The hot tropical deserts and subdeserts.
Die Stimmen von Marrakesch (1967)
1 Geology stripped bare
1. Both drought and heat
1.1 The hot dry deserts
One criterion to define the world's hot deserts and subdeserts is that the average annual temperature is above 50[degrees]F (10[degrees]C). Yet the climatic parameters of deserts show great variability. Taking the Sahara, the world's largest desert, as an example, it can be divided into three areas: a mountainous Sahara that is cool or even cold, depending on altitude and latitude; a torrid Sahara in the south, where the average annual temperature is 86[degrees]F (30[degrees]C) or slightly more; and a temperate Sahara along the Atlantic coastline where temperatures never fall below the freezing point but where the highest temperatures do not exceed 77-86[degrees]F (25-30[degrees]C). Despite this diversity, the Sahara is a geographical entity that has to be treated as such, even at the risk of losing some of the finer details. The same arguments apply to most other deserts and subdeserts. The average annual temperature barely exceeds 59[degrees]F (15[degrees]C) in the coastal deserts of the Namib, Chile, or Peru, while it may reach 86[degrees]F (30[degrees]C) in the southern Sahara, the Persian Gulf, and on the coasts of the Red Sea (Dallol Depression, 394 ft [120 m] below sea level, and the Djibouti region).
The dominant high temperatures
With a few exceptions, the winter temperatures of the hot deserts are relatively mild, but depending on altitude and latitude, their winters may be hot, temperate, or cool. The average of the lowest daily temperatures in the coldest month is between 28[degrees]F (-2[degrees]C) and 23[degrees]F (-5[degrees]C) in some parts of the Iranian desert or the steppic highlands of the Near East or North Africa, while it is 59-68[degrees]F (15-20[degrees]C) in the Persian Gulf, the equatorial deserts and subdeserts of eastern Africa, northeastern Brazil, and those in Venezuela, Colombia, and Ecuador. The winters may be cold, cool, temperate, or hot, while the summers are hot or very hot, except in the oceanic coastal deserts. The average maximum temperatures of the hottest month varies from 77[degrees]F (25[degrees]C) in the coastal oceanic deserts to 113[degrees]F (45[degrees]C) in the southern Sahara, the area around the Red Sea, and in the Shatt al Arab. The southern Sahara, the northern Sahel, and the shores of the Red Sea and the Persian Gulf are, without a doubt, the hottest regions on Earth. The absolute maximum temperature may often reach 131[degrees]F (55[degrees]C) in many sites in the central Sahara. The absolute maximum temperature recorded on standardized equipment was 136[degrees]F (58[degrees]C) in Al-Aziziyah in northwestern Libya in September 1922 and at San Luis Rio Colorado in Sonora State in northwest Mexico (on the border with the state of Arizona, United States), in August 1933. In 1928 nonstandardized measurements of 149[degrees]F (65[degrees]C) in the shade and 167[degrees]F (75[degrees]C) in the sun were recorded in the Dallol Depression in the Danakil Desert (northeastern Ethiopia).
These high temperatures were reached because the desert areas receive very high levels of sunshine due to the low atmospheric humidity. Total sunshine in the central Sahara and Arabia, where the levels of sunshine are the highest on the planet, is 200,000-230,000 cal/[cm.sup.2] per year, 266-306 W/[m.sup.2] per year, or 0.84-0.97 MJ/[cm.sup.2] per year. Under these conditions, soil temperatures can easily reach 154-158[degrees]F (68-70[degrees]C), values that have frequently been measured on the surface of dunes in the Sahara. In coastal oceanic deserts, with the frequent fog and cloud cover caused by upwellings of deep cold waters (the Humboldt, Benguela, Canary, and California currents), total radiation does not exceed 140,000-180,000 cal/[cm.sup.2] per year, so temperatures are more benign, with an average minimum in the coldest month of 45-50[degrees]F (7-10[degrees]C) and an average maximum in the hottest month of 68-77[degrees]F (20-25[degrees]C); the average annual temperature is 59-68[degrees]F (1520[degrees]C). The low atmospheric humidity in deserts means that the temperature changes greatly over the course of the day and may even be greater than the difference between average temperatures for the different seasons. Differences of almost 30[degrees]C between daytime and nighttime temperatures are not uncommon, and this contrast is even greater in winter than in summer. At the In Salah station in the Algerian Sahara, diurnal variations of up to 29[degrees]C have been recorded in the summer months, and in the winter months oscillations of up to 38[degrees]C have been measured (37.5[degrees]C daytime maximum, and minimum of -0.5[degrees]C) on a single day in December.
The extreme scarcity of rains
The most distinctive feature of the climates of hot deserts and subdeserts is the scarcity of rainfall and the length of the dry season, when normally not a single drop of rain falls. Furthermore, total annual rainfall varies greatly from one year to another, with deviations from the mean of more than 40%. Extreme cases of this variability can be seen quite clearly in the data for Yuma, Arizona, where only 1 in (25 mm) of rain fell in 1899, compared with 11 in (280 mm) in 1905, and Helwan (or Hulwan, just south of Cairo), where not even a tenth of an inch of rain (only 2 mm) fell in 1934 compared with 5 in (125 mm) in 1946. Deserts may also have very different seasonal rainfall regimes. Some deserts have solstitial rains (Mediterranean deserts with winter rains, and tropical deserts with summer rains), while others have equinoctial rains (subtropical deserts with rains in the spring and autumn); both types of rainfall regime may be unimodal, as in the two first examples, or bimodal, as in equatorial deserts with spring and autumn rainfall.
High potential evapotranspiration
The ombrothermic (rainfall and temperature) diagrams of hyperarid zones are characterized by the fact that the rainfall curve is below the temperature curve in every month of the year. However, the diapneic curve, obtained by multiplying the potential evapotranspiration ([PE.sub.t]) by the constant 0.35, is above the rainfall curve. There are, however, some exceptions. For example, Mendoza (Argentina) and Djibouti fulfill these conditions, though they are not in the hyperarid zone. Simply, arid zones are characterized by a dry season 7-11 months long, while semiarid zones have a shorter dry period of 6-9 months. In many arid areas, [PE.sub.t] is about 1,400-1,600 mm per year, but unlike rainfall, it varies very little within a given region. The result is that in a given region where the [PE.sub.t] varies little, the aridity can be assessed simply on the basis of the annual average rainfall. Naturally, this cannot be done when comparing regions with very different [PE.sub.t] values; in these cases, the aridity is assessed by comparing their rainfall/evapotranspiration ratios (P/[PE.sub.t] where P = annual rainfall).
In hyperarid zones, annual [PE.sub.t] is at least 10-20 times greater than annual rainfall; to put it another way, in these regions the annual solar radiation can evaporate more than 10 times the annual rainfall. Practically, this corresponds to the area with the isoline of 100[+ or -]50 mm annual rainfall.
The violent and persistent wind
The force and persistence of the wind is another characteristic feature of hot deserts, especially continental ones. The hot winds, known locally by different names (harmattan, sirocco, khamsin, ghibli, chergi, chinook, Santa Ana), may blow for 100-300 days a year.
The wind is responsible for aeolian erosion, the main factor directly shaping a desert's surface morphology and landscapes. This type of erosion leads to the formation of desert pavements and wind deposits. The wind's effectiveness at shaping the landscape depends on many factors, above all on the wind's force, direction, persistence, and turbulence. The wind's effect also depends on the characteristics of the substrate, including its coherence, compactness, structure, texture, and humidity. Pre-dictably, the aridity of the climate is also a major factor, as the effectiveness of wind erosion increases proportionally with aridity. The plant cover of the soil is an important factor as well, and wind erosion is inversely proportional to the perennial plant cover (annual species play only a relatively minor role). Finally, soil use and cultivation practice must be considered; for example, tilling with a disk harrow has great impact on sandy soils.
It is not necessary, though, to take all these complex interactions into account. Experimental studies in laboratory wind tunnels have shown that the soils most at risk of erosion are sands with a particle diameter of 0.10-0.25 mm. For these diameters, the threshold velocity for erosion to occur is a wind speed of about 10 mph (4.4 m/s) at 12 in (30 cm) above ground level; the effect of deflation (the process of removal of dry material from the land surface) is greatest at wind speeds above 6 m/s. The threshold velocity decreases when turbulence increases. However, a suspension of fine particles may be created at any speed greater than 1 m/s, forming dust-bearing winds that often limit visibility in desert areas. There is an important threshold in the plant cover--25% cover by a layer of perennial vegetation. Above this threshold, there is little or no erosion, but erosion increases exponentially when the perennial cover declines below this value. Aeolian erosion involves five different processes: reptation (creep) over eroded surfaces by the largest particles (diameter greater than 0.5 mm); saltation of the smallest particles (diameter 0.25-0.5 mm); deflation of the fine grains (diameter 0.1-0.25 mm); suspension of the fine particles (diameters below 0.1 mm); and corrasion, when obstacles are blasted by the impact of wind-borne particles (mainly due to saltation and deflation).
The role of the wind is very important in the central and southern Sahara, where the harmattan wind blows from the northwest to the southwest from November to May at an average speed of 2.5-3.5 m/s. It has been responsible for the sandy formations of the Sahel, which formed during several hyperarid episodes in the Pleistocene.
1.2 Distance from the sea and the hot desert climate
Total radiation, annual rainfall, and the rainfall/evapotranspiration ratio are the most relevant climatic differences between the different hot deserts and subdeserts, but they are not the only ones. There is a whole series of other parameters that may vary, depending largely on how continental a desert's climate is (its temperature, range of temperature variation, and relative humidity).
The oceanic influence
The special features of the oceanic coastal deserts and subdeserts of Chile and Peru, California, the western Sahara, and the easternmost Canary Islands, as well as those in Angola, Namibia, and South Africa are caused by the cold currents flowing along their coastline (the Humboldt, California, Canaries, and Benguela currents, respectively). The oceanic coastal deserts and subdeserts are very unusual zones, where the moderate temperatures, cloud cover, and hidden precipitation (caused by the frequent fogs and the dew) reduce their aridity. These deserts cannot be considered true hot deserts and subdeserts, nor cold ones, as there are never frosts.
Oceanic coastal deserts and subdeserts are characterized by their high relative humidity (annual average greater than 70%), very frequent fogs (30-120 days a year), and a hidden input of water and condensation derived from fog and dew (less than half an inch to 8 in [10-200 mm] per year, and sometimes much more). The effect of the fog is greatest at those times of year that correspond to the rainy season at the same latitude--the winter in the Mediterranean area and the summer in the tropical zones. The average annual temperature is low, between 50-68[degrees]F (10-20[degrees]C), never reaching freezing or high temperatures above 86[degrees]F (30[degrees]C). Cloud cover is very abundant, and total incoming radiation is relatively low: 120-140 kcal/[cm.sup.2] per year (about 60-80 kcal/[cm.sup.2] per year net radiation), as opposed to the 160-200 kcal/[cm.sup.2] per year in continental areas at the same latitude (80-120 kcal/[cm.sup.2] per year net radiation). The potential evapotranspiration is low (35-39 in [900-1,100 mm] per year, compared with 59-98 in [1,500-2,500 mm] per year in continental areas at the same latitude). The rainfall/evapotranspiration ratio is very low because the rainfall is very low.
The coastal oceanic deserts of the coastlines of Namibia, South Africa, Chile, and Peru are very narrow. The coastal effect is most intense in a strip reaching about 6 mi (10 km) inland and gradually declines to nothing at 62 mi (100 km) inland, or even less. Farther inland, fogs are less common, relative humidity is lower, temperatures are higher, there is an increase in the range of temperatures over the course of the day and of the year, and potential evapotranspiration increases; the temperatures also tend to increase as hidden forms of condensation diminish. The landform may, however, increase or decrease the influence of the sea. Fogs tend to flow along valleys perpendicular to the shoreline and may have a considerable effect in these deserts on slopes facing the sea, as happens in the lomas in the Peruvian desert, where the vegetation is often completely dependent on hidden precipitation. In some cases, there is a fog zone in mountains running parallel to the coastline. This can be seen in the mountains around the Red Sea and the Gulf of Oman, where, depending on the site, the fog zone is situated at elevations of 3,281-5,577 ft (1,000-1,700 m). The same phenomenon also occurs in the Cape Verde Islands and the Canary Islands in the Atlantic, where the mountainous slopes exposed to the trade winds from the northeast often receive considerable condensation (which may be 1.5-5 times greater than the quantity of rainfall recorded), while the southwest-facing slopes show a strong feohn effect, which disperses clouds and fogs on the opposite side of the mountain range.
The effects of continentality
The characteristics of the continental hot deserts and subdeserts contrast sharply with those of the coastal ones, and the more continental the desert, the greater the contrast.
There are several ways of estimating how continental the climate is in a given site. The simplest is to estimate the mean annual temperature range (A), which is obtained by calculating A = M-m, where M is the mean annual maximum temperature of the hottest month, and m the mean minimum temperature in the coldest month. A may vary from less than 54[degrees]F (12[degrees]C) (on oceanic islands) to more than 104[degrees]F (40[degrees]C) (in the central Sahara or the Mojave Desert). Gorczynski's index of continentality (K) is also often used. K= =1.3 A/sin y -36.3, where A is annual temperature range and y is the latitude. K may vary from 10 to almost 50.
In general terms, the average annual temperature in hot continental deserts and subdeserts is 64[degrees]F (18[degrees]C) or greater and may reach 86[degrees]F (30[degrees]C) locally. The average annual heat range exceeds 68[degrees]F (20[degrees]C) and may even exceed 104[degrees]F (40[degrees]C). The total radiation is 140-230 kcal/[cm.sup.2] per year. Annual rainfall is 0-16 in (0-400 mm) and potential evapotranspiration is 47-98 in (1,200-2,500 mm) per year. The rainfall/evapotranspiration ratio may be 0-30%, and the Budyko xerophytic index may be between three and more than 50. The number of hours of sunshine is 2,500-4,500, compared to values of 2,0002,500 in cold deserts and subdeserts.
2. Little soil but many stones
2.1 The difficulties of soil formation
In the arid conditions of the hot deserts, soil formation processes (edaphogenesis) are extremely slow. This might suggest that soils with differentiated profiles would be uncommon. In practice, the situation is not always as might be expected, since most present-day deserts have not always been as arid as they are now, and soil formation may have occurred in pulses during periods of high rainfall that are not reflected in annual rainfall data.
The factors leading to soil formation
In deserts, the soil formation factors that most affect the soil's chemical properties are the parent material, the climate, the relief, and the duration of soil formation. The vegetation does not, however, play a decisive role in arid environments because, when present, it is only open scrub or annual herbaceous plants. The organic matter content of the soils is always quite low, as the few plant remains decompose rapidly when temperatures are high and water is available after rainfall. Soil horizon formation is very slow in deserts. Most soils with well-developed subsurface horizons such as argillic horizons (Bt, with accumulation of clay) were formed in a warmer climate, probably thousands of years ago. Horizons showing accumulations of calcium carbonate or calcium sulfate, known as calcic and gypsic horizons, develop faster than argillic ones. Chemical weathering of primary minerals is also slow under these conditions.
In areas with arid and hyperarid climates, soils are chemically neutral or alkaline, with base saturation of more than 90%, a surface layer with less than 1% organic matter, a subsurface calcic horizon at a depth of less than 7 ft (2 m), and a clay fraction dominated by 2:1 type phyllosilicates.
Accumulations of carbonates, gypsum, salts, and silica
Two of the most characteristic features of soils in arid regions are subsurface horizons and calcic or gypsic endopedons. They are due to soil formation processes and are not geological formations, as they have sometimes been interpreted.
Absent or poorly developed in deep sands and in clays, their development is clearest in permeable gravel soils occupying a stable position in the landscape, where calcium carbonate or gypsum (calcium sulfate) has accumulated in the soil.
Carbonate or gypsum accumulations may form in different ways: as nodules, a layer covering larger structures, horizontal layers, vermiform gypsum, pseudomycelia of calcium carbonate, imbricated crystals (desert roses, see photo 27) within the horizon, or as generalized dusty or floury accumulations. These accumulations may be soft or cemented into a solid horizon as hard as solid rock. They are known as petrocalcic or petrogypsic horizons, depending on the cementing agent.
Calcic horizons are due to the accumulation of calcium carbonate and, to a lesser extent, the accumulation of magnesium carbonate. These subsurface horizons contain more carbonates than the parent material from which they are considered to have formed. The carbonates may derive from the washing of surface horizons, deposition of calcium or carbonates from the atmosphere, from other soils or from the sea (cyclic salts), or even by the upward movement of water rich in carbonates from the water table. Calcic horizons are typical of calcisols.
Gypsic horizons, typical of gypsisols, are very widespread in arid regions. Petrogypsic horizons are rare in the Americas but more common in northern Africa and in the cold deserts of central and middle Asia. Gypsum accumulation may have been caused by several different processes, including the washing from surface horizons, the presence of a water table saturated in calcium sulfate, the arrival of dissolved gypsum in surface runoff, the in situ breakdown of gypsum deposits, or even transport by wind. If accumulation is due to washing of gypsum from the upper horizons, a calcic horizon often forms above the gypsic one because calcium carbonate is less soluble and precipitates first.
Other horizons that are found in arid environments include salic horizons, natric horizons, and those cemented by silica. Salic horizons are surface or subsurface horizons that are found mainly in endorheic depressions and also in coastal deserts where cyclic salts more soluble than gypsum are deposited. Accumulation of salts in unirrigated soils often coincides with the presence of calcium carbonate or gypsum. Salic horizons are the distinctive feature of most solonchaks.
Subsurface horizons cemented by silica found less than 3 ft (1 m) below the soil surface are known as duripans. In arid regions, they are often calcic, and the soils containing them are therefore classified as duripan phases of calcisols. In Australia, most duripans are found on the surface as desert crusts known as silcrete, which form a solid cover in the table relief of the continent's interior. On weathering, the silcrete forms a desert pavement known as "billy gibbers."
Natric horizons are typical of the soils known as solonetz, and are restricted to areas with semiarid to subhumid environments. All the natric horizons are also argillic, signifying they have to be very old for the clay to have accumulated. The distinctive features of natric horizons are 1) their columnar, or prismatic, structure, 2) their high percentage of exchangeable sodium, and 3) their low permeability to water.
The slow weathering processes
Desert soils are little developed except for those that formed in the past in wetter conditions or those that have been stable for decades, centuries, or even millennia, as is the case of many Australian desert soils. Physical and chemical weathering is slow in arid regions, meaning that soils are generally coarse-textured, with a poorly developed structure, a high proportion of macropores, and low structural stability due to their low content of organic matter. They are highly susceptible to erosion by water because of the scarce plant cover. Wind erosion is very active due to the combination of sparse vegetation and the sandy texture of the surface horizons.
The weathering of the original materials is due mainly to the sharp daily temperature differences that may exceed 122[degrees]F (50[degrees]C) and cause the physical breakdown of the rocks. The chemical action of the water--in the form of the occasional rains or the dew (that may be of great importance, as in the westernmost strip of the Namib Desert)--favors to some extent the chemical weathering and the dissolution and mobilization of soluble components within the landscape. This occurs especially in the zones where the water is concentrated, as happens in the sabkhas, along the watercourses, and near the springs.
The texture of desert soils is very variable, ranging from sand to clay, but there is a geographical zonation from which the differences can be deduced. Soils on plateaus are almost always sandy, gravelly, or stony, due to the low rate of weathering of the parent material and the rock fragments deposited on the alluvial slopes of hills and mountains.
It should be remembered that clay formation, which may only take a few decades in the wet tropics, may take thousands of years in the deserts. Water is clearly the limiting factor. Clay soils are now found in present-day or former endorheic depressions where water bearing fine particles has provided a wetter environment--one that is more favorable for clay formation. The size of the depressions varies greatly; they may be as small as the playas of the High Plains of Texas or as large as the former lakes and seas in the Sahara.
Almost all the soils in arid regions that have formed under current climatic conditions show only moderately developed structures, except those cemented by carbonates or silica. The soils with the weakest structures are found on the hyperarid plains, while those with the most developed structures are found in the wettest environments in the arid climate zone, in the depressions and watercourses. The main factors responsible for this poor structural development of the soil are its low organic matter and clay content.
Vulnerability to erosion
As stony soils are more widespread than sand dunes in arid regions, water erosion is a more important factor in shaping the landscape than wind erosion. This is not the case in hyperarid zones, where the extreme rarity of rain means that wind erosion is more important, and where erosion processes are not greatly influenced by humans. This erosion is the result of the natural forces acting on Earth's surface, and little can be done to reduce it. Any attempt to do so is doomed to failure unless the characteristics of the environment change. The situation is different in areas with an arid climate where there is some agricultural or stockraising activity such as grazing, which, when excessive, can cause accelerated erosion of anthropogenic origin. This erosion may reduce the area's productivity by increasing the risk of flooding or silting of reservoirs. These effects are greatest in sites where natural erosion is already high. Erosion caused or increased by humans can be controlled, though in many cases it may not be economically worthwhile for the people who live in the affected areas.
The soil surface in arid regions is often covered by a thin crust--hard enough to make seed germination and growth more difficult and impermeable enough to increase runoff, at least for a short period of time. These crusts are formed by two processes: the impact of raindrops, which destroy the aggregates, and the filling of the soil pores by the particles suspended in the splashes. Rain drops (and even sprinkler irrigation) break up the aggregates when the water hits the soil, dispersing fine particles into the air that then fill up the surface pores, thus reducing the water that can infiltrate. Runoff water and irrigation have similar effects when they filter into the soil, as they both deposit their sediments on the surface, forming thin layers that are less permeable than the soil below. When they dry out, they form a hard seal and make seed germination difficult.
Erosion by the wind is a normal phenomenon in hot and cold deserts. Aeolian erosion is highly effective in abrading rocks and soils through the impact of the grains of sand that are carried. At the base of slopes or rock cliffs, the phenomenon known as alveolization occurs: erosion is more active at the base of the wall where the shade favors humidity, so that the wind gradually undermines the rock. This leads to the formation of cavities and hollows known as tafoni. Natural wind erosion in deserts is responsible for the large deposits of loess in Asia, Europe, and North America. Loess deposits consist of huge accumulations of silt blown by the wind from deserts (or from areas affected by the last ice age) and deposited in semiarid regions where the vegetation traps this very fine dust. This process is still active in regions such as the Sahara. The Sahara Desert acts as a source of fine dust that is deposited in the Middle East and forms the parental material of the loess soils. The world's deepest deposits of loess are in China.
The unusual geomorphology of the subdeserts
The subdeserts show a special geomorphological feature: the synclinal slopes are occupied by sloping erosion embankments (glacis, or pediments, three to five in number, that developed during the Quaternary). These embankments are restricted to patches and are often topped by a hard formation or crust that causes them to fossilize. The crusts are calcareous or gypsum in the Mediterranean area, and ferruginous in the tropics and in noncalcareous environments (crystalline plinths, eruptive rocks, noncalcareous sediments). But calcareous and gypsum crusts also occur in the tropics (Somalia, Kalahari), and there are some outcrops of Mio-Pliocene crusts in the Mediterranean area. These formations are relicts of the ancient, middle, and recent Quaternary and are located on erosion embankments and alluvial terraces, generally three or four in number. Erosion embankments are normally covered by skeletal soils and are thus unsuitable for natural vegetation and crops alike. Alluvial embankments and terraces, however, have relatively well-developed, deep soils.
The most important features of subdesert soils are those that reduce the aridity of the climate by allowing the soil to accumulate the scarce rainfall, and then make that water available to the plants. These include the location within the relief (the gain or loss of water through runoff), the texture (mainly that of the surface horizon, which conditions permeability and thus filtration of water), the state of the soil surface (such as the presence or absence of a biological crust), the water's rate of filtration into the soil, the soil depth (which affects its storage capacity in the water balance and thus the production), the absence of toxicity and of dissolved salts in unfavorable quantities, and the chemical balance (this also affects fertility, especially the levels of phosphorus). It is worth pointing out that for plants, water is not always the most limiting factor in subdesert soils; the lack of nutrients (N, P, K, Ca, Mg) or trace elements (Cu, Mn, Zn, Mo, Fe) may occasionally limit productivity. The low quantity of organic matter and the soil's poor microflora, microfauna, and mesofauna, are often also factors limiting productivity, especially on degraded soils.
The role of biological crusts
Biological crusts play a major role in soil formation, nitrogen fixation, and the reduction of erosion by water and wind. Except for mobile dunes, lichen and bacterial communities occur in almost all environments in arid zones, however inhospitable they may be. Lichens are well adapted to extremely arid environments, as they can tolerate very high levels of sunshine and intense drought. They can survive astonishingly long periods without water by entering a latent state, then can renew activity after periods of up to several decades. Furthermore, lichens can photosynthesize at very low levels of humidity--levels at which algae alone cannot photosynthesize.
Algal and bacterial crusts (see vol. 10, pp. 335-338) only form in the periods when there is standing water on the soil surface, as they are more sensitive to dry conditions than lichens. When they dry out, algal and bacterial carpets curve upwards like mud or clay crusts. They are about 0.2 mm thick, but if they contain trapped sand, they may be 2-3 mm thick. Microalgae and cyanobacteria are tolerant of low temperatures, but they do not tolerate high temperatures very well. In arid regions, algal and bacterial carpets reduce erosion by wind and water after rainfall. They also reduce soil water loss by evaporation. These effects are not, however, very important in deserts because these short-lived crusts only last until they dry up. Dry crusts may even increase erosion by reducing the speed at which water filters into the soil. The ability of these crusts to fix nitrogen is limited--it exists but is not significant.
2.2 The soil types
The characteristics of the soils in the hot deserts vary with the climate, the location within the landscape (enclosed depressions, mountains, hills, plains), and the age of the soil. The cliche that deserts are huge spaces covered by sand dunes is incorrect. This is only true of 25% of the Sahara and barely 50% of the Arabian, Namibian, and Iranian deserts. Sand deposits only occupy a small area in the hot deserts of North America and the Atacama Desert.
In the climatic deserts in hyperarid regions (the Sahara, Namibian, and Arabian deserts), the soils consist mainly of shattered rocks (regs, hamadas, sand) or, in a few areas, the soils may be derived from lake deposits, which are not very developed, clayish, or loamy. Most soils in these extremely arid regions--where annual rainfall is less than 4 in (100 mm)--are covered by stones and gravel.
Soils in mountain deserts
In areas with abrupt relief, the dominant landforms are slopes, where the lack of water prevents weathering and the establishment of vegetation. The surface runoff water is linked to irregular but often intense torrential rains, meaning that erosion eliminates all the materials formed by physical weathering. The characteristics of the slopes are thus controlled by the rate of weathering. The soils are skeletal, stony, and highly superficial, usually with many rocky outcrops, often with a black varnish of iron and manganese oxides.
The materials resulting from erosion and water transport give rise to accumulations of materials at the base of slopes and alluvial fans that fill hollows. In these lower positions, thanks to the supply of preformed soil, soil formation may have been faster during the wettest cycles of the Quaternary.
Sand, less abundant in deserts than is often thought, accumulates in large rolling areas known as ergs that cover the smooth previous relief. Ergs usually occupy large areas: the Grand Erg Oriental in the Sahara, for example, occupies 75,676 mi2 (196,000 [km.sup.2]) in Algeria and Tunisia.
Sand consists mainly of quartz but contains a small percentage of grains that can be weathered. They vary in diameter from 0.15-1.4 mm and are generally yellow to dark red in color. The color indicates the erg's age; the redder colors correspond to older sands, up to 500 million years old, while the lighter sands are more recent.
Most desert soil sands form part of sandy coverings of considerable thickness, as in the Sahel region and the Kalahari desert, or they form sand dunes, as in the Rub' al-Khali Desert (Saudi Arabia). These materials are mobile in the wind, so soil formation does not occur. Arenosols can only form in the most stable sites. The texture of the sand cover, with a flat or slightly rolling relief, varies from coarse sandy loam to sandy loam or sandy. In general, the surface horizons tend to be finer textured in less arid regions, where soil formation processes are more important than in drier regions.
Erg soils are often 3 ft (1 m) deep, lack a calcareous surface, and contain very little organic matter. The only signs of soil formation are a slight accumulation of organic matter on the surface and slight enrichment of the deeper horizons in clay. Their sandy texture means they retain little water, and their fertility is low. These soils are very prone to erosion by wind and may also be affected by water erosion after heavy rains.
Soils in stony deserts--regs
Regs or serirs (stony deserts) are characterized by a cover of gravel and pebbles, which are normally rounded. Regs cover large desert plains of gravel in the Middle East, North Africa, and in the world's other very arid regions. In many cases they are flat plains, but they may form on gentle slopes or on alluvial fans. Regs are the result of the action of the wind (deflation), which has sorted the materials by particle size, leaving the largest particles--those not blown away by the wind--on the surface.
One of the characteristics of these soils is that the layer of gravel and pebbles forms a cover resistant to erosion by both water and wind. This layer is known as a desert pavement and by other different names in different deserts. They often contain stones called dreikanters, which have been shaped by the different prevailing winds. The gravel is frequently covered by a very thin, dark, shiny film known as desert varnish, consisting of iron and manganese oxides precipitated on the surface and mixed with clay minerals, silica, and carbonates. Some soil scientists feel that desert varnish is due to a chemical process of solution of the oxides followed by precipitation, while others think its formation is due largely to microbial activity.
Wherever the landscape was stable enough during the wetter paleoclimatic periods, there may have been translocation of calcium carbonate leading to the formation of cemented accumulations within the soil (petrocalcic endopedons). In other cases, after the washing of the carbonates, the clay may have been illuviated to form an argillic horizon--one of considerable thickness but containing only a little clay. Repetition of these processes during the wet periods of the Quaternary made it possible for these soils to acquire more complex horizons. The result is the presence of argillic or stony calcisols, either on the surface or buried by newer materials. The gravels of the modern reg may thus be covering soils that have undergone a very intricate formation process.
Reg soils cover 15% of the Negev and Sinai deserts, regions where annual rainfall rarely exceeds 3 in (65 mm). Some authors see a clear relationship between the age of the geomorphological surface and the soil characteristics (such as the development of cambic, salic, and gypsic horizons). Younger soils of thick desert alluvium are found in the drier beds of the wadis and are only a few thousand years old; their soil profile shows little differentiation into horizons, and they are generally not saline.
Higher terraces 70,000-12,000 years old show clear differentiation into profiles, and a cambic endopedon may be present. These saline soils sometimes contain gypsum. True reg soils are found, for example, in the oldest and highest geomorphological surfaces in the Ha'arava watercourse to the south of the Dead Sea and are several hundreds of thousands of years old. Their profile shows clear differentiation into cambic, salic, and gypsic horizons. Under the desert pavement, which protects them from accelerated erosion, the horizons of typical reg soils contain almost no stones. The subsurface (B) horizons are highly saline, and at depth there may be a gypsic or petrogypsic endopedon.
Soils in rocky deserts--hamadas
The rocky deserts known as hamadas are desert platforms consisting of hard rocky outcrops. In the Thinghert hamada (Algeria), the outcrops are white Cretaceous limestone. In the northwestern Sahara they consist of mixed sedimentary materials that may give rise to a smooth relief with slopes. Block hamadas are formed from sedimentary rocks and are covered by fragments of the underlying rock. Stone hamadas develop from hard rocks and are covered with dreikanters, stones showing deflation. Fissures in the rock may accumulate materials that are coarser at the top and finer at depth, as there is some illuviation of the clay.
Sabkhas (saline depressions) form in the lowest areas of depressions or endorheic drainage basins. They have different names depending on the region in which they occur: sabkha, sebkhah, or chott in the Near East and northern Africa, pan in Australia and South Africa, mier in southern Africa, and salina or salar in South America. Sabkha formations occupy only about 1% of the total area of deserts, but they play a major part in the landscape because of their role in the circulation of water, in landscape formation, and land use, as some types of oasis are located on the edge of a sabkha. Water accumulates, temporarily waterlogging the site, and then evaporates, leaving behind the fine particles in the water and precipitating the salts and dissolved compounds either within the soil or on its surface. The distribution of these compounds is conditioned by their solubility; since the least soluble components precipitate first, there is a concentric zonation of accumulations of carbonates, gypsum, and more soluble salts from the edge to the center of the sabkha and from the deepest horizons toward the soil surface.
Playa soils are deep, fine-textured, alkaline, calcareous, show low permeability, and are frequently saline. They are classified as vertisols or solonchaks in the FAO classification system and have very little agricultural potential. Most lack plant cover due to the low rainfall or because they are waterlogged for long periods. If there is any vegetation, it usually consists of special communities adapted to salinity or high gypsum levels.
Clay and alluvial soils
Today's clay plains formed under lakes or seas. They are found in several Saharan countries such as Sudan, Chad, and Mauritania, in the Near East, and in the Thar Desert in India and Pakistan. The soils of clay plains are very similar to those of sabkhas, though they may occupy large plateaus such as the Gezira region of Sudan, where high levels of cotton production may be achieved on irrigated land. These soils are thick and form deep wide cracks when they desiccate. Great care has to be taken to avoid their waterlogging and suffering salinization. Most soils on these plains are classified as vertisols, though there are inclusions of saline or sodic soils, known respectively as solonchaks and solonetz.
Desert watercourses are usually ephemeral; they may be small streams or cover a large area. Known as arroyos or wadis, they normally bear water only after long intense rains in the upper areas of the basins they drain. Though their volume of flow is usually low, it may occasionally be much larger and may bear large rocks downstream and form deep gullies. The flow of water often percolates into the soil and gives rise to underground water flows that continue draining toward the depressions, the sabkhas, where they eventually evaporate. When flow is violent, the materials deposited are blocks, gravel, stones, and coarse sand, but in the case of normal runoff the materials borne in suspension are clays, mud, and fine- to medium-sized sand. As a result, these alluvial soils vary greatly in texture, both horizontally and vertically, depending on the characteristics of the flow that deposited the materials. These soils are normally classified as fluvisols.
3. The world's hot deserts and subdeserts
3.1 The hot deserts and subdeserts of the Old World
The hot deserts of the Old World, north and south of the equator, are all very different. The African deserts occur in three different areas of the continent: 1) in North Africa, the Sahara Desert straddles the Tropic of Cancer; 2) in southern Africa, straddling the Tropic of Capricorn and running along the southwestern Atlantic coast, lies the Namib Desert and, further inland, the Karoo and the Kalahari; and 3) in the east of the continent, along the Red Sea and in the Horn of Africa, is the Danakil-Somali Desert. One of the world's most extensive areas of hot desert and subdesert is located at the same latitude as the Sahara. It runs from the coastline of the Red Sea to the Indus Valley and includes most of the Arabian Peninsula, much of Mesopotamia, the southern half of Iran (especially the lowlands and the regions closest to the coast), and the lower and middle valley of the Indus.
Despite the differences between them, the Old World deserts all have their own highly diverse flora and vegetation. The deserts in Arabia and the Near East are relatively closely related to those of northern Africa (almost half the 2,800 plant species occurring in the Sahara also occur in Arabia), while the Sind and Thar are tropical deserts related to the Sahel, the deserts of the southern Sahara, southern Arabia (Oman and Yemen), and eastern Africa (Ethiopia, Somalia, and Kenya). The flora of the Sind and Thar deserts in the eastern Indus Valley, astride the India-Pakistan border, is 80% tropical. Although the expression Saharo-Sindian floristic element, often used in phytogeography, seems totally inappropriate when discussing the Old World deserts (as the Sahara has only 30% tropical species, the Arabian has the same, while more than 50% of the species have Mediterranean affinities), it can be used because of the regional flora's genetic homogeneity. The flora of the deserts of southern Africa also shows tropical affinities, but their flora is much richer than that of the Sahara or the deserts of eastern Africa because of the proximity of the Cape floral region.
The vegetation shows a special pattern of distribution, with perennial plants concentrated along watercourses and in different types of open and closed depressions, especially in pebble and gravel soils and on clay plains, while they are absent from the interfluvial areas. In subdeserts, however, perennial plants are scattered, that is to say their distribution is regular, though scarce, throughout the landscape.
The fauna of these areas includes goats (Capra hircus), Barbary sheep or aoudad (Ammotragus lervia), and antelopes such as the gazelles (Gazella), addax (Addax nasomaculatus), oryx (Oryx), dik-diks (Madoqua), cobs (Kobus kob), kudus (Tragelaphus strepsiceros, T. imberbis), and impala (Aepyceros melampus). There are also Cape buffalo (Syncerus caffer), Grevy's zebra (Equus grevyi), cheetahs (Acinonyx jubatus), sand cats (Felis margarita), fennecs (Fennecus zerda), African hunting dogs (Lycaon pictus), and jackals (Canis adustus, C. aureus). The only desert species of the camel family, the dromedary (Camelus dromedarius), was originally only from the Arabian deserts.
The Sahara Desert
The Sahara, the world's largest desert, covers an area of approximately 8.6 million [km.sup.2], including the area below the 4 in (100 mm) mean annual rainfall isohyet. It stretches 3,418 mi (5,500 km) from east to west along the 20th parallel from Port Sudan (Sudan) on the Red Sea at 37[degrees]E to Nouadhibou (Mauritania) on the Atlantic coastline at 17[degrees]W. The longest distance north-south, along the 6[degrees]E meridian between 17[degrees]N and 35[degrees]N, runs almost 1,243 mi (2,000 km) from Agadez (Niger) to Biskra (Algeria). This immense area, the size of Australia, is very diverse in terms of relief, geology, mineral resources, climate, geography and physiography, fauna, flora, and human populations.
The Sahara's geology includes all the world's most common rock types from all periods, ranging from the pre-Cambrian metamorphic basal complex, which is more than three billion years old, to Holocene lake deposits less than 10,000 years old. In the upper Palaeozoic, when the Gondwana landmass began to separate from Pangaea, the area that is now the Sahara and the rest of the African continent were very near the South Pole. From the Devonian (about 400 million years ago), however, the landmass started to drift north, and the continent occupied its current location by the Miocene (about 25 million years ago). The thick Devonian sandstones of the Tassili N-'Ajjer Massif surrounding the mountains of the central Sahara have been a major source of the sandy materials that erosion, and then long-distance transport by past rivers, moved to other parts of the desert. These materials were eroded by the wind and rain and in the Pleistocene formed the great sand deserts to the north and the sandy substrates of the Sahel to the south. The soils show little development and contain very little organic matter. Saline and gypsic soils are very common on sedimentary substrates, especially in the northern and central Sahara. Yet microbial activity can be observed in even the driest gravel soils, showing detectable C[O.sub.2] production by the bacteria, fungi, and algae.
Large areas of the northern Sahara are covered by outcrops of Mesozoic limestones, marls, and schists. The continental Mesozoic-Cenozoic sandstones have played a major role in Algeria and Tunisia in the conservation of water resources by forming a deep aquifer that covers an area of subsoil of 0.8 million [km.sup.2], with a flow of 21.5 [m.sup.3]/s. This aquifer flows to about 2,200 wells, many foggara (see fig. 117), springs, and salt lakes that may contain a total of 6 x [10.sup.13] [m.sup.3]. Outcrops of Nubian sandstones in the eastern Sahara, also dating from the Mesozoic to the Cenozoic, cover 1.8 million [km.sup.2] in Libya, Egypt, and Sudan and supply water to many typical oases and depressions (Kharga, Bahariya, Farafra, Siwa, Qattara, Al-Kufrah), with an artificial flow of 4 [m.sup.3]/s in the Al-Kufrah basin, but apparently with a much greater potential discharge and an unknown, but seemingly immense, capacity. The waters of the continental sandstones of the northwestern Sahara are 4,000-8,000 years old and their quality (for irrigation) is good to poor, depending on the local situation. The waters of the Nubian sandstones of the Al-Kufrah basin are 20,000-30,000 years old; their quality ranges from good to excellent and they are, at least partly, derived from the waters of the Nile in the Pleistocene.
The area's elevation varies from 449 ft (137 m) below sea level in the Qattara Depression in northwestern Egypt (and -56 ft [-17 m] to 66 ft [20 m] in large chott areas of southeastern Algeria and southwestern Tunisia) to 9,843 ft (3,000 m) in the Ahaggar Massif in the central Sahara in southern Algeria, and 11,155 ft (3,400 m) in the Tibesti Massif in northern Chad. Most of the desert, however, consists of rolling plains at elevations of less than 1,640 ft (500 m). Mountains and hills occupy 0.8 million [km.sup.2] (approximately 10% of the Sahara's total area); denuded plateaus, pavements, and stone and gravel plains occupy 5.8 million [km.sup.2] (68%), while dunes and sandy areas occupy 1.9 million [km.sup.2] (22%). Despite the popular impression, dunes and sandy areas cover less than a quarter of the Sahara.
As might be expected in such a large area, the Sahara's climate varies greatly in aridity and seasonality. The northern Sahara receives only winter rains; its climate is an extreme form of the Mediterranean climate, with rains caused by cyclonic depressions linked to the polar front. The southern Sahara, however, is almost exclusively subject to summer rains generated in the Intertropical Convergence Zone. The oceanic western Sahara occupies a coastal strip 31 mi (50 km) wide and is an attenuated coastal desert, characterized by moderate temperatures, high levels of humidity, cloud cover, the frequent presence of mists, strong sea breezes, and a relatively low potential evapotranspiration (40% less than in inland areas). All these phenomena are associated with the upwelling of the deep cold waters of the Canaries current, which flows north-south along the coastlines of Morocco and Mauritania between latitudes 21[degrees]N and 28[degrees]N (see also vol. 10, p. 39).
In the Sahara, temperatures vary greatly with latitude, altitude, and the degree of continentality. Surprisingly, the average annual temperatures in the Sahara are not the highest in Africa, as the average annual temperatures in the Sahel and the shores of the Red Sea may locally exceed 86[degrees]F (30[degrees]C), higher than those of the Sahara. The absolute maximum temperatures recorded in the Sahara are, however, among the highest recorded on the planet, 122-131[degrees]F (50-55[degrees]C). Except for the oceanic strip, the western Sahara with its average annual temperature of 82[degrees]F (28[degrees]C), is hotter than the eastern Sahara (except for the Aswan and Wadi Halfa region), with an annual average temperature of 77[degrees]F (25[degrees]C). In the mountains, above 4,921 ft (1,500 m), the average annual temperature declines to less than 68[degrees]F (20[degrees]C) and frosts are common in winter. (Temperatures descend as low as 14[degrees]F [-10[degrees]C] on peaks in the Ahaggar and Tibesti massifs at altitudes of about 9,850 ft [3,000 m].) Most areas in the northern Sahara, from southeastern Algeria to southwestern Libya, including southern Tunisia, experience frequent slight frosts in winter.
The Sahara's flora is richer than one might expect. About 2,800 species of flowering plants have been described, half of them also found in the Arabian Desert. About 500 species are endemic, that is to say, their distribution is restricted to the Sahara. The flora of the northern Sahara is closely related to the Mediter-ranean flora, as 70% of the species belong to Palaearctic families and genera, and less than 10% are from tropical areas. In sharp contrast, the flora of the southern Sahara is 70% tropical and only 5% Mediterranean. The flora of the plains of the central Sahara is very poor (with only 500 species), derived half from the Mediterranean and half from the tropics. Thus, the distribution pattern of the flora is similar to that of climatic factors. Vegetation is normally associated with the drainage network, except for dunes, cliffs, and areas with a water table close to the surface. The vegetation consists of shrubs, grasses, and other annual and perennial herbaceous plants. The northern Sahara is dominated by small shrubs, with many small chenopods marking the transition into subdesert steppes, also dominated by these plants. Tall trees and shrubs are normally restricted to the drainage network. Tropical perennial grasses dominate the southern Sahara (Panicum turgidum, for example), while spiny trees and shrubs (Acacia, Balanites, Ziziphus) mark the transition to the Sahel scrub and savannah farther to the south.
The fauna also shows both tropical and Mediterranean affinities, but its distribution may depend more on the zoological group in question than on geography. Most of the large mammals show Afro-tropical affinities, while the rodents are mainly Mediterranean. The same applies to other groups such as birds, reptiles, and insects. The Sahara has 110 species of mammal (20 large mammals and 90 small ones), but most of the large mammals that were common in the late nineteenth century are now extinct or endangered. There are still 45 species of rodent and 22 species of bat. In addition, there are 250 species of bird (90 of them sedentary), 100 species of reptile (70 lizards and 30 snakes), 10 species of amphibian, and 20 species of fish. There are many insects, particularly hymenopterans (66 species of ant and 30 of termite) and beetles (Carabidae, Curciolinidae, Scarabaei-dae, and especially Tenebrionidae--ground beetles, weevils, dung beetles, and darkling beetles).
The deserts of the Middle East and the Indian subcontinent
The Arabian Peninsula is a large block of territory covering three million [km.sup.2]. It separated from the Afro-Arabian plate in the middle Tertiary with the formation of the Red Sea, the northern extension of the Great Rift Valley. The peninsula's characteristics are very similar in every way (geology, human population, climate, fauna, etc.) to the Sahara. A considerable part of the peninsula is covered by sand deserts; among them are the An Nafud (a red sand desert in the northwest) and the Rub' al-Khali (in the southwest, whose 650,000 [km.sup.2] make it the world's largest sand desert). The two are joined by a strip of sandy desert, the Ad Dahna Desert, in the form of a eastward-facing convex arc more than 620 mi (1,000 km) long and 6-62 mi (10-100 km) wide. The western, southern, and southeastern coastlines are bordered by mountains, but only the mountainous regions to the south receive enough rainfall to prevent them from being totally arid. (This is because of their height and the influence of the Indian monsoon regime.) The entire inland plateau (central Saudi Arabia), called the Najd or Nejd, is a desert. The Syrian Desert lies to the north and the northeast and all along the coastal regions of the Persian Gulf (also known as the Arabian Gulf or simply "the Gulf" among Arabs).
At the peninsula's northwestern tip, the Sinai Peninsula and the Negev Desert extend the desert area of the Arabian Peninsula and connect with the Egyptian deserts. To the northeast, the Arabian deserts join up with the deserts and subdeserts of Mesopotamia and Khuzestan. On the Iranian shoreline of the Gulf and in the deserts of Baluchistan, the desert conditions prevailing are not very different from those in the Arabian Peninsula, without winter rains or harsh frosts. This feature distinguishes them from the deserts and subdeserts of the Iranian Plateau and of Afghanistan, which receive their scarce rainfall in the winter and experience harsh frosts in the cold season; these are considered cold deserts, even though the most southerly ones (such as the Sistan region and Dasht-e-Lut) are at a latitude of about 30[degrees]N. The irrigated Indus Valley separates the Baluchistan deserts from the Thar Desert in the India-Pakistan border region.
These areas all show great biogeographical affinities with the southern Sahara and the Sahel (with which they form the Sudano-Zambezian floristic region), rather than with the neighboring deserts and subdeserts of the plateaus of Anatolia, Iran, and Afghanistan. The flora and fauna of the latter show much greater affinities with those of central Asia, and there is a strong Mediterranean influence in their flora as well. Features they share with the southern Saharan vegetation include the growth after rainfall of characteristic communities of annual plants in depressions between dunes (acheb) in sandy deserts, the presence of different species of Acacia and Prosopis (A. flava, A. hamulosa, A. tortilis, P. cineraria [=P. spicigera]), which form thickets where the subsoil water is accessible, several cactiform euphorbias (Euphorbia schimperi, E. triaculeata), and shrubby euphorbias that shed their leaves at the height of the dry season (E. balsamifera, E. caducifolia).
The Danakil Desert
The Danakil Desert lies between the Red Sea and the Ethiopian Highlands, between 11[degrees]N and 14[degrees]N, almost from the latitude of Masawa (Eritrea) to the north and that of Djibouti to the south, covering an area of about 150,000 [km.sup.2]. The average annual rainfall is 1-5 in (30-130 mm): Aseb 40 mm, Assal 60 mm, Obock 80 mm, Djibouti 130 mm. This is probably the hottest area on Earth, because the average annual temperature is close to 86[degrees]F (30[degrees]C), or even higher in the Dallol Depression (394 ft [120 m] below sea level).
The Danakil Desert is an active volcanic area, the site of tectonic activity and continental rift, with active and recent lava flows, sulfide and chlorate deposits, hot water springs, geysers, and saline depressions. The area has genuine potential for exploitation as a source of geothermal energy. The mountainous areas of Ethiopia to the west, which rise 3,281-6,562 ft (1,000-2,000 m) above the desert, are the sources of temporary rivers, water resources, and flood meadows that allow cattle to be raised in the desert. There is almost no agriculture, except in the small oases around Djibouti and through the development of irrigation projects on about 150,000 ha (1 ha=2.5 acres) along the middle and lower course of the Awash River, with finance from the Ethiopian government.
The flora is very rich (with 800 species of flowering plants) and typically tropical. The abundant vegetation includes the remains of juniper forests at higher elevations (4,0007,000 ft [1,200-2,200 m]). There are also some productive grasslands in lower zones and in the flood plains of temporary rivers or permanent underground rivers with sources in the Ethiopian Highlands. The Awash River, the only permanent river, runs south-north into Abbe Lake. In 1974, the remains of "Lucy" were discovered in the valley of this river, probably the most important hominid find so far (vol. 1, pp. 253-254).
The Somali Desert
The Somali Desert can be divided into two clearly separate areas with contrasting characteristics: the coast of the Gulf of Aden and the Bari Nugaal Plains. The desert occupying the coast of the Gulf of Aden runs from Djibouti to the tip of the Horn of Africa (Cape Caseyr), forming a narrow strip 0.6-37 mi (1-60 km) wide and 620 mi (1,000 km) long between the sea and the foothills of the Somali plateau (which is about 2,625-7,875 ft [800-2,400 m] above sea level). This coastal desert occupies an area of 8,000-10,000 [km.sup.2]. The Bari Nugaal plains lie to the south of the Horn of Africa and at the base of the plateau, running 311 mi (500 km) along the shoreline of the Indian Ocean in a strip 31-217 mi (50-350 km) wide (Qardho, Iskushuban, Eyl, Hurdiyo, Weyla, Galkacyo, Laascanood, Caynabo).
The Somali Desert on the coastline of the Gulf of Aden is hot and dry, with an annual average temperature greater than 86[degrees]F (30[degrees]C). The average annual rainfall is 2 in (50 mm) in Berbera and less than 1 in (20 mm) in Boosaaso and shows a bimodal seasonal distribution over the course of the year, with rains in spring and autumn; potential annual evapotranspiration is about 98 in (2,500 mm). Mountain areas, which receive more rain, are the source of the many temporary rivers flowing across the sand and gravel and supply it with water. The area contains the remarkable Conocarpus lancifolius (Combretaceae), a fast-growing tree that indicates the presence of relatively shallow aquifers. The Bari Nugaal plains desert is less hot and has fewer temporary watercourses, but the annual average rainfall is much greater, at 2-5 in (50-120 mm). The area contains many shallow calcareous soils and gypsum deposits. This desert covers an area of 80,000 [km.sup.2] (Bari Nugaal and Nugaal Valley).
The foothills of the Somali plateau rising above the desert of the Gulf of Aden's coastline support incense trees (Boswellia carteria, B. frereana, B. sacra), which grow on bare rocks in a narrow altitudinal band--4,000-5,575 ft (1,200-1,700 m)--often affected by fogs and sea mists; at higher elevations, the presence of cliffs allows the formation of a relatively degraded forest of African pencil cedar (Juniperus procera), with lichens of the genus Usnea and other epiphytes, as well as some other interesting tree species such as Ceratonia oreothauma, Pistacia emarginata, and Sideroxylon spp. In general, the flora is rich and original with a large number of endemic species of Acacia (Leguminosae), Commiphora (Burseraceae), and a range of many as-yet-undescribed species.
The fauna includes a large number of rare species of large mammals: the ass (Equus hemionus), the dibatag or Clarke's gazelle (Ammodorcas clarkei), the gerenuk or giraffe antelope (Litocranius walleri), and many small dik-dik antelopes (Madoqua). The desert's water resources allow the establishment of stockraisers and camel herders, the Issa and the Somali, but the ground is not cultivated. The coastal city of Berbera is the center of the active livestock trade between Somalia and the Arabian Peninsula.
The Namib, Karoo, and Kalahari deserts
The Namib Desert covers an area of 250,000 [km.sup.2], running north-south along a coastal strip 75-125 mi (120-200 km) wide that runs from Mocamedes in Angola to the Olifants River in South Africa and includes much of Namibia. It is basically a coastal plain between the Atlantic Ocean and the Namibian Great Escarpment, which runs parallel to the coast 62-93 mi (100-150 km) inland. The Namib Desert rises from sea level to 2,133 ft (650 m) at the base of the escarpment, which rises 2,625-4,921 ft (800-1,500 m) above the plain. The northern zone, from Mocamedes to Walvis Bay, consists mainly of rocks, gypsum, and gravel. The central part, from Walvis Bay to Luderitz, is almost entirely covered by dunes, the tallest reaching a height of 984 ft (300 m). The southern part, south of the Orange River, consists of areas of gravel and pebbles alternating with dunes and saline sediments.
The average annual rainfall on the coast is very low (8-20 mm), reaching 4 in (100 mm) at the base of the escarpment. North of Luderitz (26.4[degrees]S), rainfall occurs in the summer, while to the south it falls in the winter. The temperatures on the coast are moderate to cold (average annual temperature of 59[degrees]F [15[degrees]C]), due to the effects of the upwelling of the cold waters of the Benguela current. The temperature inversion caused by this cooling leads to cloudiness, high atmospheric humidity, and 100-250 days of fog per year in a strip 31 mi (50 km) inland from the coast. Fog frequency declines with distance from the coast, but fogs may reach the escarpment a couple of times a year. It has been calculated that the condensation of fog supplies the Namib with the annual equivalent of 2 in (50 mm) of rain. These climatic conditions are clearly very similar to those of the oceanic western Sahara, but the comparison of climatic data shows that the conditions are much more favorable for life in the Namib. The average annual values for temperature, potential evapotranspiration, wind velocity, and water vapor pressure are all lower than in the western Sahara. However, the relative humidity, as might be expected, is higher.
The Namib is famous among ecologists and naturalists for its richness, originality, and diversity and for the extraordinary adaptations to aridity of many of its plants and animals. Many species obtain all or part of the water resources they need from the condensation of the fog. As in the Somali and Danakil deserts, many temporary rivers, and some permanent or subpermanent ones, run from sources in the Namibian plateau across the Namib Desert, bringing life to the lines of oases running east-west across the coastal plain. As the old saying goes, this is one of the few places where a lion might come face to face with a seal.
The Karoo and southern Kalahari, inland extensions of the Namib Desert, are very moderate deserts in comparison with the Sahara and even with the Namib. The rains decrease from northeast to southwest, where about 6 in (150 mm) fall in a true rainy season at the end of the southern summer. Most of the Kalahari is not a true desert, as it is occupied by grassy savannahs with patches of scrub and woody savannahs. This region dried out in the cold period that affected the southern polar regions about 12 million years ago; since then, the sand, blown by the dominant winds, has formed belts of orange or reddish dunes that occupy a total of about 250,000 [km.sup.2] in the Kalahari and Karoo deserts.
3.2 The hot deserts and subdeserts of the Americas
Unlike the compact and extensive deserts of the Old World, the American hot deserts and subdeserts are much more scattered and cover a relatively small area. In the New World, deserts are present in the Northern and Southern hemispheres and show some similarities to each other, though each one shows a high level of endemism. The Northern Hemisphere deserts are concentrated in northern Mexico and the areas of the United States adjoining the Mexican border. The main Southern Hemisphere desert and subdesert areas are on the Peruvian and Chilean coastlines and in the Argentinean monte, in western and eastern South America respectively, but there are also subdesert areas on the Caribbean coastline of Colombia and Venezuela and in northeast Brazil (see map 38, p. 74).
The deserts in the Americas
The desert regions only cover 5% of the land area of North America, about 800,000 [km.sup.2]. They fall into two large groups: the Sonoran Desert, west of the Sierra Madre Occidental (including the desert and subdesert areas of the California Peninsula and the south of California State), and the Chihuahuan Desert to the east of the mountain range. To the north of the Sonoran Desert, the Mojave Desert is a zone of transition with the two cold desert and subdesert regions of the Great Basin Desert. Moving northwest-southeast from the Mojave are the Colorado Desert and the Gila Desert, still within the United States (in California and Arizona, respectively). The Colorado Desert is joined to the south with the subdeserts of Baja California (lower Colorado Desert, the Vizcaino Desert, the central coast of the Gulf of California, Magdalena), while the Gila Desert is joined to the Sonoran Desert (Altar, Sonoran plains, and the central Sonoran). Finally, farther east, the Chihuahuan Desert lies on the other side of the Sierra Madre Occidental, forming the second large group.
Unlike the situation in North America, where--with few distinctions--there is a degree of unity and continuity between the different deserts, in South America the various deserts are clearly separated and very dissimilar. The Andes separate the two main desert regions, the Peruvian-Chilean coastal desert and the Argentinean monte; the other small areas of the Caribbean and northeastern Brazil are separated by the immense Amazon region, as well as by other natural obstacles.
The American hot deserts are characterized by the presence of hundreds of species of the cactus family, the agave family (such as yuccas [Yucca], agaves [Agave], and sotols [Dasylirion]), and the only known species of the Simmondsiaceae, the jojoba (Simmondsia chinensis). Other abundant plants include creosote bushes (Larrea, Zygophyllaceae), mimosoid legumes such as mesquites (Prosopis), acacias (Acacia), caesalpinoid legumes such as Jerusalem thorn (Parkinsonia), palo verde (Cercidium), Fabaceae such as chanar (Geoffroea), desert ironwood (Olneya), tarbrush, (Flourensia, Aster-aceae), and frankenias (Frankenia, Frankeniaceae).
The Mojave, Sonoran, and Chihuahuan deserts
The Mojave Desert covers 140,000 [km.sup.2], including the lowest point in the Western Hemisphere, Death Valley, (282 ft [86 m] below sea level). The desert occupies the southern tip of Nevada, the basin of the River Mojave and all the endorheic basins in California southeast of the Sierra Nevada, and a small portion of the Colorado Valley from the southwest tip of Utah to the Mojave Mountains in northwestern Arizona. The Mojave Desert receives most of its scarce rainfall (2-5 in [50-120 mm] per year) in the winter, like the neighboring Mediterranean area of California. It is often considered a transition between the hot and cold deserts, since its northern border with the cold deserts of the Great Basin is essentially a matter of altitude: 4,925-5,575 ft (1,500-1,700 m) to the south of Nevada, the creosote bush (Larrea divaricata tridentata), characteristic of the hot American deserts, is displaced by shadescale or saltbush (Atriplex confertifolia) and the common sagebrush (Artemisia tridentata), characteristic plants of cold deserts. Although its southern limit is rather unclear, more than a quarter of its flora is endemic, and nearly 80% of its annual species are endemic. Unlike other neighboring hot deserts, the Mojave is dominated by a sort of open scrub, poor in species, lacking large cacti, but with localized populations of yuccas, especially the Joshua tree (Yucca brevifolia), for which California's Joshua Tree National Park on the northern foothills of the San Bernardino Mountains has been named.
The rest of the lower valley of the Colorado River is occupied by the northern sector of the Sonoran Desert, the lower Colorado Deserts (Colorado, Gila, Altar). The Sonoran Desert (275,000 [km.sup.2]) is the most varied of the hot American deserts, as shown by the great variety of terms used to describe the different sectors. Rainfall (2-14 in [50-350 mm]) is often greater than in the Mojave Desert and shows a biseasonal distribution, partly dominated by winter rains in the west and summer rains in the east. It is also the desert with the lowest elevation and a location closest to the sea (on the coastline itself in Sonora and Baja California). This helps explain why, though temperatures in the Sonora are usually higher, the heat regime is less harsh than in other more continental deserts. The most characteristic feature of this desert is the shrubs, including the creosote bush, together with wolf or squaw berries (Lycium andersoni, and other species) and small trees such as mesquites (Prosopis), palo verde (Cercidium floridum), and palo azul (C. microphyllum). These trees grow in association with cacti of every size, both large (Carnegiea, Pachycereus) and small (Opuntia). Farther south, toward the edges of the Sonoran plains and at higher elevations on the edges of the highlands of Arizona or the Sierra Madre, the desert increasingly resembles a very open woodland of small trees such as those mentioned above--Olneya, Cercidium floridum, C. microphyllum, and the guayacan (Guaiacum culteri).
The last desert in this group, the Chihuahuan Desert (453,000 [km.sup.2]), lies east of the Sierra Madre Occidental on the northern Mexican Plateau, spreading over the frontier formed by the Rio Grande to roughly the Pecos River. It blends into the short-grass prairies of the high plains of southern New Mexico and southeastern Arizona, where its northwestern tip also connects (to the east of Tucson) with the Arizona highland sector of the Sonoran Desert. It continues south into the Mexican states of Chihuahua, Coahuila, Durango, Zacatecas, San Luis Potosi, the southeastern tip of Nuevo Leon, the northern tip of Guanajuato, and some isolated patches even farther south, in Queretaro and Hidalgo. The Chihuahuan Desert's high elevation--it may reach 6,562 ft (2,000 m) at its southern tip and only descends below 1,640 ft (500 m) on the banks of the Rio Grande--means that it is colder than the Mojave, although it is still a hot desert. Rainfall (6-16 in [150-400 mm]) is in summer, arriving from the Gulf of Mexico, and is slightly more abundant than in other North American deserts. Together with the dominant calcareous substrates, this favors abundant herbaceous plants, especially grasses, distinguishing this desert from the other North American deserts. Another typical feature is the presence of patches of gypsic soils and materials of volcanic origin, with a characteristic flora rich in endemic species.
The Peruvian-Chilean coastal deserts: the Sechura, Tamarugal, and Atacama deserts
The coastal desert region of Peru and Chile forms a continuous strip that runs from almost the Peru-Ecuador border (5[degrees]S) to the valley of the Elqui River in Chile (30[degrees]S). Its runs along 2,175 mi (3,500 km) of coastline and occupies about 290,000 [km.sup.2]. The region's aridity is due to the combined influence of the cold Humboldt current from the South Pole, the South Pacific subtropical anticyclone, and the foehn effect exercised by the Coastal Cordillera and the Andes, which together prevent the arrival of moist air masses from the ocean and from the Amazon Basin. The uprising of the central Andes, together with the formation of the Humboldt current 13-15 million years ago due to intensification of Antarctic upwellings, seems to have caused the development in the mid-Miocene of climatic conditions similar to those now prevailing in the region. The system receives some moisture from the occasional rainfall in autumn and winter (between May and August) and from wet air masses that form over the Pacific and discharge their moisture as a fine drizzle (garua) or thick fog (camanchaca). Even so, much of the water supply of human populations and settlements in the more arid area comes from subterranean sources, apparently from a large fossil deposit that formed during the Pleistocene.
The annual rainfall is extremely low and varies greatly from one year to the next, but in some years rainfall is far greater than the long-term average. This appears to be related to the El Nino Southern Oscillation (ENSO), as a result of which air masses coming from the Pacific reach farther inland before discharging their moisture, often as torrential storms with high winds (see vol. 10, pp. 40-43).
On the northern coastline of Peru, from the Ecuadorian border to about 8[degrees]S, the fog belt only reaches an elevation of 1,640 ft (500 m), occasionally extending 2,625 ft (800 m). The flatter coastal relief (at its widest at Punta Mal Nombre [6[degrees]S], the coastal plain is only 106 mi [170 km] wide) allows the development of a sizeable arid zone, largely without vegetation, consisting of sandy plains and active dunes (medanos). This zone is known as the Sechura Desert. Around the plain, the foothills of the Andes are occupied by a stony desert that is spectacularly rich in cacti above about 3,280 ft (1,000 m).
From Trujillo (8[degrees]S) to the southern tip of the desert region, the coastal area's relief is more abrupt, and a discontinuous mountain range, the Coastal Cordillera, lies between the coast and the slopes at the base of the foothills of the Andes. The lomas are successions of hills and plains (pampas) before the abrupt foothills of the Coastal Cordillera, which, where it is interrupted, may extend to the foothills of the Andes. The loma environment is subject to garuas, with some peculiar perennial plants adapted to use the atmospheric humidity. Among the adaptations are the air plants (Tillandsia, Bromeliaceae); some cacti with prostrate, low, or spherical appearance, often covered with fine woolly hairs or lichens (which retain the humidity), and a rich flora of ephemerophytes that grow and flower when abundant rain falls. At an elevation of 984-1,969 ft (300-600 m), there are some local populations of shrubs such as faique (Acacia macracantha) and tara (Caesalpinia spinosa).
In the Chilean sector, the values for annual rainfall and air temperature, but not the relative humidity, show a latitudinal gradient. While the average temperatures decrease at higher latitudes, the annual average rainfall increases from virtually zero at the northern edge (18[degrees]S) to 4.7 in (118.4 mm) in the coastal sector and 5.4 in (134.6 mm) in the inland sector at the southern tip (29-30[degrees]S). On the coast, the nearby ocean buffers the differences in air temperature between day and night. (These are much greater inland.) The humidity caused by the ocean also shows limited penetration inland because of the unusual coastal topography, with terraces or coastal plains, the Coastal Cordillera (where it is present), and the plains and slopes separating the range from the Andes, giving rise to many areas with distinct environmental conditions.
The stretch of the Coastal Cordillera in northwestern Peru is represented by the Cerro Illesca, the Silla de Paita, and the Cerros de Amotape. In southern Peru (between the Paracas Peninsula at 14[degrees]S and the frontier with Chile), its average altitude is 3,281-5,249 ft (1,000-1,600 m), and it is 9-19 mi (15-30 km) wide; in Chile it reappears about 6 mi (10 km) south of Africa (18[degrees]S), where its maximum altitude is 1,640-2,625 ft (500-800 m). The coastal ranges are not continuous: in some areas there are depressions that are known locally as salares if they have a visible saline crust or pampas if they do not. At the southernmost tip of the desert region, 27-30[degrees]S, the Coastal Cordillera disappears, while the Andes tend to branch transversely toward the sea, forming a landscape of small plains (pampitas), mountains, and transverse valleys.
The area on the eastern slopes of the Coastal Cordillera varies from flat to slightly rolling or hilly and runs to the base of the Andes. It varies in width from about 20-30 mi (30-50 km) in some parts of southern Peru to 6 mi (10 km) at the Peru-Chile frontier. South of Pisagua (19[degrees]36'S), it widens again into a central plain known as the Depresion Intermedia (Intermediate Depression), containing, from north to south, the Pampa del Tamarugal (roughly 19-22[degrees]S), the Pampa Ondulada (22-24[degrees]S), and the Atacama Desert (24-26[degrees]S). The vegetation is very scarce, except in valleys, oases, and thickets that survive on underground water. The interfluve of the Copiapo and Elqui rivers (27-29[degrees]S) marks the beginning of the transitional inland desert, with considerably more plant cover than that of the more northerly inland desert. At the southern tip of the Chilean desert region, the Intermediate Depression disappears and is replaced by a complicated relief, high and abrupt, clearly recalling the Andes. Toward the mountains at 5,900-11,500 ft (1,800-3,500 m) is the Andean Desert, which is widest at the latitude of the Atacama Desert.
The subdeserts of the Argentinean monte
The Argentinean monte occupies a large region of western and central Argentina between the foothills of the Andes and the Pampean Sierras. It shows many similarities with the hot deserts of North America, specifically the Sonoran Desert, especially because of the creosote bushes (Larrea) that dominate the landscape. They form a sort of open scrub that is only locally displaced by other vegetation types such as cacti on stony soils, riparian vegetation around springs and along the rivers crossing the monte as they flow down from the Andes, and halophytes in the saline depressions (salares). The monte runs almost 1,243 mi (2,000 km) north-south from the south of the Argentinean province of Salta, through northwestern Tucuman, through almost the entire eastern half of Catamarca, into a small sector of the southwest of Santiago del Estero, through almost all of the La Rioja, San Juan, and Mendoza provinces (except for the areas in the Andes), then through part of northwestern Cordoba, the western half of the provinces of San Luis and La Pampa, through the eastern third of Neuquen, the northeastern half of Rio Negro, and into small fragments of Buenos Aires to northeastern Chubut.
An area covering such a long north-south distance and varying in altitude from almost 9,843 ft (3,000 m) in the north to sea level in the south is bound to show considerable variations. The rainfall regime in the northern zone is subtropical (with rains during the summer), while the southern area can be considered Mediterranean (with rain distributed irregularly over the course of the year, but with a minimum in the summer and relative peaks in autumn and spring). Average annual rainfall is very low--3-10 in (80-250 mm) for most of the area--with a latitudinal gradient from a minimum in San Juan and northern Mendoza and increasing to the north and to the south, but with local differences in function of exposure and height.
Yet the flora and fauna of the monte is very uniform, and the basically flat or rolling landscape is little more than a monotonous succession of patches of creosote bushes, enlivened by sporadic retamo (Bulnesia retama, Zygophyllaceae), a leafless shrub with a treelike appearance and dark green stems (it is now rare because of its over-exploitation for wax), some algorrobo cuyano or algorrobo dulce (Prosopis flexuosa), and an occasional chainar such as Geoffroea decorticans. Locally, on the rocky escarpments, columnar cacti such as pasacana (Trichocereus pasacana) may be found; sites with a water table near the surface support pajonales of perennial grasses; river banks and points where the water table is shallow but not at the surface support algorrobales of Prosopis flexuosa and other species of the same genus; the edges of the salares support jumiales of chenopods; and in the highlands of the Pampean Sierras, scattered between the depressions (bolsones), are grasslands of coirones or fescues (Festuca), similar to those in the Andes.
The subdeserts of northeastern Brazil and Venezuela
Much of northeastern Brazil (about 800,000 [km.sup.2] in the states of Alagoas, Bahia, Cear, Paraiba, Pernambuco, Piaui, Rio Grande do Norte, and Sergipe) receives annual average rainfall of more than 14 in (350 mm), but with great year-to-year variation and always concentrated in the summer months, especially between March and April. There is no rainfall between August and December and an almost permanently low atmospheric humidity. Permeable soils and the cultivation of sugar cane (most destructive in South America) have converted these areas, otherwise comparable to savannahs, into genuine subdeserts, rich in cacti and spiny terrestrial bromeliads. The area is partly occupied by caatinga (see vol. 3, p. 59, and vol. 2, p. 50) and partly by sertao. True caatinga is a very diverse deciduous forest, with many columnar and even arborescent cacti such as Cereus jacamaru, and other species of the same genus, terrestrial bromeliads, and even the sporadic presence of barrigudas (Cavanillesia arborea, Chorisia ventricosa, both Bombacaceae) and some palms such as the urucuri palm (Syagrus [=Cocos] schizophylla), catole (S. [=Cocos] comosa), and carnauba palm (Copernicia cerifera). True sertao contains no trees and is dominated by clumps of low spiny plants, especially small cacti. A large number of intermediate shrubby variants of caatinga have been described from this region of northeast Brazil.
On the Colombian and Venezuelan coastline of the Caribbean, from the Sinu River to Puerto Cabello and the Netherlands Antilles (off the Venezuelan shore), climatic conditions are similar to those of northeastern Brazil. The same is true, farther east along the coastline, of Barcelona (Venezuela) and a part of the Araya Peninsula and the neighboring islands (such as Margarita Island) and extends as far as the southern faces of the northeastern tip of the Andes that descend to the llanos of Barcelona in Venezuela. The vegetation consists of espinares (spiny forests of small-leaved mimosoid and caesalpinoid members of the Leguminosae that shed their leaves during the six months or more when there are no rains) and cardonales (thickets of columnar or candelabra-form cacti to 26 ft [8 m] tall) accompanied by some trees from the espinares and a low layer of smaller cacti.
3.3 The hot deserts and subdeserts of Australia
Much of the center of the Australian landmass is occupied by deserts and subdeserts. An area is considered arid in Australia if it receives average annual rainfall of less than 250 mm (350 mm in the north of the continent), and this applied to 70% of Australia. Yet this includes many semiarid areas with plant cover dominated by acacias and she oaks (Casuarina) that are hard to define as deserts or subdeserts. Australia has five great deserts--the Great Sandy Desert, the Gibson Desert, the Great Victoria Desert, the Simpson Desert (or Arunta Desert) and the Sturt Desert--that occupy 20% of the continent. These, together with the other desert and subdesert areas (except those occupied by a scrubland of eucalyptus, acacias, and she oaks), cover a little less than half the continent. Australia's deserts are not as extremely arid as the central Sahara and show more similarities to the Sahel or the wastelands and thyme scrub of the arid areas of Mediterranean Africa.
The Great Sandy Desert, Gibson Desert, Great Victoria Desert, Simpson Desert, and the Sturt Desert
The Great Sandy Desert occupies roughly 400,000 [km.sup.2] of the sedimentary Canning Basin in the north of Western Australia (18-23[degrees]S), north of the Tropic of Capricorn and thus has rainfall in the summer. One of the most inhospitable regions of Australia, it is mainly covered by dune fields with scarce vegetation dominated by hummock grasses of the genus Triodia. The Gibson Desert, to the south on the Tropic of Capricorn and occupying the lower part of the Western Australian Shield, is a sandy and stony desert. It was virtually unexplored until the 1960s, except by aboriginal tribes who lived in the territory.
South of the Gibson Desert, separated from it by the Warburton Range and other smaller ranges, is the Great Victoria Desert, located on the border between Western Australia and South Australia. The Nullarbor Plain, probably the largest flat rock surface in the world, lies to the south of the Great Victoria Desert, separating it from the Great Australian Bight (a bay on Australia's southern coast). The Great Victoria Desert is mainly a sand desert covered by belts of dunes, partly fixed by spinifex hummock grasses (Triodia). On the calcareous Nullarbor Plain, wherever a little clay accumulates on the surface or in cracks in the rock, there are patches of a typical halophytic scrub with blue bush (Maireana [=Kochia] sedifolia), oldman saltbush (Atriplex vesicaria), and other species of the same genus known generically as saltbushes, along with several species of copper burr (Bassia).
The Simpson Desert (or Arunta Desert, its Aboriginal name), occupies a triangular basin of about 150,000 [km.sup.2] around the junction of the state frontiers of Northern Territory, Queensland, and South Australia, with its southern tip in the area around Lake Eyre. It was crossed for the first time in 1939 by a European riding a caravan of dromedaries. The Simpson Desert is mainly a sand desert covered by hundreds of dune cordons running NNW-SSE with the ever-present clumps of spinifex grass. Farther southeast, the Sturt Desert occupies the area near the junction of South Australia, New South Wales, and Queensland, separated from the Simpson Desert by the stony desert plains lying between the Diamantina and Cooper Creek rivers.
24 Utter desolation dominates the landscape of the most extreme hot deserts. Yet desert landscapes have their own strange beauty, sometimes featured in films that have become classics, including Zabriskie Point (1969) by the Italian director Michelan-gelo Antonioni (1912), which was filmed near the area of Death Valley (United States) shown in the photo. Eroded by occasional downpours and strong persistent winds, the bare rocks are eventually carved into strange shapes or unusual reliefs.
[Photo: Ramon Folch / ERF]
25 Sandstorms, such as the one in this photo of the city of Gao (Mali), are one of the most unpleasant aspects of living in the desert. Inside the sandstorm, the sand borne on the wind blankets all sound and blocks all light, so visibility may be reduced to little more than 3 ft (1 m). Very strong winds may suddenly raise a solid wall of sand taller than a man and almost 300 mi (500 km) long. Some of these sandstorms travel hundreds of kilometers and reach a wind speed of 30 mph (50 km/h).
[Photo: Tony Crocetta / Still Pictures]
26 The desert Ahaggar Massif reaches 9,843 ft (3,000 m) above sea level. It is in the center of the Sahara, almost exactly on the Tropic of Cancer and about 994 mi (1,600 km) from the Mediter-ranean and Atlantic coastlines. The extreme continentality and high altitude lead to very severe conditions and create an unusual climate, especially in the central region of Atakor, which is dotted with steep igneous extrusions (phonolites). A cloud layer covers the massif for much of the year. Spreading cyclone fronts are responsible for the cloud cover in spring and autumn, while the cover in winter results from the remains of the masses of moist tropical air. Each day, from May to October, the cloud cover starts to form early in the morning, and by about 3:00 in the afternoon it completely covers the sky. It then rains, but only a few drops reach the ground, mainly at elevations greater than 6,562-8,202 ft (2,000-2,500 m), and virtually none at the lower elevations of the foothills, which are extremely arid.
[Photo: Crispin Hughes / The Hutchison Library]
27 Desert roses are clumps of gypsum crystals that include fine reddish particles. The interlocking and overlapping of the gypsum crystals makes them look like flowers. They form in soil horizons that are subject to fluctuations in a water table that is saturated in calcium sulfate; when the wind blows away the horizon covering them, they are left on the surface. The best-known desert roses are found in the Sahara, but they also form in other arid regions such as the deserts of Arizona and New Mexico (United States).
[Photo: Joaquim Reberte & Montserrat Guillamon]
28 Continued erosion by wind and water of fine-grained rocks may create forms as surprising as these Jurassic sandstones in Ante-lope Canyon in Arizona (United States). Water wears away the materials and dissolves them, while the wind abrades them because the sand and dust it bears blast away at the rocks. The resulting highly diverse shapes depend on the erosive forces, their opportunities to act, and the nature of the material being eroded.
[Photo: Francois Gohier / Ardea London]
29 Highly eroded petrified dunes on the banks of the former Lake Mungo, now dry, in Mungo National Park in New South Wales (Australia). These dunes, known as the Walls of China, were deposited in very different climatic conditions from those now prevailing; since then, erosion by water has led to gully formation. The high content of salts prevents plants from colonizing the dunes, leaving them bare and exposed to erosion by the water.
[Photo: ANT / NHPA]
30 In areas of erg (a sea of sand) winds create a relief consisting of dunes, which vary greatly in shape, size, and also in orientation, as shown by this photo of the desert in Tunisia. The simplest dunes are merely accumulations of sand leeward of a shrub, rock, or hill or at the bottom of a shallow depression. Other dunes are formed by sudden gusts of wind that deposit the suspended particles in small heaps. Once formed, a pile of sand tends to grow leeward, as it is an obstacle to the force of the wind. This is why dunes often occur in regular groups, all of them the same shape and height (see photos 35 and 141). Only in erg regions where winds are variable do the layers of sand form irregular dunes or no relief at all.
[Photo: William Fautre / Bios / Still Pictures]
31 The stony region of the Namib Desert extends to the north of the Kuiseb River, which flows into the Atlantic Ocean at Walvis Bay. It is an extremely arid zone, totally covered by the stones and gravel that the water and wind have not washed or blown away. This has led to the formation of a surface desert pavement that protects the underlying soil from intense erosion. The almost nonexistent rains (the Namib is the driest desert in Africa, as average annual rainfall does not exceed 1 in [25 mm]) prevent this pavement from being covered by a layer of green (see also photo 37).
[Photo: Michael & Patricia Fogden]
32 Vast salt flats, the remains of ancient lakes, are the most characteristic feature of the Atacama Desert. The Salar de Atacama, or Atacama Salt Flat (Chile), one of these saline basins, is enclosed between mountains in the Domeyko Cordillera. This salt flat is at an elevation of 7,546-7,874 ft (2,300-2,400 m) above sea level. It receives little rain (the Atacama Desert is the driest desert in South America), but in some regions halo-philous plant communities grow.
[Photo: Tom Till / Auscape International]
33 Alluvial soil strata exposed by the erosive action of a wadi near the Dead Sea (Israel). Wadis are seasonal river valleys, normally running between steep walls; they are dry for most of the time but fill up dangerously quickly after intense rains. Wadis are very common in northern Africa.
[Photo: Cyril Ruoso / Still Pictures]
34 The hot deserts of the Old World are in Africa (northern Africa, southern Africa, eastern Africa, and Madagascar), the Arabian Peninsula, and the Indian subcontinent. The largest is the Sahara (9 million [km.sup.2]), which runs from the Red Sea to the Atlantic Coast and from the southern shores of the Mediterranean to about 10[degrees]N. Logically, the landscape varies greatly in such a large area (with hamadas, ergs, mountain areas, etc.), and there is great diversity of climate, as shown by the temperature and rainfall diagrams (climograms) for three different sites: Smara inland in the western Sahara, Agadez in the south next to the Sahel, and the Siwa Oasis in the east. The Arabian Desert is another huge Old World desert, occupying the entire interior of the Arabian Peninsula and covering 2.3 million [km.sup.2]. The civilizations that developed in the valleys of the rivers that cross this desert have had a very important influence on Western culture and society, and their enormous energy reserves, in the form of oil, make them one of the most disputed areas on the planet. The much smaller Thar Desert, 200,000 [km.sup.2], on the India-Pakistan border, is characterized by enormous sand dunes; as shown by the climograms for Bikaner, the Thar Desert is very moist, with average annual rainfall of 4-20 in (100-500 mm), though strong winds, high temperatures--the annual average temperature in Bikaner is 79[degrees]F (26[degrees]C)--and the shortage of water all make it inhospitable. To the west of the valley of the Indus River, the Thar Desert continues as an arid coastal strip that runs as far as the Iraqi border and also connects with the deserts of the Iranian Plateau. The other deserts are much smaller, among them the deserts in the Horn of Africa (the Ogaden and others) and those in southern Africa (the Kalahari and the Namib).
[Drawing: IDEM, from several sources]
35 The great sea of dunes, the Great Western Erg, in the northwestern Sahara (northern central Algeria) covers an area of 78,000 [km.sup.2]. Surrounded by mountains, the Great Western Erg basin has for thousands of years (at least 10,000) acted as a trap for particles blowing in the wind. Dust and sand have accumulated and formed thick deposits; if all the sand were leveled out, the sand layer would be at least 141 ft (43 m) thick. The surface sand is subject to the action of the wind, which piles sand into dunes, barchans, and huge sand mountains that can reach a height of 394 ft (120 m) (see also photo 141).
[Photo: Jean Roche / Bios / Still Pictures]
36 The Dallol Depression is the lowest area of the Danakil Desert: most of it is about 394 ft (120 m) below sea level. The little rain that falls evaporates very quickly, leaving layers of saline sediments that are 3,281 ft (1,000 m) thick in some places. The active volcanoes in the area continually emit smoke and gases and have left deposits of sulfides and chlorates, while the denuded substrate is often painted bright colors by the minerals dissolved in the springs of hot water.
[Photo: Dieter & Mary Plage / Survival Anglia / Oxford Scientific Films]
37 In the Namib Desert, the aloe (Aloe dichotoma), and other plants adapted to the aridity, manage to survive with few problems, although it is one of the most inhospitable places on Earth. Fogs are almost the only source of moisture in the Namib (see photo 31), but it is enough for the great diversity of plants and animals living there.
[Photo: J.L. Klein & M.L. Hubert / Bios / Still Pictures]
38 The hot deserts and subdeserts of the Americas, unlike those of the Old World, are highly dispersed and cover only a relatively small area. The North American deserts are in northern Mexi-co and the southwestern United States and form two major centers: the Sonoran Desert, west of the Sierra Madre Occidental, and the Chihuahuan Desert, east of the Sierra Madre Occidental. To the north of the Sonoran Desert, the Mojave Desert is a transition to the cold deserts and subdeserts of the Great Basin Desert. In South America, the main desert and subdesert areas are on the Peruvian-Chilean coastline (west of the southern Andes) and the Argen-tinean monte (east of the Andes). Some areas on the Caribbean coastline of Columbia, Venezuela, and northeastern Brazil can also be considered subdeserts. The deserts of South America are much more arid than those of North America, because the Andes prevent the passage of moist air masses from the Pacific, and the Cordillera de la Costa blocks moisture from Amazo-nia, casting a rain shadow. The only moisture reaching these deserts is from fogs and occasional rainfall. In some regions, it is not at all unusual for many years to pass without a single drop of rain falling, as shown by the climogram for Antofagasta. El Paso, Texas, receives much more rainfall than the other desert regions of North America, as moist air masses arrive from the Gulf of Mexico. Temperatures are surprisingly lower in El Paso than in Death Valley, which is in a transition zone into the cold deserts. Death Valley, however, is below sea level, while El Paso is at an altitude of 3,917 ft (1,194 m).
[Drawing: IDEM, from several sources]
39 The deserts of Baja California (Mexico) are dominated by cacti and other cactiform plants. One very abundant species is the "cardo pelon" or "cardon" (Pachycereus pringlei, to the right), one of the largest cacti; it can reach a height of 49 ft (15 m) and weigh several tons. Its grooved stem allows it to expand in wet periods and contract in times of drought (see drawing 56). Another very typical plant of the region is the boojum tree (Fouquieria [=Idria] columnaris, Fouquieriaceae, to the left). The boojum tree only bears leaves immediately after rainfall, shedding them when the soil dries out to avoid water loss by transpiration. In the background (bottom center), there are some yuccas (Yucca, Agavaceae), which are also highly characteristic of the hot deserts of North America.
[Photo: Francois Gohier / Ardea London]
40 Air plants (Tillandsia, Bromeliaceae) grow in scattered groups on the soil in the National Reserve of Paracas (Peru). Like the epiphytic species of the same genus, terrestrial air plants can survive on just the atmospheric humidity absorbed by the dense layer of specialized trichomes covering their leaves. Their roots, often invisibly small, only anchor the plant to the substrate; they do not absorb water.
[Photo: Adolf de Sostoa & Xavier Ferrer]
41 The monte landscape in Valles Calchaquies in northwest Argentina shows the typical scattered patches of jarillas or creosote bushes (Larrea) that dominate this subdesert. The monte formed in the Pleistocene (probably less than 1.5 million years ago) in the last orogenic cycle of the formation of the Pampean Sierras. It is thus a very young formation compared with other arid regions. The plant formations of the monte are derived from those of the chaco--or at least some of its more important species are.
[Photo: Ildefonso Barrera]
42 The xerophytic and spiny plant communities of the caatinga dominate the landscape of 800,000 [km.sup.2] in northeastern Brazil. Cacti, terrestrial bromeliads, and, to a lesser extent, palms and members of the Bomba-caceae dominate the plant cover of this subdesert derived from true savannahs. Most caatinga plants grow on calcareous soils and are well adapted to drought (showing succulence, spininess, water storage organs, and other adaptations). Rainfall is not particularly low in the zone (the annual average is 16 in [400 mm]), but it is distributed very irregularly over the course of the year.
[Photo: Tony Morrison / South American Pictures]
43 Deserts and subdeserts occupy almost half of Australia, especially its center. The five most important are: the Great Sandy Desert and Gibson Desert in the northeast of Western Australia; the Great Victoria Desert in Western Australia and South Australia; the Simpson Desert, in the southeast of Northern Territory; and the Sturt Desert, where the state frontiers of Queens-land, South Australia, and New South Wales meet. The climate of the Australian deserts is not excessively arid, as shown by the climograms for Alice Springs (Northern Territory) and Kalgoorlie (Western Australia). In Kalgo-orlie, the rainfall curve even rises above the temperature curve for a few months, some-thing that never happens in an extreme desert climate.
[Drawing: IDEM, from several sources]
44 Clumps of spinifex grass (Triodia) cover hundreds of square kilometers of the Gibson Desert in Western Australia. In addition to keeping the sand relatively stable, these clumps of bunch grasses are a suitable shelter for many desert animals such as small reptiles and marsupials. The sharp, rigid, thorn-like leaves of Triodia spp. protect them from being eaten by herbivores.
[Photo: J.M. La Roque / Auscape International]
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|Publication:||Encyclopedia of the Biosphere|
|Date:||Apr 1, 2000|
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