Alta S. Walker *
GLOBAL ATMOSPHERIC CIRCULATION
We live at the bottom of a gaseous envelope that is gravitationally
bound to the planet Earth. Our atmosphere is generally separated into
two zones-the homosphere at low altitudes where mixing produces
a uniform composition of gases, and the upper heterosphere. The
homosphere consists of 78 percent nitrogen, 21 percent oxygen, and
0.9 percent argon. Water vapor, carbon dioxide (0.03%), and
other molecules are present as minor constituents. The lowest layer
of the homosphere, which contains about 80 to 90 percent of the mass
of the atmosphere, is the troposphere, which is defined as the layer
where temperature decreases with increasing elevation. Most of the
Earth´s weather occurs in the troposphere. The heterosphere
is divided into four subzones, each with a distinct composition. Above
the heterosphere is the exosphere, which is the top of the atmosphere.
It begins about 600 km above the surface of the Earth and extends up
to the altitude at which the average density of the atmosphere falls to
the average density of the interplanetary medium. Using this criterion,
the upper boundary of the atmosphere is about 20000 km above the
surface of the Earth (Smith and West, 1983).
The circulation of a planetary atmosphere is a complex process
because of rotation of the planet and inclination of its axis. The
Earth´s axis is inclined 23 1/2° from the ecliptic.
Vertical rays of the Sun strike 23 1/2°N latitude, the
Tropic of Cancer, at summer solstice in late June. At winter solstice,
the vertical rays strike 23 1/2°S, the Tropic of Capricorn.
The inclination of the axis causes seasonal changes in the global
circulation system.
The circulation of the atmosphere is further affected by the
rotation of the Earth, which introduces the Coriolis Effect. The
Coriolis Effect illustrates Newton´s first law: a body in
motion will remain in motion in the same direction unless acted
on by some outside force. The rotational velocity of a planet is
zero at the poles and maximum at the equator. If a missile were
fired true north from the equator, it would acquire the rotational
velocity of the equator. As the missile progresses northward, it
would retain the rotational velocity of the equator. This velocity
would be greater than the rotational velocity of the Earth under it.
Inasmuch as the Earth rotates west to east, the missile would be
deflected toward the east. The Coriolis Effect causes moving
objects to be deflected toward the right in the northern hemisphere
and toward the left in the southern hemisphere.
The global circulation pattern of Earth is illustrated in
Figure 8.1. The
circulation of air between the hot equatorial region and the cold
polar regions creates pressure belts that influence weather. In
1735, George Hadley suggested that one large convection cell
existed in each hemisphere for a nonrotating Earth. According to
Hadley, the simple convection cell consisted of hot air rising at
the equator, cooling as it moves toward the poles, descending
as cold air over the poles, and warming as it moves over the
surface toward the equator.
In the zone between the equator and about 30° latitude,
the surface atmospheric flow is toward the equator, and the
flow aloft is poleward. This area is referred to as the Intertropical
Convergence Zone (ITCZ). It is an equatorial low and is a
zone of calms or light variable winds, known to mariners as
the doldrums. The location of the ITCZ varies seasonally and
has been measured as far north as 40°N during a northern
hemisphere summer (Lutgens and Tarbuck, 1979). Its migration
significantly modifies precipitation, with the length of the dry season
depending primarily on the distance of an area from the ITCZ.
Around 30° latitude, the poleward airflow begins to
subside in subtropical high-pressure belts. The
subsiding air is relatively dry because its moisture has
been released near the equator in the tropical rainforests.
Near the center of this high-pressure zone of
descending air, called the Horse Latitudes, the winds at
the surface are weak and variable. The name for this area
is believed to have been given by Colonial sailors who,
when sometimes becalmed at these latitudes while crossing
the oceans with horses as cargo, were forced to throw a
few of them overboard to conserve water. Over the
continents, this subsidence zone is the site of many of the
world´s deserts.
The subsiding air that then flows equatorward from
the subtropical high-pressure belts is deflected
toward the west in both hemispheres by the Coriolis Effect.
Because wind direction is the direction from which the wind
is blowing, these winds are referred to as the northeast trade
winds in the northern hemisphere and the southeast trade
winds in the southern hemisphere. The trade winds meet at
the doldrums.
The poleward surface flow away from the equator
beyond the Horse Latitudes produces winds referred to as
westerlies. They meet easterlies from the polar highs at a
subpolar low-convergence zone at about 50 to 60
degrees latitude.
The atmospheric circulation systems of other terrestrial
planets with gaseous envelopes also depend on the orbit and
inclination of the planets and the composition parameters of
the atmospheres. Leovy (1977) finds that the atmosphere of
Mars behaves like a rarefied version of the Earth´s
atmosphere. Because of its retrograde rotation and extremely
high pressure, the atmospheric dynamics of Venus are
considerably different from those of the Earth. Seasonal
variations on Venus are negligible because of its small orbital
eccentricity and the 3° inclination of its axis from the ecliptic.
In addition, the 243-day rotation minimizes the Coriolis
Effect. At cloud height, the atmosphere circulates the planet in
about 4 days. The complex global circulation patterns of the
atmosphere of Venus are not fully understood (Schubert, 1983).
Fujita (1981) proposes that a planet´s atmospheric
motion be divided into five scales. The largest scale is the
equator length, for the Earth, 40 000 km. The dimensions of
the scales decrease by two orders of magnitude per scale, and
the smallest scale on Earth has a maximum length of 40 cm.
Wind is a stream of air flowing because of a difference in
atmospheric pressure between two points. Winds blow from
areas of high pressure to areas of low pressure. Near the
ground, wind is affected by friction and by changes in topography.
Winds may be seasonal, sporadic, or diurnal. Their intensities on
Earth range from gentle breezes to tornadoes with speeds in excess
of 60 m/sec (Fujita, 1981).
A BRIEF INTRODUCTION TO DESERTS
Approximately one-third of the land surface of this
planet is considered to be desert-barren land with meager
rainfall that can support only sparse vegetation and a limited
population. A widely accepted arid-zone classification
system was suggested by Meigs (1953) and is shown in
Figure 8-2.
Desert regions on Earth have been divided into three categories
according to the amount of precipitation they receive. Extremely
arid lands have at least 12 consecutive months without rainfall,
arid lands have less than 250 mm of annual rainfall, and semiarid
lands have a mean annual precipitation between 250 and 500 mm.
Arid and extremely arid land are deserts, whereas semiarid
grasslands generally are referred to as steppes. Savannas, which
are treeless plains with distinct wet and dry seasons, lie between
deserts and tropical rain forests.
Trade-Wind Deserts
The trade winds are two belts on the equatorial sides of
the Horse Latitudes. As these surface winds move from
higher to lower latitudes, they become warmer. In addition
to being dry, the winds dissipate cloud cover, allowing more
sunlight to heat the land. Trade-wind deserts are
sometimes referred to as low-latitude deserts.
Most of the major deserts of the world lie in areas crossed
by the trade winds. The world´s largest desert, the Sahara
of North Africa, with a maximum recorded air temperature of 57°C
(Cooke and Warren, 1973), is a trade-wind desert. The
Taklimakan Desert of China is predominantly a trade-wind
desert although its northern and southern borders are in the rain
shadows of two mountain ranges.
Midlatitude Deserts
The midlatitude deserts occur between 30 to 50° latitude,
poleward of the subtropical high-pressure zones. They
have a wide temperature range with lower winter temperatures than
those found in trade-wind deserts. Midlatitude deserts
normally develop far from the ocean in interior drainage basins.
The Lut Desert in Iran and the Sonoran Desert in the United States
and Mexico are midlatitude deserts.
Rain-Shadow Deserts
Rain-shadow deserts are created by mountain ranges
that prevent moisture-rich clouds from reaching areas in
the lee of the range. Rain or snow is precipitated as the air rises
over the mountains, and a desert is formed on the lee-side
"shadow" of the range. Rain-shadow deserts
are recognized easily from space by the absence of vegetation on
the lee side of the mountains. These deserts sometimes are called
orographic deserts. The Turpan Depression, Plate E-24, is a
rain-shadow desert.
Coastal Deserts
Coastal deserts generally are found on the western edges
of continents near the tropics of Cancer and Capricorn. They
have the smallest temperature range of all deserts (Meigs, 1966).
These deserts are affected by cold ocean currents parallel to the
coast. These currents create high-pressure zones that lower
the temperatures and cause temperature inversions. In coastal
deserts, local wind systems often override the trade winds.
These deserts are, therefore, less stable than other deserts and
are dependent on ocean currents (McCauley et al., 1977).
Winter fogs, which reduce solar radiation and decrease the
temperature, are frequent in coastal deserts. Many coastal sand
dunes are relatively complex because they are at the interface of
terrestrial, oceanic, and atmospheric systems (Pye, 1983). The
world´s driest desert is the Atacama of South America, a
coastal desert shown on Plate E-10.
Monsoon Deserts
Changes in the position of the ITCZ lead to monsoons
in low latitudes. The term monsoon is derived from an Arabic
word meaning season and refers to a wind system with pronounced
seasonal reversals. Monsoons develop in response to
temperature variations between continents and oceans. The two
major monsoon systems on Earth are in northern Australia and
West Africa. Lesser monsoon systems affect the deserts of central
Asia. The southeast trade winds of the southern Indian Ocean
provide heavy summer rains in India when they move onshore. As
the monsoon crosses India, it loses moisture on the eastern slopes
of the Aravalli Range. The Rajasthan Desert of India and the Thar
Desert of Pakistan (Plate E-21) are parts of a monsoon desert
region west of the range.
Polar Deserts
Polar deserts are in currently nonglaciated areas with
annual precipitation of less than 250 mm, and a mean warmest
month temperature of less than 10°C. The polar deserts on Earth,
which have a total area of nearly 5 M km2, generally
consist of bedrock or gravel plains. Sand dunes are not
prominent features, but snow dunes are common in areas
where precipitation is locally more abundant.
Because most moisture in polar deserts is in the solid
rather than the liquid state, there are differences between
polar and other deserts (Péwé, 1974).
Temperatures in polar deserts often fluctuate above and
below the freezing point of water. Alternate freezing and
thawing creates patterned textures, such as contraction
polygons that may be 15 to 30 m in diameter. These
sand-wedge polygons are not usually seen in other
deserts.
Paleodeserts
Patterns characteristic of dunes in sand seas may be
recognized on images even though they are located in
presently nonarid environments (see, for example, Plate
F-8). Many such relict dunes are in areas in which annual
rainfall is now 700 to 1500 mm, and some extend into areas
presently occupied by tropical rain forests (Goudie, 1983).
Ancient sand seas, changing lake basins, archaeological
data, relict fluvial topography, and vegetation analyses
indicate that climatic conditions have changed over vast
areas of the Earth. Goudie (1983) suggests that, during
most cold phases of the Pleistocene, parts of the deserts
were more arid than they are today. Sarnthein (1978) notes
that 10 percent of the land between 30°N and 30°S is
presently covered by sand seas. About 18 000 years ago,
sand seas in two vast belts occupied almost 50 percent of
this land area. As today, tropical rain forests and savannas
existed between the two belts.
Many sandstones are considered to be eolianites,
sedimentary rock formed by consolidation of sediment
deposited by wind. Some investigators hold that distinction
between eolianites and certain fluvial sediments can be very
difficult, but eolianites may often be recognized by large-
scale cross bedding with high-angle dips, ripple marks,
slump marks, and contorted bedding characteristic of dry sand
(McKee, 1979a).
As described by Glennie (1970), vector analysis and
stereographic plots may be used to determine prevailing
paleowind directions when the dunes were formed.
Bigarella (1972) considers that the paleolatitude of eolianites
may also be determined. He refers to a latitude that marks the
mean boundary between the trade winds and the westerlies as
the "wheel-round latitude" (Bigarella,
1972, p. 41). This latitude is marked by a distinct change in
the alignment of sand dunes and dune movement. He argues
that the present wheel-round latitude in northern Africa is
25°N, but comments that other investigators have placed
the Pleistocene wheel-round latitude at 15°N. This
difference suggests a climatic boundary shift of more than 1000 km.
Arid-land geomorphic features are usually excellent
indicators of paleoclimatic conditions. In some environments,
however, differentiating between response to a global climatic
change and response to regional uplift of an associated mountain
range is difficult. Stoertz and Ericksen (1974) address this
problem by noting that regional uplift causes the crowding of
playas, salt crusts, lake shores, and piedmont alluvial slopes in
parts of Chilean basins. Other anomalies they observed in one
basin include the presence of lake sediments along the eastern
side high above the western playas and the diminishing height
of terrace scarps around salt crusts toward the west. Climatic
change, as opposed to regional uplift, is indicated by the absence
of extensive and consistent regional asymmetry.
The Nebraska Sand Hills, Plate E-14, is an inactive,
probably Holocene, 57 000-km2 dune field in central
Nebraska. The largest sand sea in the western hemisphere,
it is presently stabilized by vegetation and has an annual
precipitation of 406 to 610 mm (Ahlbrandt and Fryberger,
1980). These authors found the dunes to be of crescentic
and dome types as high as 91 m. They have a maximum
thickness extending to 34 m below interdune surfaces.
Present-day effective winds, calculated from weather
station data, differ from paleowind regimes inferred from
the internal structures of the dunes. A modern analog may be
Lake Chad, Plate E-8, where large-scale crescentic
dunes occur in an arid area in which the climate has changed
episodically.
Although paleoeolian depositional features have been
recognized widely, little work has been done until recently in
recognizing paleoeolian erosional features. Only with the
advent of space imagery was the regional extent of some
eolian erosional features recognized (McCauley et al.,
1977; Mainguet, 1983).
Martian Sand Seas
Although Mars has a diameter of about one-
half that of Earth and a surface atmospheric pressure
about 1 percent that of Earth, it has a north circumpolar
sand sea with an area of at least 700 000 km2, greater
than the Empty Quarter, which is the largest sand sea on
Earth (Breed et al., 1979b). The martian sand sea,
Plate E-15, consists predominantly of crescentic dunes and is
visible on plains south of the perennial ice cap of the north
polar area. Smaller dune fields are found elsewhere on the
planet, including inside many large craters in the north and
south polar regions. Eolian depositional features of polar
deserts on Earth are studied as analogs for martian
geomorphology (Morris et al., 1972).
Eolian Processes on Venus
Venus is frequently called the Earth´s twin
because it is close to the Earth in radius, mass, density,
and surface gravity. The relation is only sororal, however,
Venus has a retrograde rotation period of 243 days, a mean
surface temperature of 480°C, and a predominantly
carbon dioxide atmosphere with a surface pressure almost
100 times that of the Earth.
Orbiting U.S. and Soviet spacecraft have acquired
radar data of the surface of the planet, but due to low
resolution, evidence for global eolian activity has not
been noted yet. The Soviets have landed several spacecraft
on the surface of the planet. Images from these landers
generally show flat layered rocks centimeters in diameter.
After examining several Venera images, Greeley and Iversen
(1985) point to a bimodal-size distribution of the particles
on the surface as indicative of fluid transport. Inasmuch as
liquid water cannot exist on Venus, they assume that the fluid
involved is the atmosphere.
DESERT FEATURES
The deserts of the world are not restricted by latitude,
longitude, or elevation. The People´s Republic of
China has both the highest desert-the Qaidam
Depression (3000 m above sea level)-and one of the
lowest deserts-the Turpan Depression (100 m below
sea level).
Only about 20 percent of the Earth´s desert area is
covered with sand (Holmes, 1965). Much of the sand collects
in sand sheets and sand seas-vast regions of undulating
dunes resembling an ocean surface caught in an instant of time.
Sand seas are called ergs in North Africa (Wilson, 1973).
Nearly 50 percent of desert surfaces are plains where
eolian deflation has exposed loose unsorted residual gravels
predominantly of pebble size (4 to 64 mm in diameter) but
with occasional cobbles (64 to 256 mm in diameter). An
extensive flat desert surface composed of pebbles on bedrock
is called a hamada. Bagnold (1941) calls coarse-grained
residues remaining after the passage of sand a whaleback.
The remaining surfaces of arid lands are composed of
fluvial deposits, including alluvial fans, playas, desert lakes
and oasis, and exposed bedrock outcrops and desert soils.
Bedrock outcrops commonly stand up as inselbergs, which
are small mountains surrounded by extensive erosional plains.
Soils and Caliche
Soils that form in an arid regime, in which the potential
evapotranspiration exceeds the precipitation during most of the
year, are termed "aridisols." They are
predominantly mineral soils with a low organic content. The
episodic accumulation of water in some aridisols favors the
development of distinct salt horizons. Calcium carbonate
precipitated from solution may cement sand and gravel into
hard horizons called "calcrete." Calcrete
forms layers as thick as 50 m in the regoliths of some arid
environments (Goudie and Wilkinson, 1977). As reported by
Birkeland (1984), carbonate horizons in aridisols may be
subdivided into morphological stages that correlate with the
age of the parent material.
Caliche is a layer of secondary carbonate accumulation
found in many aridisols. Believed to be of pedogenic origin,
caliche commonly occurs as disseminated nodules or as coatings
on mineral grains, and is formed by a complex interaction
between water and carbon dioxide released by the decay of
organic material or by plant roots during respiration. In areas
of low rainfall, the caliche zone is close to the surface. As
rainfall increases, however, it descends to greater depth.
When annual rainfall exceeds 100 cm, caliche disappears
from the soil profile (Blatt et al., 1980). Ancient caliche is
difficult to recognize because it is susceptible to recrystallization
and chemical changes by ground water.
Plants
Deserts typically have a plant cover that is sparse but
of enormous diversity. The Sonoran Desert has the most
complex desert vegetation (Crosswhite and Crosswhite,
1982). Most desert plants are xerophytes (drought- or
salt-resisting plants), succulents (which store water in
their leaves, roots, and stems), or phreatophytes (with long
tap roots that penetrate the water table). Plant roots that anchor
the soil inhibit deflation, and the stems and leaves of some
plants check the surface velocity of sand-carrying wind
to prevent saltation and to cause deposition. A sand dune
protected from wind by vegetation is referred to as a fixed dune.
Desert Water
Rain occasionally falls in deserts, and desert storms are
often violent. In the Sahara Desert, 300 mm of rain were
once recorded to have fallen within 24 hours. Large Saharan
storms may have an intensity of precipitation of 1 mm/min
(Cooke and Warren, 1973). As originally pointed out by Gerster
(1960), more people drown in deserts than die of thirst.
Though deficient in precipitation, deserts receive runoff from
ephemeral streams fed by rainfall and snow meltwater from
adjacent higher elevation areas. They commonly transport considerable
quantities of sediment for 1 or 2 days, filling the channel
with a slurry of mud. A dry former stream channel is a wadi or
arroyo. Although most deserts are locked into basins with interior
drainage, few deserts are crossed by "exotic"
rivers that derive their water from outside the desert. Such rivers
and evaporate on their journeys through the deserts, but their
volume is sufficient to maintain their integrity. The Nile, Colorado,
and Yellow are exotic rivers.
Lakes form where rainfall or meltwater in interior
drainage basins is sufficient. Desert lakes (playas) are
generally shallow, temporary, and salty. Their water
levels fluctuate and may dry up, leaving a salt crust or
claypan. Because playa lakes are shallow and have a
low bottom gradient, wind stress may cause the advance
and retreat of lake waters over many square kilometers
(Torgersen, 1984). Ground water feeds some desert lakes.
Most playas in the United States are relics of Pleistocene
lakes (Cooke and Warren, 1973). Playas are one of the arid
landforms that may have been inherited from a wetter past
and thus contain clues to climatic change (Street and Grove,
1976).
A large multilingual terminology describes desert-
lake deposits. Investigators with experience in parts of the
Middle East usually call a flat area of clay, silt, or sand
encrusted with salt a sabkha (Glennie, 1970). In North
America, it is a playa, salt pan, or hardpan. In South America,
such a lake is a salina or salar. In parts of North Africa, it is a
chott; and in the U.S.S.R., it is a solonchach. Glennie
separates desert lakes into coastal and inland sabkhas (see
Plate C-21). The former develop from seepage of ground
water or where water flowing in wadis intermittently floods
low-lying depressions. Coastal sabkhas, which he
defines as coastal flat areas just above high tide, are
characterized by algal mats and deposits of halite, gypsum,
and dolomite.
Playas are potential sources of mineral wealth formed
by evaporation (see Plate KL-13). In addition, their flat
terrains make excellent runways for air and spacecraft. The
Space Shuttle frequently lands on Rogers Lake Playa at
Edwards Air Force Base, California.
EOLIAN EROSION
Wind erodes by deflation-the removal of loose,
fine-grained incoherent particles by the turbulent
eddy action of the wind-and by abrasion-the
wearing down of surfaces by the grinding action of
windborne particles.
Evaporation
Water is the base level for eolian erosion, in that it
provides cohesion for surface particles, and thus prevents
deflation. In a desert basin, wind blowing in a hot, dry
atmosphere increases the rate of evaporation, the process
by which a substance passes from the liquid or solid state to
the vapor state. Evaporite deposits form in closed basins in
which water flows with large quantities of salts. As water
evaporates, minerals such as gypsum and various nitrates,
chlorides, and borates are precipitated. The minerals deposited
depend on the temperature and composition of the saline
waters at the time of crystallization. Many of these deposits
are economically significant. Plate E-13 illustrates some
evaporite deposits in the Great Basin Desert.
In describing evaporites, Blatt (1982) states that modern
evaporite deposits and belts of higher salinity in the oceans tend
to occur more frequently at about 30° latitude as a consequence
of global circulation patterns. He suggests that fossil evaporite
deposits may be used to narrow down paleolatitudes of
associated ancient landforms developed on drifting terranes
or continents.
Ventifacts
Wind uses saltating and suspended grains as agents
of abrasion. During frictional contact, particles carried
along flow lines and in centers of vorticity (Whitney, 1978)
create grooves or depressions in material exposed on the
ground in the path of the wind. Ventifacts are aerodynamically
shaped rocks up to meters in size that have been cut, and
sometimes polished, by the abrasive action of wind.
McCauley et al. (1979) note that the pitted and fluted
rocks in the Western Desert of Egypt resemble the pitted rocks
seen at the Viking Lander sites on Mars. They surmise that
susceptible materials on the martian surface may have been
modified by a large amount of eolian erosion.
Blowouts
A blowout, which is a deflation basin, is a hollow
formed by the removal of particles by wind. Blowouts
generally remain small, but may enlarge up to kilometers
in diameter. Glennie (1970) discusses a circular 2- to
3-km diameter blowout in Libya 60 to 70 m deep.
Although he notes that a few short wadis drain into the
hollow, he believes that the hollow was formed predominantly
by the wind.
Yardangs
Yardangs are sculpted landforms streamlined by desert winds.
Up to tens of meters high and kilometers long, yardangs are
found in many deserts of the Earth and also on Mars (McCauley
et al., 1977). Some investigators suggest that the
Sphinx of Egypt was constructed out of a yardang. Yardangs
were first described northwest of Lop Nur, China, by Swedish
explorer Sven Hedin (1903). The photographs accompanying
Plate E-25 show the Lop Nur yardangs carved from lakebed
sediments. McCauley et al. (1977) show that yardangs
can be carved in rocks of all types, including crystalline bedrock,
and therefore are not limited to soft sediments as formerly supposed.
In wind-tunnel simulations, Ward and Greeley (1984) find that yardangs
form by abrasion concentrated at the windward end of a structure.
Deflation and reverse air flow control the formation near the middle
and downstream ends.
Mariner and Viking images show yardangs up to 50 km
long and 200 m wide that lie in the equatorial region on Mars (
Ward, 1979). Ward notes that the azimuths of some martian
yardangs are not parallel to wind-streak directions,
indicating that the yardangs were formed earlier by winds from
a different direction than those that formed the adjacent, more
recent wind streaks.
Desert Pavement
Most eolian deflation zones are composed of desert
pavement, a sheet-like surface of rock fragments that
remain after wind and sheetwash have removed the fine
particles. The rock mantle in desert pavement protects the
underlying material from deflation. Because stony deserts
are ubiquitous, a diverse terminology describes their
surfaces. Stony deserts in Africa are called lag gravels or
reg if fine material remains, or serir if no fine material is
present. In Australia, a stony desert is a gibber, and in the
People´s Republic of China, it is a gobi. (The adjective
"gobi" should not be confused with the Gobi
Desert in the Mongolian People´s Republic.) Most
investigators consider that many of these stony desert plains
are inherited from previous fluvial cycles and represent surfaces
merely modified by winds.
Desert pavement is easily identified in space images (e.g.,
in Plate E-24, the Turpan Depression). Desert pavement
enhances saltation due to the increased elastic rebound of grains
bounding along the coarse surface (Greeley and Iversen, 1985).
Desert Varnish
Desert varnish is a dark shiny stain found on the
surfaces of many desert rocks. Manganese, iron oxides,
hydroxides, and clay minerals comprise most varnishes
and provide the shine. These constituents are believed to
come from sources external to the rocks. Many investigators
consider atmospheric dust to be the source of the manganese
found in the varnish. Dorn and Oberlander (1982) favor a
biologic origin for desert varnish. These authors note that,
although varnish is found predominantly in arid environments,
it also occurs in alpine, arctic, fluvial, and other humid
environments, and they prefer to use the term "rock
varnish."
EOLIAN TRANSPORTATION
Particles are transported in the wind by suspension,
saltation, and creep. In a classic work on the physics of
windblown sand, Ralph Bagnold (1941) pointed out that
an object falling from rest through any fluid (air, water,
oil, etc.) will reach a constant terminal velocity of fall. The
net force on an object as it falls is the result of the pull of
gravity on the particle and the force of the fluid acting in
the direction opposite to that of the fall. When these two
forces are equal, the velocity of the object has reached a
constant value-the terminal velocity of fall.
A lifted particle whose terminal velocity of fall is less
than the velocity of upward eddy currents within a surface
wind is susceptible to transport in suspension. For typical
winds near the surface of the Earth, particles having
diameters less than 0.20 mm may be kept in suspension
and scattered as dust or haze.
With the advent of the space age, scientists can investigate
eolian processes on planets having atmospheres of significantly
different compositions and pressures. Although Mars and
Venus have predominantly carbon dioxide atmospheres, the
atmospheric surface pressure on Mars is nearly 100 times less
than that of Earth, and the pressure on the surface of Venus is
nearly 100 times greater than that of Earth. Iversen et al.
(1976) and Greeley and Iversen (1985) have calculated a
threshold friction speed, which is the minimum wind speed
required to initiate particle motion for each planet. According
their calculations, a particle 0.1 cm in diameter would have a
threshold friction speed of about 8 cm/sec on Venus, 60
cm/sec on Earth, and 600 cm/sec on Mars. These
investigators calculate a particle diameter for the saltation/
suspension boundary of 30, 52, and 210 microns for Venus,
Earth, and Mars, respectively.
Saltation and Creep
Saltation is the downwind movement of particles
through a series of jumps initiated by granular impact.
The particles move in a curved path and strike the ground
at angles between 10 to 16°. Bagnold (1941) found
experimentally that saltation proceeds at one-half to
one-third the speed of the wind and that the mean
height of saltation of grains with an average size of 0.25
mm is less than 1 cm on Earth.
When a saltating grain impacts other grains on the
surface, a fraction of the energy it has acquired is passed
on to grains that are ejected upward and continue the
saltation. However, most of the energy is dissipated among
a large number of grains, and a slow forward surface creep
of the surface grains is initiated. Through creep, a saltating
particle can move a surface grain more than 200 times its
own weight. Bagnold (1941) estimated that as much as 25
percent of grain movement in a desert is due to surface creep.
Dust and Loess
Dust storms are eolian turbidity currents. When rain falls
from a cumulonimbus cloud, the air is cooled significantly
as the precipitation passes through it. Because this air is
cooler and denser than the surrounding air, it sinks. When it
reaches the ground, this cold air is deflected forward, sweeping
up surface debris in its turbulence. Dust storms have been
reported since 1150 B.C. in China (Liu et al., 1981)
and since biblical times in the Middle East (Péwé,
1981). Villages, crops, people, and possibly even climates are
affected by dust storms. Peterson and Junge (1971) estimate
that 500 x 106 tons of windblown dust are carried from deserts
annually. McCauley et al, (1981) report that millions of tons of
valuable topsoil were removed from the U.S. High Plains by a
single dust storm in 1977. This was the first dust storm whose
entire progression was recorded in satellite images. Their
geostationary orbits and wide fields of view allow the NOAA
meteorological satellites to monitor the entire course of regional and
continental dust storms.
On Earth, great dust storms, called haboobs from the
Arabic word for violent winds, sometimes rise to elevations
around 2500 m and advance at speeds up to 200 m/sec (
Idso et al., 1972; Idso, 1976). Some dust storms are
intercontinental, a few may circle the globe (Péwé,
1981), and some may engulf entire planets. When Mariner 9
arrived at Mars in 1971, the planet was enshrouded in global
dust. Dust storms on Mars have been observed to last up to
100 days. The poles and topographically high regions are the
first to clear as the dust settles (Greeley, 1982).
Dust devils are small vortices that whirl around arid lands
and are thought to be related to very intense local heating and
instabilities (Cooke and Warren, 1973). Dust devils on Earth
may extend up to 1 km high (Péwé, 1981). Using
shadow measurement of Viking Orbiter images, Thomas and
Gierasch (1985) recently identified 94 dust devils on Mars.
They determine that some dust devils are up to 6 km high and
suggest that dust devils are common near the subsolar point.
Péwé (1981) discusses two particle size
ranges for dust. Dust devils, dust storms, and loess deposits
are composed of particles 5 to 50µm in diameter. Particles
less than 10µm in diameter are tropospherically sorted dust t
hat moves as an aerosol and remains suspended in air until
brought down by rainfall.
Loess is a deposit of homogeneous, nonstratified, and
unconsolidated windblown silt that may bury the existing
topography. The banks of slopes cut in dry loess generally
stand vertically, but can slump when wet. The thickness and
mean size of windblown silt varies inversely with distance
from the dust source (Pye, 1984). The thickest known loess
deposit, 335 m, is on the 300 000 km2 Loess Plateau
in China (Derbyshire, 1983), a small portion of which is shown
in Plate E-22 (see also Plates F-2 and F-3). In Europe and in the
Americas, loess thicknesses may exceed 60 m, but 20 to 30 m is
more common (Pye, 1984). As reported in Derbyshire (1983),
geomagnetic and thermoluminescence dating of loess deposits
fixes the times when the climatic conditions produced the
environment of deposition. The majority of European and North
American loess is of glacial and periglacial origin, and the loess of
China is believed to have been produced from the deserts (Goudie,
1978; Pye, 1984).
EOLIAN DEPOSITION
Wind-deposited sand bodies are found in many
parts of the Earth´s deserts. Wilson (1973) contends
that sand deposits occur in bed forms at three scales:
ripples, dunes, and large sand bed forms that he calls draas.
In addition, sand sheets are irregular accumulations surfaced
by grains that may be too large for saltation. Sheets are low,
flat, gently undulating sandy strips with no slipfaces (Tsoar,
1983). Sand sheets comprise approximately 40 percent of
eolian depositional surfaces (Fryberger and Goudie, 1981).
One of the largest sand sheets on Earth is the 60 000-
km2 Selima Sand Sheet of southern Egypt and northern
Sudan. Haynes (1982) describes it as absolutely flat in some
places. He notes that active dunes are moving over the sand sheet.
Ripples
As the wind blows on a sand surface, the surface becomes
rippled, with ripple crests and troughs being perpendicular to
the wind direction. The mean length of jumps during saltation
corresponds to the wavelength of ripples (Wilson, 1972). These
wave-lengths and the heights of the crests above the troughs
increase with increasing wind velocity. In ripples, the coarsest
materials collect at the crests. Bagnold (1941) reports that this
characteristic distinguishes the small-scale surface ripples from
large-scale dunes (see Figure E-11.1) in which the coarsest
materials are generally in the troughs. Sharp (1963) distinguishes
between sand ripples composed of grains with median diameters
of 0.30 to 0.43 mm and granule ripples composed of grains with
diameters greater than 1 mm.
Dunes
A dune is an accumulation of sediment blown by the wind
into a mound or ridge. Dunes have gentle upwind slopes on the
stoss, or wind-facing side. The downwind portion of the
dune, on the lee slope, is commonly a steep avalanche slope
referred to as a slipface; dunes may have more than one
slipface. The slipface stands at the angle of repose, which is the
maximum angle at which loose material is stable (30 to 34° for
sand). Bagnold (1941) observed that the minimum height of a
slipface is about 30 cm. The brink of the dune is the top of its
slipface, which may or may not be at the highest point-
the crest.
Sand particles are transported up the gentle stoss slope
of the dune by saltation and creep. When a load of particles
reaching the brink exceeds the angle of repose, a small
avalanche takes place that reforms the slipface. As the
avalanching continues, the dune migrates in the direction
of the wind.
Bagnold (1941) recognized two basic dune types in the
Egyptian desert, the crescentic dune, which he called barchan,
and the linear dune, which he called longitudinal or seif (the
Arabic word for sword). Local names have been applied to
these and many other varieties of dunes in sandy deserts,
which has resulted in a bewildering terminology. However,
certain basic types that occur widely as small dunes were
defined by McKee (1957, 1979a) on the basis of their internal
structures as longitudinal, transverse, barchan, star, dome, and
parabolic.
Breed and Grow (1979) showed that these basic types
occur globally as very large "mega-dunes"
that form the bulk of the Earth´s sand seas. Based on a
worldwide inventory of deserts using Landsat images and aerial
and satellite photographs, these authors categorize dunes
according to their shapes in plan view and the relative positions
of their slipfaces, and classified them as linear, crescentic, dome,
star, or parabolic. This classification system, which has global
applicability and transcends regional language, was tailored for
space observations and is used in this chapter.
Crescentic dunes are crescent-shaped mounds or
segments of ridges
(Figure 8.3a). Each segment is generally wider than it
is long and is bounded on its concave side by a slip face.
These dunes form under dominantly unidirectional winds
and are referred to as barchan, barchanoid, or transverse
by some writers. At the downwind ends of crescentic
dunes are horns that have no slipfaces. Crescentic dunes
move faster than any other dune type. A set of dunes was
measured to move an average of 100 m per year between
1954 and 1959 in Ningxia Province, China (Li, 1962), and
similar rates are recorded in the Western Desert of Egypt
(Embabi, 1982). Breed and Grow (1979) note that the
argest crescentic dunes are in the Taklimakan Desert, where
their horn-to-horn mean width is 3.24 km.
Movement of such large dunes is probably so slow that it is
practically imperceptible over the short run. The most
common dune form on Earth and on Mars is the crescentic
(Breed et al., 1979a, 1979b).
Hills (1940) described unusual 6- to 10-m
high crescentic dunes on the lee shores of some of the lakes
in arid Australia. These dunes are generally composed of
clay, and Hills called them lunettes. Langford-Smith
(1982) suggests that lunettes are formed when seasonal
exposure of saline lake beds induces clay particles to
combine in aggregates. The aggregates are deflated from
lake beds and deposited as clay dunes that form arcuate
mounds along the downwind lake shore.
Linear dunes are straight or slightly sinuous longitudinally
symmetrical sand ridges typically much longer than they are wide
(Figure 8.3b).
Slipfaces occur alternately on both sides (Tsoar, 1978). Linear
dunes cover more area in the sand seas of Earth than any other
dune type, but are rare on Mars (Breed et al., 1979b).
These dunes have been called seif dunes, sand ridges, or
longitudinal dunes.
Linear dunes may occur as isolated ridges, but they
generally form sets of parallel ridges separated by kilometers
of sand, gravel, or rocky interdune corridors. The origin of
linear dunes is controversial. Bagnold (1941) suggested that a
change in wind direction could preferentially advance one of
the horns of the crescentic dune, forming a linear dune. Many
workers have noted similarities between the axial alignment of
linear dunes and the prevailing direction of the regional winds.
Some conclude that linear dunes are built by unidirectional
winds. Other investigators, such as McKee (1979b), Lancaster
(1982), Tsoar (1983), and Breed et al. (1984), provide
field evidence that many linear dunes are the product of
bidirectional or multidirectional wind regimes and that the long
axis of each dune extends in the resultant direction of sand
movement.
Most deposition on linear dunes in bidirectional wind
regimes is known to occur by lee-side accretion. The
effect of dominance of a wind blowing from one direction
rather than the other is to concentrate erosion and deposition
at the same locations on the dune. If the wind regime becomes
essentially unidirectional, Lancaster (1980), suggests that a
linear dune would be reformed into a series of individual or
linked crescentic dunes, as may be happening in the northern
parts of the Namib sand sea (Plate E-9).
Linear dunes are commonly more than 100 km long (Breed
et al., 1979a). Some linear dunes have "Y"
connections (Plates E-17 and E-18). Langford-Smith (1982)
reports measuring a linear dune in the Simpson Desert, Australia,
that was longer than 300 km.
Linear dunes, as well as crescentic dunes, are known to
form in submarine canyons. Valentine et al, (1984) investigated
3-m high linear dunes in a canyon on the continental shelf.
Those dunes have 20-cm wavelength ripples subparallel
to the slope of the walls on their gentle faces. These authors find
that the crests are oriented across the canyon axis.
Star dunes are radially symmetrical pyramidal sand
mounds with slipfaces on three or more arms that radiate
the high center part of the mound (Figure 8.3c). They tend to accumulate
in areas with multiwind direction and are sometimes referred
to as ghords, pyramid dunes, radial dunes, or rhourds. Star
dunes grow upward rather than migrate laterally. According
to Breed and Grow (1979), star dunes are the dominant dunes
in the Grand Erg Oriental of the Sahara (Plate E-6). In other
deserts, they occur around the margins of the sand seas,
particularly near topographic barriers (Breed et al.,
1979a, 1984). The highest sand dunes on Earth may be the
star dunes in the Badain Jaran Desert of China, which are up
to 500 m tall (Zhu et al., 1980).
Parabolic dunes are U-shaped mounds of sand with
convex noses trailed by elongated arms. The nose and the arms
are bounded by slipfaces on their outer convex sides (Figure 8.3d). They are sometimes
called U-shaped, blowout, or hairpin dunes and are well
known in coastal deserts. These dunes form in unidirectional
wind regimes, but they are opposite to the geometry of crescentic
dunes in that their horns point upwind. Although their elongated
arms trail behind because they have been fixed by vegetation, the
bulk of the sand migrates forward. The largest parabolic dune
measured by Breed and Grow (1979). is near the
Colombia/Venezuela border and has a trailing arm 12 km
long. Parabolic dunes are illustrated in Plate E-21, the Thar Desert.
Blowout dunes are small (meter size) dunes that form
downwind from deflation hollows-cup-shaped
depressions or "blowouts" scooped out by
wind erosion of a sand deposit. Many investigators suggest
that blowouts evolve into parabolic dunes when a prevailing
wind advances a section of the rim and vegetation stabilizes
the arms (McKee, 1979b).Dome dunes are semicircular
mounds that lack slipfaces
(Figure 8-3e). Small dome-shaped dunes are normally
not visible on Landsat images, but a few large dome-shaped
dunes have been observed. Plate E-1 shows some dome dunes in
Saudi Arabia. Bigarella (1972) discusses one 137- by
128-m dome dune that is 5.5 m high.
Reversing dunes are a variety of any of the foregoing types
that may occur wherever winds periodically reverse direction.
These dunes typically have major and minor slipfaces oriented in
opposite directions. They tend to grow in height but migrate only
slowly, if at all. McKee (1979a) considers them to be intermediate
in character between a star dune and a linear dune. Breed et al.
(1984) show reversing dunes that are modified crescentic dunes.
Simple, Compound, and Complex Dunes
Breed and Grow (1979), deduce that the foregoing dune
types are scale-independent and occur in three forms-
simple, compound, and complex. Simple dunes are basic forms
with a minimum number of slipfaces that define the geometric
type. They represent a wind regime that has not changed in
intensity or direction since the formation of the dune. Compound
dunes are large dunes on which smaller dunes of similar type and
slipface orientation are superimposed. An example is a crescentic
dune with smaller crescentic dunes on its gentle slope. Compound
dunes probably represent a wind regime that has decreased in
intensity, but not in direction, since the larger basal dune was
formed. Possibly, however, the entire feature may be built by
contemporaneous processes.
Complex dunes are combinations of two or more dune
types. A crescentic dune with a star dune superimposed on its
crest or a star dune with an arm evolving into a crescentic
dune are examples of complex dunes. The most common
complex dunes noted by Breed and Grow (1979), are linear or
crescentic dunes with star dunes along their crests. Such
complex dunes indicate that the wind regime has changed
significantly with respect to intensity and direction.
Understanding the parameters involved in the eolian regimes
of complex dunes may provide evidence for the direction of
paleoclimatic changes.
RESOURCES IN DESERTS
Some mineral deposits are improved, preserved, or created
by geologic processes that occur as a consequence of an arid
climate (Smith, 1977). Evaporation in arid lands is a process that
may enrich mineral accumulation in lakes. Leaching of ore
minerals and redeposition in zones near the water table may
convert minerals to minable grade ore. Of the 15 major deposits
in the western hemisphere strongly affected by ground water,
13 are in deserts (Smith, 1977).
Minable mineral salts concentrate in sediments and near-
surface brines formed by evaporation of inland bodies of water,
often during geologically recent times. The known amount of
available lithium in the world, for example, nearly doubled when
the mineral was found in the saline deposits in the Qaidam Basin
of west-central China. Landsat views of significant evaporite
deposits are illustrated in Plate E-13, the Great Basin and Mojave
Deserts, and Plate E-10, the Atacama Desert.
Owing to scarcity of vegetation, arid lands are especially
amenable to spectral remote sensing. Limonitic rocks have a
composition that may be identified readily from Landsat
Multispectral Scanner (MSS) data. By extending the wavelength
range of scanners further into the infrared, additional surface
materials may be identified. Using airborne spectrometer surveys
of the Mojave Desert, Raines et al. (1984) discovered
three previously unknown molybdenum stockworks. Field
reconnaissance should determine whether these finds are
economically important.
The Landsat Thematic Mapper (TM) increases the
capability to detect hydrothermally altered volcanic rocks
in arid and semiarid lands. Kahle (1982), recognizes the
presence of hydrous minerals using data from an airborne
TM simulator sensor. She points out that recognition of
hydrous phases sometimes allows mapping of hydrothermal
alteration zones, which is important in economic mineral
exploration. Podwysocki et al., (1984) investigated
altered rocks in the Mojave Desert, drawing upon data
digitally processed to enhance areas of hydrothermally
altered volcanic rocks. They used a color-composite
image composed of three band ratios to recognize several
types of altered rock on the image. These investigators found
that some deposits were confused with bleached altered rocks,
thus indicating the necessity for field work to investigate the
significance of TM anomalies.
REMOTE SENSING OF ARID LANDS
The world´s deserts are generally isolated,
inaccessible, and inhospitable. They are also excellent
laboratories in which to study all aspects of eolian
geomorphology and the interaction of wind with other
elements of the environment.
Modern technology and new perspectives made
available by remote sensing systems are proving to be
especially helpful in the effort to understand arid lands.
Landsat satellites provide excellent data with which to
monitor temporal variations in a given area. For example,
comparing data over a period of several years may indicate
changes in eolian features or an increase or decrease in
productive land near the desert fringes.
One of the most exciting results of the first Shuttle
Imaging Radar System (SIR-A) is that images of the
Western Desert in Egypt and the Sudan show buried fluvial
topography, faults, and intrusive bodies that otherwise are
concealed beneath sand sheets and dunes (see Plate I-3). Most
of these features are not visible in the field. The radar signal
that penetrates the loose, dry, surficial sediment is being used
to find previously unknown archaeologic sites and potential
sources of potable water in the arid environment (McCauley
et al., 1982; Schaber et al., 1986).
The high-resolution and forward overlap of the
30.5-cm focal-length lens on the Large Format
Camera (LFC) allows investigators to acquire accurate to
pographic data previously unavailable for poorly mapped
arid lands. Using stereo LFC photos, experimental topographic
profiles have been constructed for the sand sea in the Namib
Desert and for the Qattara Depression of the Western Desert of
Egypt (Walker, 1986). Desert color, which often reflects the
composition of exposed rock surfaces and sand and which may
be a significant age marker for eolian sands on a regional and a
local scale, also may be investigated with the mapping-
quality natural-color photographs from the LFC.
Desert studies still are hampered in many regions by
lack of accurate climatic data. Most desert weather stations
are in oasis surrounded by trees and buildings and have
been subjected to many height and location changes
throughout the duration of the station. Data from oasis do
not reflect data from the raw desert. In addition, a wide
variety of instrumentation has been used. Measurements are
recorded over varying lengths of time and in different formats,
making them difficult to interpret and compare.
To overcome some of these problems in deserts of the
American Southwest, the U.S. Geological Survey (USGS)
established a Desert Winds Project to uniformly measure
several key meteorologic parameters of arid lands (McCauley
et al., 1984). This group has successfully established
geometeorologic (Geomet) stations to measure windspeeds,
including peak gusts, which do most of the geologic work.
The stations also measure wind direction, precipitation,
humidity, soil and air temperatures, barometric pressure, and
blowing dust particles at specific heights above the surface by
using low-maintenance automatic solar-powered
sensors. Data are sampled at 6-minute intervals and
transmitted every 30 minutes to a Geostationary Operational
Environmental Satellite (GOES). From GOES, the data are
transmitted in real time to a receiving station and are distributed
to the USGS laboratory in Flagstaff, Arizona.
These investigators have been recording weather data
from four different types of deserts in Arizona and will add a
Geomet site in the Chihuahua Desert in New Mexico. They
have recorded a windstorm near Vicksburg, Arizona, with
peak gusts of almost 144 km/hour (McCauley et al.,
1984, p. 17). The investigators will combine analysis of
quantitative meteorologic data acquired from the Geomet stations
over several years with detailed geologic field studies and
repetitive aerial and ground photographs and Landsat imagery.
They will study long-term changes produced by wind in
deserts of differing geologic and climatic types.
DESERTIFICATION
The main bodies of the world's great deserts were formed
naturally by processes interacting over long periods of time.
During those times, deserts expanded and shrunk independent of
human activities. Evidence is abundant that large sand seas, now
inactive because they are "fixed" by vegetation,
once extended well beyond the present margins of core deserts,
such as the Sahara. In some regions, deserts are separated sharply
from the surrounding less arid areas by elevation changes that
reflect structural differences in the regional geology. In other
regions, the desert fringes, the transition to a more humid
environment is gradational, and the desert border is difficult to
define. These arid transition zones have very fragile, delicately
balanced ecosystems. Hare (1983) discusses the desert fringes
as a mosaic of microclimates in which small enclaves support
vegetation that picks up heat from the hot winds. After rainfall,
vegetation serves as a gigantic wet bulb and is distinctly cooler
than its surroundings. He notes that, in areas having high
livestock densities, the pounding of soil by hooves increases the
proportion of fine material and reduces the percolation rate, thus
encouraging wind and water erosion. Browsing by livestock and
collection of firewood by man reduce or eliminate plant cover that
provides good soil binding.
In these areas, and in similar semiarid environments,
human activity is commonly superimposed on the natural
processes that tend to shrink or expand deserts. These
human activities may stress the ecosystem beyond its
tolerance limit, resulting in general and semipermanent
degradation of the land. This degradation of formerly
productive land, referred to as desertification, is a complex
process that can occur from different causes and at varying
rates in different climatic regimes. Desertification may
exacerbate a general climatic trend toward increasing aridity,
or as some investigators propose, it may initiate local climatic
change (Charney, 1975; Charney et al., 1977).
The process of desertification was first brought to world
attention in the 1930s when parts of the Great Plains of the
United States turned into the Dust Bowl as a result of poor
farming practices initiated during a "wet"
cycle. Millions were forced to abandon their farms and their
livelihoods. The term was popularized by Aubreville (1949).
By 1973, drought that began in 1968 in the Sahelian countries
of Africa was responsible for the deaths of between 100 000 and
250 000 individuals, the disruption of millions of other lives,
and the collapse of the agricultural bases of five countries. More
than $200 million in cash and food were donated by private
individuals, governments, and the United Nations in an effort
to prevent mass starvation. Reining (1978) estimates that
desertification affects more than 600 million people and one-
third of the Earth´s land surface. The 1977 UN Conference
on Desertification estimated that 31.6 x l06 km2 of the surface
of the Earth has been subjected to desertification (Kates et al.,
1977). Cross (1983) concludes that the southern edge of the Sahara
has expanded by 650 000 km2 in the past 50 years.
Scientists at the Lanzhou Institute of Desert Research
of the Chinese Academy of Sciences consider 170 000 km2
in China to be desertified and an additional 170 000 km2 to
be in danger of desertification. They recently calculated that 50 000
km2 of formerly productive land in China has been desertified
within the past 50 years owing to misuse of the land. Chinese
scientists refer to deserts that they think are entirely manmade as
"sandy lands." Part of the Mu Us Sandy Land
(Plate E-22) falls into this category.
Desertification does not proceed in linear, easily mappable
patterns. Deserts advance erratically, forming patches on their
borders. Areas far from deserts may quickly degrade to barren
soil, rock, or sand through lack of sound land management
practices. Desert encroachment does not occur because of a
change in only one measurable parameter in a given area. Rather,
it is a process to which many factors contribute. Usually, an area
undergoing desertification is brought to public attention only after
the process is well under way. Often little or no data are available
to indicate the previous state of the ecosystem or the rate of
degradation.
Landsat sensors provide extensive (34 000 km2 per
image) repetitive coverage of the Earth´s surface in uniform
scale and in uniform format. The sensors provide data well suited
for monitoring desertification. Using Landsat reflectance data,
Otterman (1977) analyzed the impact of people and animals on
the surface albedo of the Earth. Low albedo is a result of dark
plant debris accumulating on the crusted soil surface, and a
higher albedo is produced when collecting firewood and
overgrazing results in a trampled, crumbled soil. Otterman
believes that overgrazing has a significantly regional and global
effect and may have raised the surface albedo of the Earth as
much as 10 percent. Hare (1983) and Charney et al., (1977)
argue that the feedback processes of albedo increase may intensify
and extend drought, but may not induce permanent change.
In a review of the concept of desertification, Glantz and
Orlovsky (1983) argue that the term has numerous definitions.
Some researchers consider it a process of change, while other
investigators view it as the end result of a process. Also,
researchers are unable to agree as to where desertification can
occur.
According to Glantz and Orlovsky (1983), some investigators
consider climate to be the major contributor to desertification
processes, with human factors contributing only a minor role.
Other researchers reverse the significance, and a third group
blames climate and humans equally. The importance of a long-
term climatic change, when compared to a few years of fluctuating
precipitation, is not yet known. Certain investigators, as summarized
in Kerr (1984), suggest that drought in the American Great Plains
may be related to the superposition of an 18.6-year lunar cycle
of changing declination and the 22-year sunspot cycle. Some
investigators consider desertification to be irreversible during a
season or a few seasons, but reversible on a scale of decades or
centuries.
On a local level, individuals living on desertified land
may do much to reclaim the land. The wind regime near the
face of dunes can be interrupted by covering the dunes with
large boulders to prevent sand from blowing. In another
approach, meter-size straw grids are constructed to
decrease the surface wind velocity on some dunes, as is
illustrated in Plate E-23 of the Tengger Desert. Shrubs and
trees planted within the grids are protected by the straw until
they take root. In areas where some water is available, dunes
have been stabilized with shrubs planted on the lower one-
third of a dune´s windward side. By decreasing the wind
velocity, this vegetation prevents much of the sand from moving
near the base of the dune. The higher velocity winds at the top of
the dune level it off. Trees are then planted on top of the flattened
dune.
Oasis and farmland in windy terrain are sometimes protected
by rimming the area with specially selected grasses and shrubs.
Sand that manages to pass through the grass belts is caught in the
adjacent forest belts, which are strips of poplars planted 50-
to 100-m apart as windbreaks. Small plots of trees may be
scattered inside oasis to further stabilize the area. In China, a Green
Wall is being planted in the northeast to protect the sandy lands. When
complete, the Green Wall will be 5700 km long.
Efficient utilization of water resources and control of
salinization are other effective tools for improving arid lands.
Research is being done to exploit surface water resources,
primarily seasonal runoff from adjacent highlands, to find and
tap ground-water resources, and to develop effective
irrigation procedures for arid lands. Other research on the
reclamation of deserts is now focusing on how sand-fixing
plants maybe adapted to local environments and how the resources
of grazing land and water may be effectively exploited without
being overused.
CONCLUDING REMARKS
In a report comparing the agricultural situation in Africa
and Asia, Norman (1985) notes that, although the rate of
growth of food production in Asia increased by 20 percent
from the 1960s to the 1970s, crop yields in Africa have not
kept pace with population growth. During the 1970s, the
population in Africa grew twice as fast as its food production.
A new wave of desertification presently is threatening millions
of people in the Sahel. According to Glantz (1980), solutions
to several causes of desertification are known, but because of
the political, economic, cultural, or social situation, they are
not applied.
Dr. Mostafa Kamal Tolba, the Executive Director of the
United Nations Environmental Programme, recently discussed
the present Sahel drought (Tolba, 1984, p. 3):
"The message now is the same as it was in 1977:
when we cope with desertification we can cope with drought.
The difference between now and then is that now we have no
excuse. The framework for applying the known remedies for
desertification has existed for over six years; the suffering
of the millions of poor in the semiarid regions should be on all
our consciences."
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