and bring light rainfall in winter or unexpected convectional storms may bless the parched lands with brief showers in summer. For example, Kashgar in western China in the Gobi Desert, as illustrated in Fig. 20.2(b), has most of its 89 mm (3.5 inches) of annual precipitation in the summer. Due to their coldness and elevation, snow falls in winter.

From Fig. 20.2(b), it is clear that summers are very hot with an average temperature of 27°C (70°F) in July at Kashgar and winters are extremely cold with two months below freezing point. The annual range of temperature is 32°C (58°F), much greater than that of the hot deserts. Continentality accounts for these extremes in temperature. Winters are often severe, freezing lakes and rivers, and strong cold winds blow all the time. When the ice thaws in early summer, floods occur in many places. The greatest inhibiting factors to settlement are the winter cold and the permanent aridity, besides remoteness from the sea.

 

Natural Vegetation

All deserts have some form of vegetation such as grass, scrub, herbs, weeds, roots or bulbs. Though they may not appear green and fresh all the time, they lie dormant in the soil awaiting rain which comes at irregular intervals or once in many years. The environment, so lacking in moisture and so excessive in heat, is most unfavourable for plant growth and a significant vegetation cannot be expected. But very rarely are there deserts where nothing grows.

The predominant vegetation of both hot and mid-latitude deserts is xerophytic or drought resistant scrub. This includes the bulbous cacti, thorny bushes, long-rooted wiry grasses and scattered dwarf acacias. Except for date palms in oases and well-watered river courses, trees are rare because intense insolation, very low relative humidity and rapid radiation would dry out their foliage and kill them. Along the western coastal deserts washed by cold currents, as in the Atacama Desert, the mists and fogs, formed by the chilling of warm air over cold currents, roll inland and nourish a thin cover of vegetation.

Plants that exist in deserts have highly specialized means of adapting themselves to an arid environment. Intense evaporation increases the salinity of the soil so that the dissolved salts tend to accumulate on the surface, thereby forming hard pans. There are plants called halophytes (salt lovers) that are adapted to survive in such saline conditions. Desert soils are deficient in humus since there is little vegetation to decay and the absence of moisture retards the rate of decomposition. Plants, whether annual or perennials, must struggle for survival against both aridity and poor soil.

Most desert shrubs have long roots and are well spaced out to gather moisture and search for ground water. Plants have few or no leaves and the foliage is either waxy, leathery hairy or needle-shaped, to reduce the loss of water through transpiration. Some of them are entirely leafless, with pricks or thorns. Others like the cacti have thick succulent stems to store up water for long droughts. There are still others that shed their leaves during droughts. The seeds of many species of grasses and herbs have thick, tough skins to protect them while they lie dormant. They germinate at once when their seeds are moistened by the next rain. In short, all plants must adapt themselves to survive in such an inhospitable region as the desert.

 

Life in the Desert

Despite its inhospitality, the desert has always been peopled by different groups of inhabitants. They struggle against an environment that is deficient in water, food and other means of livelihood. Some, like the Egyptians, have attained a high level of civilization, others, like the Bedouin Arabs, have fared quite well with their flocks of sheep, goats, camels and horses. The Bushmen of the Kalahari and the Bindibu of Australia remain so primitive in their mode of living that they barely survive. They are, in fact, a relic of the Old Stone Age in the modern world. The desert inhabitants may be grouped under the following categories.

1) The primitive hunters and collectors: Of the primitive tribes, the Bushmen and the Bindibu are the best known. Both tribes are nomadic hunters and food gatherers, they do not grow crops or domesticate animals. The Bushmen roam the Kalahari Desert with their bows and poisoned arrows, spears, traps and snares. They are not only skilful and strong but have great endurance. In order to capture their prey, they have to be very patient and if necessary run many kilometres to track down the wounded animals. In this way, they hunt antelope and other smaller animals. The women and children collect insects, rodents and lizards, and gather honey,roots, grass and grubs. Great skill is required in obtaining water in the desert. Dew is carefully gathered from leaves early in the morning and stored in ostrich shells. Very often, it is necessary to dig a hole in damp sand and suck the water up from the ground through a hollow reed. This is often a very slow process. Bushmen either wear a loin cloth or go virtually naked. They travel in small family groups and live together in open sherms. This is a hollow, dug out beneath a thorn tree, surrounded by bushes. At night, a fire is lit to keep the family warm.

The Bindibu or Aborigines of Australia live in very much the same way as the Bushmen. They are lean and dark but healthy. They are skilled trackers and some of them use wooden throwing sticks or boomerangs and spears. They also domesticate the dingo, a wild dog that assists them in tracking down kangaroos, rabbits and birds. The women gather grass, roots, seeds, berries, moles and insects, to supplement their diet. Like the wandering Bushmen, the Bindibu move in family groups in search of fresh hunting grounds. But one distinct difference is that they always stay close to a water source as they still have not devised a means of tapping and storing water. They live in wurlies, simple shelters made of branches and tufts of grass.

 

2) The nomadic herdsmen: These people represent a more advanced group of desert dwellers who pursue a livestock economy. They ride on animals instead of walking and are heavily clad against the blazing sun, the stormy winds and the chilly nights. The Bedouin of Arabia ride on horses and live in tents; the Tuaregs of the Sahara are camel riders and dwell in grass zeribas, while the Gobi Mongols ride on horses to herd their yaks and live in portable yurts (a kind of tent). The Bendouin are the best examples of a desert group who have fared well as nomadic herdsmen. Besides keeping large flocks of animals, they are also engaged in trade with the caravan merchants and the oases people.

All round the year, the Bedioun wander with their herds in search of water and green pastures. Their wealth is their animals: sheep, goats, camels and perhaps a few horses. The animals provide them with all they require, daily milk and cheese and on occasions meat as well. Their skin is used for hides or leather, for making tents, clothing, belts, footwear and water bags. From the hair and wool, the Bediouns make clothes, mats, ropes, rugs and carpets. These can also be exchanged at trading stations or oases for other necessities of life which the desert herdsmen cannot produce, like dates, grain, beverages, medicines, firearms and other manufactured articles.

The Bedouin move in groups of about a hundred. They follow a regular pattern of routes, along which are scanty patches of pastures, wells or springs. In this manner, one group rarely clashes with the other in their search for new grounds to pitch their tents. Since they move so often, they always travel light and only essentials are carried along.

 

3) The caravan traders: These were the travelling merchants of the desert. Their journeys across the wastes of the Sahara or Russian Turkestan sometimes lasted months or even years. They travelled at night as a team and were well armed. They carried a wide range of goods into remote interiors where their merchandise was highly sought after. These goods were sold or exchanged for hides, rugs, carpets and other valuable products of the deserts. Though profits were high, the risk were equally great.

The pack-animal used by the caravans to carry their goods across the desert was the camel, aptly described as the ‘ship of the desert’. Unlike horses which have sharp hooves that sink readily in the sandy desert, the camel has broad padded feet that will not slip in the sand. A pack-camel, adequately fed, can carry a load of 159 kg (350 pounds) and travel 80 km (50 miles) a day and a racing camel can do twice the distance when ridden! The camel has several other advantages: it provides milk and hair, it can store up water in its stomach, and fat in its hump, so that it can go for long periods without either food or water.

With the introduction of modern air, road and rail transport, the role of the caravan traders is greatly reduced. Goods can be conveyed much more cheaply and with greater security by desert jeeps, vans or trucks. But between the interior oases and scattered outposts beyond the reach of roads, the caravan routes remain the only form of available transport.

 

4) The settled cultivator: For crops to be cultivated in the desert,irrigation is indispensable. This is obtained either from oases, rivers or dams, or through a network of canals. In Egypt, the Nile supports a population of 25 million, mainly concentrated in the Nile Valley and delta. The life-giving waters of the Nile made it possible for the Egyptians to raise many crops as early as 5000 years ago. When the Nile flooded in summer, the overflowed water was caught in basins with raised banks and led into the fields to irrigate the crops. This was basin irrigation which was widely practised by the ancient Egyptian farmers and is still employed by the Egyptians today. Rice and cotton are cultivated in summer, followed by wheat, barley, beans and other minor crops in winter. Modern concrete dams constructed across the Nile e.g. Aswan and Sennar Dams, are even more effective for extensive irrigation works. The flood water can be held back and controlled and then released at any part of the year to feed the fields that yield two, three or even more crops a year. In the same way, desert cultivators rely on the Indus in West Pakistan, the Tigris-Euphrates in Iraq and the Colorado in the Imperial Valley of California, and are able to irrigate millions of hectares of arid land for crop production.

In the deserts, wherever there are oases some form of settled life is bound to follow. There are depressions of varying sizes, where underground water reaches the surface. Some of them are abnormally large like the Tafilalet Oasis in Morocco which measures 13 000 sq. km (5000 square miles), supporting many settlements, including large towns. Others may be so small that they are no bigger than the mining pools that we are so familiar with e.g. the Ghadames Oasis of Libya is only 2.6 sq. km (one square mile) in size. Life in an oasis is secure and well organized. A wall is usually constructed around the oasis to keep out the violent dust storms called simooms. The oasis people live in mud-brick houses with flat roofs, closely packed together. The streets are narrow and winding, and the heart of the settlement is dominated by the suq (central market place), the mosque, school and shopping blocks. Suqs may also be set up kilometres from anywhere and people come from a number of surrounding settlements on special days to trade. Around the settlements are the agricultural lands. Water is led to the field by irrigation channels or drawn up from wells by camels or mules. The most important tree is the date palm which is grown in dug-out hollows so that its roots can penetrate deep into the ground in search of water. The fruit is consumed locally and also exported. Other crops that are cultivated include maize, barley, wheat, cotton, cane sugar, fruits and vegetables. With the development of motor routes across the deserts, many sophisticated foreign goods can now reach the oases.

 

5) The mining settlers: The lure of mineral wealth has attracted many immigrants into the desert. Mining camps and isolated settlements have sprung up following the discovery of certain mineral deposits. It was gold that brought immigrants scrambling into the Great Australian Desert. Water and food supplies have to be brought 480 km (300 miles) from Perth to keep the mines going. Some of them like Kalgoorlie and Boulder, have become towns of considerable size. In the Kalahari Desert, the discovery of diamond and copper has brought many white men to the ‘thirst land’, as it is called. Even in the most arid Atacama, in northern Chile, large mining camps have been established for the mining of caliche (cemented gravels) from which sodium nitrate, a valuable fertilizer, is extracted and exported to all parts of the world. The mines are worked by local Indian labourers and supervised by foreign technicians. Besides nitrates, copper is also mined. These two products have directly or indirectly contributed to the growing size of the Chilean town of Arica, Iquique, Antofagasta and Chuquicamata. The last named is the world’s largest copper town.

The working of the rich haematite iron ore deposits east of Fort Gourand, Mauritania, is transforming the social and economic life of that country. A 640-kilometre (400 miles) railway has been built through part of the Sahara Desert to link the deep waters of Port Etienne – which is the main outlet for the ore – with the iron-fields. Apart from the heavy initial capital outlay, the cost of maintaining the track is comparatively high, as shifting dunes have to be checked and fixed often to secure a stable line. In addition, water has to be carried over long distances to the town-ships that have developed at either end of the track; even Port Etienne is supplied water from a well over 120 km (75 miles) away.

Similarly in the deserts of North America, silver is mined in Mexico, uranium in Utah and copper in Nevada. A host of other minerals and their by-products have, in fact, greatly altered the landscape and the economy of such arid regions.

In recent years, the discovery of oil in many parts of the Saharan and Arabian Deserts has transformed this forgotten part of the globe. In Algeria, wells have been sunk three kilometres deep to tap oil, so also in Libya where oil has been responsible for the country’s metamorphosis from poverty to viability. In the Middle East, pipelines over 1600 km (1000 miles) long have been laid to bring oil from the shores of the Persian Gulf across Saudi Arabia to Saida (Lebanon) and Banias (Syria) on the Mediterranean coast. With still half of the world’s reserves of oil are untapped in this region, the desert here contains promise of great riches. In Iraq, Kuwait and Saudi Arabia, the desert landscape is changing fast. New roads, huge palaces, ultra-modern hospitals, air-conditioned apartments and swimming pools are examples of a thriving new era created entirely by oil – ‘liquid gold’.