Andrew Johnson

See also: Deserted (homonymy)

A deserted is a zone sterile, or not very favourable with the life, because of the unsuitable ground, or of the weakness of the Précipitation S (less than 200 mm per annum). A desert landscape is recognized with its stripped aspect. A desert is located except ecumene.

Examples of stone and sand deserts:

  • deserted of the the deserted Sahara
  • of the the Gobi Desert Sahel
  • Deserted
  • of Deserted Namib
  • of deserted Taklamakan
  • of Atacama
  • Valley of dead the (Death Valley)

A desert can be a zone deprived of vegetation, animals and human beings:

  • Example: desert of ice and snow = the Arctic

A desert can be a zone with the close-cropped, scattered vegetation:

  • Deserted of Deserted Karakoum
  • of Thar

In Demography, a desert is an area little densément populated. Its definition varies: on a world level, one estimates that in lower part of 5 inhabitants to the km ², an area is desert. In France, the cantons populated of less than 20 inhabitants to the km ² are regarded as deserts.

Deserts marked by the aridity

Characteristics

See also: desert Climate

The aridity is the permanent lack of water which affects an area. It does not depend on the temperature: there exist arid and cold spaces then (with the poles for example). One measures the degree of aridity of an area according to the index of aridity which measures the difference between the potential evapotranspiration (EVP) and the Pluviosité.

In a general way, the desert mediums are characterized by:

  • of rare and very irregular precipitations: it often happens that it does not rain during years.
  • the morning Rosée S often constitute the only resource there Eau on the surface for the alive species present.
  • a evaporation more important than the Précipitation S.
  • a strong thermal amplitude enters the Température S diurnal and night.
  • a constant wind and often extremely
  • a ground poor and thin.
  • a Vegetation rare, low and atrophied known as Xerophytic made up in particular of succulent plants or fatty .
  • small a not very dense fauna, one finds there Insecte S, small Reptile S, Arachnide S, Rongeur S and some Oiseau X night.
  • of human weak densities.

Typology and explanation

Commonly allowed classification

The typology of Monique Mainguet proposes:
  • Deserted polar cold: these zones (the Arctic and the Antarctic) receive few precipitations indeed, because of the presence of anticyclonic cells. The ice prevents the development of the vegetation in the desert sub-polar field. The Toundra appears in the semi-arid sub-polar field

  • Deserted heats of the intertropical zone: the Sahara, Arabian desert, center of the Australia… They undergo strong a Insolation (3250 hours of sun in the the Sahel, of the very high temperatures (78°C in full sun with Tamanrasset) and a strong evaporation.

  • Deserted coastal heats: desert chilo-Peruvian, desert of Atacama, desert of Namib, Low-California, south-western Morrocan. Often misty, these deserts are created by anticyclones, cold currents (running cold of Bengela for Namib) and of the Upwelling S. They can be hyperarides (deserted of Peru and Chile).
  • Deserted of shelter of the moderate zone: these deserts are safe from a mountainous barrier which blocks the depressions come from the ocean (Grand Basin, Désert of Mojaves in the United States). The effect of Fœhn drains the air when it goes down again behind the assembly line.
  • Deserted continental: primarily located in Central Asia (, Gobi Desert Tibet, Deserted of Karakoum…) to several thousands of kilometers inside the grounds. They are characterized by a very strong thermal amplitude. It also distinguishes several arid or semi-arid mediums:
  • Saharan medium
  • medium aralien
  • Peruvian medium
  • semi-arid medium sahélien
  • Mediterranean medium.
Jean Demangeot points out that this classification must take account of the complexity of the factors. It distinguishes the polygenic deserted (Central Asia, Borkou, Sonora…) for which the causes of aridity are multiple, and the desert of altitude (Bassin of very arid Tarim, but mountains which surround it relatively sprinkled).

Classification according to the aridity

In a simpler way, one considers the deserts according to their aridity:
  • the deserts hyperarides which receive less than 50 mm of precipitations on average per annum: one finds them at certain places of the Sahara, the desert chilo-Peruvian, of Libya, of Arabia, of Namibia.
  • the arid areas (deserts) receive less than 100-150 mm of rain on average annual and know at least two months with more 30°C (deserted continental, of shelter, cold Asian deserts and some sectors of the Australian desert).
  • the semi-arid areas (center and west of Australia, etc)
  • the rain Agriculture (which does not require an irrigation) develops as from 300 mm/year.

In general, it is allowed that a medium is not Aride when the xerothermic Indice is lower than 100, semi-arid between 100 and 290, arid between 290 and 350, and hyperaride between 350 and 365.

The criterion of the evapotranspiration

FAO retains another criterion of typology: the potential evapotranspiration, associated with a vegetable formation:
  • zones hyperarides: some transitory, xerophytic bushes in the arid wadis
  • regions: hardy perennials and annual; no rain agriculture
  • semi-arid zones: open vegetable cover (steppe, bushes), hardy perennials, possible rain agriculture and extensive breeding.

List deserts

  • 8 larger deserts:

  • Other deserts:

    • North America:
      • Désert of Colorado
      • Grand Basin
      • Désert of Mojaves
      • Painted Serves
      • Désert of Sonora
    • South America:
    • Africa:
      • Deserted Kalahari Desert
      • of Libya
    • Asia:
    • Australia:
      • Simpson Serves
      • Sturt' S Stony Desert
      • Tanami Desert
      • Grand Desert of Victoria
    • Europe:
      • Deserted of Tabernas in Andalusia
      • Desierto de Las Bardenas de Navarra (by the man there creates is less than 100 years because of an intensive agriculture)

Geology and process morphogenic

Erosion

Being given the scarcity of water and vegetation in desert medium, erosion depends primarily on two processes: wind erosion and the Thermoclastie. Erosion by the thermoclastie results from the temperature variations on the rock. Those can cause, on the long run, of the cracks which increase gradually and which end up making burst the rock. The thermoclastie is all the more effective as the rock is fragile and that the thermal amplitude is important. The gélifraction (action of freezing) intervenes in the deserts of altitude.

Wind erosion attacks the rocks of the reg. by removing particles (Déflation, Abrasion) or by polishing their surface (Corrasion by wind charged with sand). It is more effective when the obstacles are non-existent and that the wind is powerful, regular and charged with dust or spray. The wind advances the Dune S (Barkhane S, ghourd) which form sometimes vast units called “Erg”.

On Channel IRD (video onlines of the Research institute for the Development): Wind dynamics. (December 2006,3 ' 01")

In the arid and semi-arid regions, the streaming can be an effective agent of erosion. The caracatère violent one and episodical of the phenomenon erodes the mountains and transports materials towards the Piedmont S, the Glacis ( sheet flood in English) and Plaine of spreading. Water streams and reaches the Talweg S to form temporary rivers, the Oued S. Their bed carts remains of various sizes (rollers, gravels, sands, suspended particles). The hyperarides mediums are marked by the absence of any river (Aréité or areism).

Effects of evaporation


  • Chott : in North Africa, a chott is a permanent salt water extent, with the changing shores, located in the semi-arid areas. The geomorphologists limit it to the sometimes ennoyée part sometimes discovered around the lake, carrying some vegetation and belonging to a unit wider than they prefer to name sebkha. The chotts are fed in a discontinuous way at the time of the rare rains, and undergo a strong evaporation, which accumulates salts on the surface of the silts, sometimes exploited.
  • Sebkha (in Africa), Playa (in the United States), salinas (in Latin America);

Vegetation

The density of the vegetation depends on the quantity of water available, of the force of the wind and the nature of the ground (salinity, reg., erg…) : only rocky the hyperarides mediums are completely deprived of vegetation (Atacama, Hoggar, reg. of Tanezrouft…). Contrary to an generally accepted idea, the plants push on the sand dunes: one finds bushes of creosote and prosopis ( Prosopis will juliflora ) in the dunes of the Vallée of dead the. The plants, the shrubs and the bushes concentrate in the beds of the wadis and around the water points. The adaptations of the desert flora mainly aim limiting the water loss, but also at obtaining as much water than the environment can provide him.

The plants succulent, also called " plants grasses" are adapted to survive in arid mediums. Among them are the Agave S, the Yucca S, the Tubéreuse S of the family of the Agavaceae and all originating in the American continent. The family of the cactacées also comes from America: their aspect is explained mainly by the adaptation to the conditions of Sécheresse, at the origin of the development of the function of storage and the reduction of surfaces of evaporation. The function of storage resulted in a thickening of the stem, and, for some species, by the development of roots tuberoses ( pterocactus tuberosus for example). She explains also the appearance of the coasts or a provision of the nipples in spiral, which allow, a little as on an accordion, the dilation and the retraction of the body of the plant to the liking of the periods of rains and dryness, without tear of the skin. The reduction of surfaces of evaporation resulted in a thickening of the skin, sometimes even covered with a kind of wax, a reduction in the number of stomata (pores allowing breathing), and especially, at much of species, the disappearance of the sheets. As for the spine S, their function is multiple: protection against the animals, but also collecting of the dew, protection of the skin against the heats of the sun, drying wind or cold of altitude…

The plants Halophyte S support salt grounds impregnated. Their adaptation, different from those of the plants Xerophytic S themselves, is related to their capacity to store Eau in the Feuille S, the Tige or the roots.

The xerophytic plants meet in very varied environments, such as the Désert S rocky but also the Canopée of the tropical forests.

  • See the article xerophytic Deserted and undergrowth

Fauna

The number of animal species is relatively relatively low in the arid zones. However, rare are the areas without any life (abiotic environments). Fauna adapted to the climatic constraints:

  • clear Pigmentation
  • Reserve: the bump of the camel contains greases
  • Temperature control of the body: in the event of hot summer days, the gangas seem to have a greater capacity to lose heat than the other birds of the desert. But this excellent adaptation to heat has its reverse: as soon as the temperature falls, the Thermogenèse must start, requesting a corresponding energy expenditure. The Oryx algazelle S can survive without water during long weeks, their Rein S preventing the water loss in urine, they can also raise the temperature of their body to avoid perspiring.
  • night Life: many animals leave only during the night to drive out and nourish (Gerboise, Addax, Oryctérope)
  • Abris: caves, burrows (the ground is an excellent heat insulator). The Addax sleep the day in basins which they dig themselves in sand, in the shade.
  • Lethargy: Estivation (yellow Souslik)

List live animals in the desert:

Occupation and exploitation by the Men

Since Prehistory, the Men always occupied and traversed all the arid deserts, in spite of the strong natural constraints. Traditionally, two lifestyles, often competitor, are present in the human society of the deserts: nomads and farmers. For two centuries, the modernization and the exploitation of the mining layers at industrial ends have transformed certain desert areas and makes emerge new challenges. Nevertheless, the desert remains one of the last really natural mediums.

Lifestyles traditional

  • Nomadism: the human groups move to seek the water points necessary to the survival of the herds. The breeding made live several Bedouin clans of in Asia or Touaregs in Africa. Today, this lifestyle is threatened to disappear because of the motorization and the assertion of the borders.

  • Cultures: since Antiquity, the irrigation makes it possible to emphasize desert or semi-desert areas in the Oasis:
    • the well makes it possible to bring back the water of the ground water to surface. The problem is that this water of fossil origin is often not renewable in the short run in the deserts.
    • the Qanat in Asia, will foggara it in Africa, is an underground system of irrigation making it possible to collect seepage waters.
    • Noria: to collect the water of the rivers in desert medium (the Nile, Tiger, Euphrate).


Great ages of the desert

Historical main roads

  • Silk route: network of trade route between the Asia and the Europe energy of Chang' year (current Xi' year) in China until Antioche, in Syria. It owes its name with the most invaluable goods which forwarded there: the silk, whose only Chinese knew the trade secret. As of Antiquity, many other products travelled on the same roads: stones and noble metals, fabrics of Wool or flax, Amber, Ivory, Lacquer, spice S, Glass, Coral, etc These roads, traversed by caravans, circumvented by north or the south the Désert of Taklamakan. These two branches had various alternatives, but all these tracks connected between them oases located at the periphery of the desert and the foot of the high mountains of the Tian Shan or Kunlun. The length of the course, the multiple dangers incurred by the travellers on these tracks subjected to the attacks of the brigands and the extreme rigor of the climate (torrid in summer and icy in winter), made very expensive the products which forwarded thus between the Mediterranean basin and the the Far East. It was one of the reasons which encouraged the Européens to seek a sea route towards the countries of the East. The Silk route was gradually abandoned at the 15th century.
  • Trans-Saharan Tracks (Africa): the caravan tracks , arranged as from the 9th century, passed by the Oasis of the Sahara: displacements were dangerous and painful because of the climatic constraints and the distances. The large convoys transported slaves since the Roman epoch but also all kinds of products which were used for the Troc.

Modern development of the desert

The extension of the perimeters cultivated in the desert depends on the techniques of irrigation. Today, the pumping of water requires electricals appliance. It thus poses the problem of the energy supply of the desert areas. The diversion of the Colorado allowed the birth of the Imperial Valley in California. The stopping of Aswan in Egypt, completed in 1970, makes it possible to irrigate 700.000 hectares of grounds.

The basement of the deserts often offers richnesses:

  • Of hydrocarbons (Deserted of the Sahara, of Saudi Arabia, Desert of Karakoum)
  • Of the Ore S: Uranium (Australia), Iron (the Sahara, Atacama), but, money (Mexico), Copper (Nevada, Atacama), Diamond S (Kalahari)
  • Of the mineral : Nitrate, Phosphate (Morocco, the Western Sahara), Borax (California), salt (Salt Lake, the Sahara,…), Gypsum.

The geographical and climatic conditions of the desert allow:

  • exploitation of solar energy and wind

The desire of expatriation and adventure of the developed companies involves the development of the tourist offer in desert medium. The town of Las Vegas developed quickly in a desert medium, thanks to water of Colorado.

Turning into a desert

  • In the past, the Sahara was not a desert before thousand-year-old IVe.
  • Of the historical examples: the Désert of Thar in India became desert between 2000 av. J. - C. and 1500 av. J. - C. At that time the river Ghaggar cease to perhaps be a river.

Writers and travellers of the desert

For a long time, the deserts, initially the the Sahara, attracted the men, in particular the Westerners. Some to explore it, chart it, discover it. Others also to find itself there vis-a-vis them-even, in a philosophical search.

Protestant desert

The " Désert" is also a term employed by the Protestants to mean the whole of the hidden places (caves, ravines, forests,…) where the Protestants French reformed were going to celebrate their worship in clandestinity during more than one century of intolerance and persecutions following the Revocation of the edict of Nantes in 1685 by Louis XIV. However, for them, this word of " Désert" simply the physical reality of the rejection did not indicate which they underwent: it was impregnated biblical reminiscences; forty years during, the Hebrews of the Exodus had wandered in the desert, place of tribulations, temptations and despairs, but also place where had been made hear the Word of the Eternal. This page of history relates to all the France but marked particularly the the Cevennes, the Gard entire, part of the Languedoc and Vivarais, where, at the beginning of the 16th century, Protestantism was largely established.

In 1910, Frank Puaux and Edmond Hugues created the " Museum of the Desert " who is established in the native house of the Chief Camisard Pierre Laporte called Roland. Located in the middle of low the the Cevennes, with the Farmhouse Soubeyran, in a hamlet of the commune of Mialet beside Anduze, it recalls this long page of the Protestant History which strongly marked the area: this period of the " Désert". He wants to also be pilot identity Protestante which appears through this history.

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