8 Deserts and Desertification

Sushmitha Baskar

Structure

9.0 Objectives

9.1 Introduction

9.2 Definitions

9.3 Characteristics of Deserts

9.4 Types of deserts and the global distribution of deserts

9.5 Factors for desert formation

9.6 Fluvial processes in deserts

9.7 Wind processes in deserts

9.8 Life forms and adaptations in deserts

9.9 Anthropogenic factors leading to desertification

9.10 Combating Desertification

9.11 SAQ

9.12 Summary

9.0 OBJECTIVES

After reading this unit, you should be able to:
define the term deserts and desertification; characteristics of deserts;
explain the fluvial and wind processes in deserts;
describe the life forms and adaptations in deserts; and
list the anthropogenic factors responsible for desertification.

9.1 Introduction

This chapter introduces the learner to deserts, their global distribution and factors for their formation. Deserts occupy large parts of the Earth’s surface and host different plants and animals that are endemic to these regions. Approximately 41% of the Earth’s land surface is covered by drylands which are inhabited by 2 billion people. In this almost 20% is degraded. So a large population is under threat from the process of desertification. Desertification processes affect approximately 1.9 billion hectares of land and 1.5 billion people world-wide. The Gobi Desert is also facing severe desertification problems. It is reported to have degraded land and many villages due to its fast rate of desert expansion. Desert ecosystems have their own biodiversity and the organisms are adapted to survive and tolerate such high temperatures and conserve water. The Thar desert in Rajasthan covers the largest state in area (10.4% of the total area) in India. The region receives little rainfall and the temperatures can go up to 55°C making it very hot and dry.

Let us now learn the definitions of deserts and desertification. We will also introduce you to the characteristics of deserts.

Weathering: Physical weathering is dominant in deserts and chemical weathering is minimal due to the absence of water. Salt wedging, thermal expansion, frost wedging, and plant rooting are some of the processes involved. Also the weathering process is much slower than in humid regions.

Poor soil development: The soil development is very poor. Salts and pedogenic carbonate development is observed. The soils are thin and lack a good amount of extensive organic matter.

Impermeable surface layers: This is due to the bedrock, salt beds and hardpans. With the result there is little infiltration and more surface runoff.

Drainage Patterns: Deserts mainly have ephemeral streams that flow either seasonally or during storms

9.4 Types of deserts and the global distribution of deserts

Let us now learn about the types of deserts and the distribution of deserts world-wide. Deserts occupy almost 25-30% of the total continental land area. Hence this forms a major part of all climatic regimes. All deserts in the world are not sandy deserts. Only 20 percent of the earth’s deserts are sandy deserts. There are also stony deserts i.e. the Kalahari Desert which consists of large exposed rocks.

(a) Hot deserts: These are situated near the equator. The temperatures during day time can be more than 38ºC and the nights are very cold. Examples of hot deserts are the Great Sandy Desert and the Simpson Desert in Australia.
(b) Cold deserts: They are found away from the equator in the centre of continents. They are characterized by high mountainous areas. As the name indicates, these deserts are covered by snow and ice during winters. Examples of cold deserts are the Patagonian Desert in Argentina and Antarctica.

(c) Inland deserts: These deserts are found far away from the sea. They form in the centre of large continents where they are protected from the blowing winds. When clouds over the oceans rise over mountains rainfall occurs. Then the air left is dry. When this air moves inland no rainfall occurs. This is the reason deserts in the centre of Kalahari and Australia are dry.
(d) Coastal deserts by the sea: They are found on the west coast of the earth’s continents. These deserts have warm summers and cool winters. Hence they are referred to as cool deserts. Some examples are the Namib Desert in south-western Africa and the Atacama Desert in South America. From cold oceans the water does not evaporate quickly in comparison to warmer oceans. When air is blown from a cold ocean to the coast it has no moisture so there is no rain. During day time the water evaporates from the land leading to desert conditions. During the night fog forms as the land cools and some moisture in the air condenses. Cold ocean currents flow up the west coast of Africa (Benguela current) and South America (Humboldt current) from Antarctica. This cold water interrupts the water cycle resulting in the formation of the Atacama and Namib deserts.

9.2 Definitions

Deserts: They are subtropical arid regions where potential evaporation is approximately >2000mm and the annual precipitation is about <200mm.

Desertification: It is the process where land is degraded and becomes relatively dry, arid and loses water bodies, vegetation and wildlife. It can be caused due to climate change and through human induced activities. The United Nations Convention to Combat Desertification (UNCCD) has defined desertification as “land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities.”

9.3 Characteristics of Deserts

Dry Climate and Nature of precipitation: In desert areas usually the rate of evaporation is more than the rate of precipitation. There are arid regions that receive precipitation <10 to 15 inches/year and semi-arid regions that receive precipitation 10-25 inches/year. Sudden heavy storm downfall and flash floods are common in deserts.

Causal Factors of Dryness: Deserts lack in vertical air movement so no air cooling and no atmospheric condensation takes place. For example the sub-tropical latitude deserts such as Sahara, Arabian and Australian. Deserts are characterized by semi-permanent high pressure systems and high level temperature inversions.

High Landscape erosion Potential: This is due to lack of vegetation and sudden intense rainfall events. So surface erosion occurs in deserts and water erosion is dominant. Weathering: Physical weathering is dominant in deserts and chemical weathering is minimal due to the absence of water. Salt wedging, thermal expansion, frost wedging, and plant rooting are some of the processes involved. Also the weathering process is much slower than in humid regions.

Poor soil development: The soil development is very poor. Salts and pedogenic carbonate development is observed. The soils are thin and lack a good amount of extensive organic matter.

Impermeable surface layers: This is due to the bedrock, salt beds and hardpans. With the result there is little infiltration and more surface runoff.Drainage Patterns: Deserts mainly have ephemeral streams that flow either seasonally or during storms

9.4 Types of deserts and the global distribution of deserts

(a) Hot deserts: These are situated near the equator. The temperatures during day time can be more than 38ºC and the nights are very cold. Examples of hot deserts are the Great Sandy Desert and the Simpson Desert in Australia.

(b) Cold deserts: They are found away from the equator in the centre of continents. They are characterized by high mountainous areas. As the name indicates, these deserts are covered by snow and ice during winters. Examples of cold deserts are the Patagonian Desert in Argentina and Antarctica.

(c) Inland deserts: These deserts are found far away from the sea. They form in the centre of large continents where they are protected from the blowing winds. When clouds over the oceans rise over mountains rainfall occurs. Then the air left is dry. When this air moves inland no rainfall occurs. This is the reason deserts in the centre of Kalahari and Australia are dry.
(d) Coastal deserts by the sea: They are found on the west coast of the earth’s continents. These deserts have warm summers and cool winters. Hence they are referred to as cool deserts. Some examples are the Namib Desert in south-western Africa and the Atacama Desert in South America. From cold oceans the water does not evaporate quickly in comparison to warmer oceans. When air is blown from a cold ocean to the coast it has no moisture so there is no rain. During day time the water evaporates from the land leading to desert conditions. During the night fog forms as the land cools and some moisture in the air condenses. Cold ocean currents flow up the west coast of Africa (Benguela current) and South America (Humboldt current) from Antarctica. This cold water interrupts the water cycle resulting in the formation of the Atacama and Namib deserts.
(e) Semiarid deserts: These deserts receive 250 to 350mm of rainfall. The summers have temperatures of 20°C and 30°C and are long and dry periods. More than one-third of Australia is a semiarid desert.

(f) a) Subtropical Deserts: These are located at latitudes near or above the Tropic of Cancer and Tropic of Capricorn: These include the following.

Tropic of Cancer: The deserts include the great Sahara desert of North Africa and the Arabian desert of the Middle East.

Tropic of Capricorn: The deserts include the deserts in the Australian interior, and the deserts in the interior Pampas of Argentina.

g) Mid-latitude Deserts: These deserts include the Gobi desert of central Asia and the deserts in the western interior of the United States.

Activity

Locate the deserts in the world using an atlas. Write down the biodiversity of these deserts.Visit the metereological department near deserts and collect the data for past few years. The data can include rainfall and temperature details.

9.5 Factors for desert formation

The planet’s air circulation patterns and wind patterns are important factors for desert formations.

a) Earth’s air circulation patterns: Most of the deserts are located between 15 to 30° north and south of the equator. Here the trade winds blow offshore. The warm air holds more moisture than cold air. Warm air rises at the equator and then cools. This cool air has less water holding ability so precipitation is common at the equator. The dry air mass moves north N30° and south S30°. At ±30° dry air sinks and warms. This warm air can hold more water resulting in evaporation of water and desert formations. Hence the equatorial region is tropical an± ±30° is desert. An example is Sahara desert.

b) Rain shadow effect: When hot rising air approaches a mountain it cools. This cool air that crosses over the mountains is dry. Hence deserts are formed on the other side of the mountain range. There is little precipitation on the leeward side also called as Rain Shadow area desert. An example is the Gobi desert.

c) Distance from the ocean: This is also an important factor for desert formation. When land is far away from oceans then deserts form. Near cold ocean currents: The cold ocean air warms as it passes over continents. The warmer air holds more moisture and causes evaporation and the formation of deserts.

9.6 Fluvial processes in desert

The significant agents of erosion and transportation in deserts are the flowing waters. Let us study in detail the fluvial processes.

Erosional Characteristics: In deserts due to the lack of vegetative cover, flash flooding, sheetwash and rilling are common. At certain times intense rainfall events lead to large amounts of sediments to be mobilized in a short span. The stream gradients are steep but the ephemeral nature of the flowing waters results in an imbalance between erosion, transportation and deposition. The streams can become heavily filled with sediments during sudden rainfalls. The drainage basins usually tend to be closed with internal drainage and basin sedimentation.
Desert Hydrology: It is generally dominated by ephemeral streams. 99% of all desert drainages are ephemeral streams. They are streams that flow during seasonal or storm events. There are also perennial streams that flow all through the year. These streams are rare and sustained by water with headlands outside the desert region in a more humid environment. Examples are the Nile river with its headlands in the East African rift valley of Kenya and Uganda, that flows north to the Mediterranean through Sudan and Egypt. Lake Basins: The perennial lakes are uncommon in deserts. Dry lake beds are common. Playa lakes found in deserts are dry lake beds. They are ephemeral and may periodically contain water. They also have high evaporation rates. Salt flats: The playa lakes contain a good quantity of dissolved salts. These precipitate salts on the lake floor when subject to evaporation. Pluvial Lakes: These are perennial lakes. They are remnants of larger lake bodies that had formed in the past in the Pleistocene. These lakes are found in some desert areas such as Great Salt Lake, Pyramid Lake of Nevada.
Fluvial Erosional Desert Landscape Features: Fluvial Erosion is the dominant landscape change process occurring in deserts. High rates of physical weathering by the fluvial process is significantly changes the desert landscape. Let us see the different features. Differential Erosion: This is the differential erodibility of different rock types under desert fluvial processes. For example, quartz sandstone is more resistant to weathering than shale rock. Due to this process less resistant rocks are weathered and the more resistant rocks remain.
a) Butte/Mesa Desert Topography: This is a result of differential erosion with resistant cap rocks holding up topographic features. This can be observed in areas with flat lying sedimentary rock layers with alternating resistances to erosion. Rocks such as sandstone and limestone often form resistant cap rocks. These topographies include the following:

Butte – They are round or oval shaped, flat topped topographic features. Mesa – They are elongated or table like, flat-topped topographic features.

Pinnacles – They are tower-like spires of rock, erosional remnants formed by cap-rocks.

b) Inselbergs: These are generally isolated, resistant rock masses that stand high in relief to surrounding topography. These topographies include the following:They are seen as erosion resistant rock masses. They stand in relief as the more easily eroded rock material is eroded from the surrounding landscape. They are found along mature/dissected mountain fronts in deserts southwest.
c) Pediments: They are gently inclined, concave up, ramp that extends outward from a mountain front, found along the lower slopes of mountains in desert regions. They are covered with a mantle of unconsolidated debris, or wind-blown sediments.
d) Badland Topography: They are intricately rilled and barren terrain in arid regions. They are mostly observed in areas underlain by horizontal strata of shale and clay formations that are poorly consolidated and subject to rilling and gullying. Badland topography is characterized by an extensive network of convoluted rills and gullies.
Fluvial Deposition in Arid Landscapes: The debris that is eroded is mostly deposited on the valley floors in form of talus slopes and alluvial deposits. The depositional sites include:

Pediment Zone: This is the ‘foot of the mountains’. It is the zone at base of desert mountain ranges which forms the site of fluvial depositions from mountain canyons. Intermontane Basins: This is the area of the internal drainage network. They are low areas between mountain ranges. They are sites of complex interactions between lake basins, aeolian processes, and fluvial regimes.

The depositional features include: Alluvial Fans: They are fan-shaped deposits of alluvial debris formed as mountain stream drainages empty onto the piedmont area. The size of the alluvial fans is a function of the area of drainage basin, the climate, and the lithology of rocks in source area.

Bajada: They are coalescing alluvial fans from adjacent mountain canyons forming a ‘fan apron’ along the mountain front.

9.7 Wind processes in deserts

Wind processes are very important and powerful forces which can help in shaping landscapes. Wind erosion processes like abrasion and deflation in deserts can change the landscape. These processes have the greatest impacts on dry soil areas as it is easier to transport light soil than wet soil. Further, deserts have less physical barriers, like hills, trees, which can reduce the wind erosion processes. The landforms shaped by wind processes include: rock pedestals, deflation hollows, oasis, sand dunes, yardangs, and desert pavements.

Wind erosion

Wind erosion is the process of breaking down and dispersing particles by wind movement. It occurs by three different methods, namely abrasion, deflation and attrition.

Wind Abrasion: This is the process of the abrasion of an object due to the impact of particles carried by the wind. It results in wearing away of an object due to the impact of particles transported by wind action. This can cause the object being hit with particles to become smooth. It is similar to sandblasting. The places most affected by this activity are those areas where there is less vegetation. Ventifacts are structures that have become smoothened by sand particle abrasion. In deserts the wind abrasion processes can also result in ridge and furrow landscapes such as in Bahrain.

Wind Deflation: It is the erosion of soil due to the removal of sand, dust, rocks by the action of wind.

Due to this the surface of the desert becomes rocky consisting with gravel-sized particles.

Wind attrition: It is the process in which wind borne particles collide with each other and wear out in the process.

Wind Deposition

The important features deposited by wind are sand dunes. Also loess are deposits of finer sediments.

Sand dunes: They are composed of a variety of minerals due to limited weathering. The dune sands are uniform in size and shape. Generally sand dunes form when there is an abundant supply of sand and steady winds. Sand dunes slope gently in the upwind direction. The wind blows sand into a network of troughs, crests and ripples that are perpendicular to the wind direction. They are the consequence of saltation. Accumulations of sand build into mounds and ridges and they become a dune when the slip face is about 30 cm high. Dunes grow as sand particles move up the gentle upwind slope by saltation and creep. They fall onto the slipface inducing movement. The wind deposits dune sands layer by layer and when the wind changes directions, cross beds are formed. They are named cross beds due to the way each layer is formed at an angle to the ground. The sand dune type is related to the quantity of sand available, the direction of the wind, and nature of the ground. Sand dunes can be crescent-shaped, star-shaped, parabolic, linear, or barchan.

Loess: The silt and clay deposited layer wise due to wind action form loess. These deposits make very fertile soils in many regions globally.

Sand sheets: These are flat areas of sand with sand grains that are too large to saltate. 45% of depositional surfaces are of this type, e.g. Selima in South Egypt.

Deflation hollows: These are formed as a result of removal of fine particles by the wind, lowering the surface and creating a hollow. Qattara Depression is an example of this formation.

9.8 Life forms and adaptations in deserts

Desert ecosystems have well adapted flora and fauna. The plants have many adaptations to survive the dry conditions.

a) Desert Plants

The plants found in deserts can be classified as annual, ephemeral and perennial species. The desert plants can also be classified according to how they survive high temperatures and how they survive uncertain precipitation: drought escapers, drought evaders, and drought resisters. The plants found in deserts are called xerophytes. Cacti are drought resisters. The desert cactus has spines that help cool the plant and protect it from the hot sun. Cacti also store moisture in their spongy stems or root tissues during periods of rain and use it during dry periods. The leaves are reduced to spines and the big wide green cortex (which is the stem) functions as the leaves, carrying out photosynthesis. Some plants like mesquite trees have long tap roots, over 100 feet that help them reach the groundwater. Some plants compete with each other for water. The plant creosote produces toxins and releases them so other plants cannot survive in that toxin affected area. This process is referred to as allelopathy. The creosote is also a drought resister. They have tiny leaves and their leaves are often coated with a waxy, resinous substance to reduce the loss of water. These are some adaptations of desert plants. Let us now study about the desert plants.

i) Annual plants: These plants are mostly herbaceous and have a life cycle of less than a year. Some examples

include: Panicum turgidum. The desert annual plants are therophytes as their life cycle is just a few weeks and they are dormant as seeds to survive dry harsh conditions.

ii) Ephemeral plants: These have a very short life cycle. An example is Alyssum alyssoides. These plants are born after occasional rains and reproduce and die before a new drought season.

iii) Perennial plants: These plants survive in dry conditions by the minimal loss of water during the hottest These plants are xerophytic. They have leaves that are leathery and coated with wax. The stomas are closed to reduce transpiration and evaporation. Most plants have a reduced foliar surface and the leaves are converted into thorns as adaptation strategies. Members of Cactaceae and Euphorbiaceae family belong to this group. Also members of the phreatophytes which have extremely long roots to take water from the groundwater belong to this group.

b) Animals of the desert

Animals also adapt to the dry conditions according to the availability of water, food and survival in the extreme weather conditions.

a) Aestivation: During the drought periods and heat some desert animals “aestivate”. These desert animals can escape drought by “sleeping” through the drought period. This dry weather “sleep” is called aestivation. For example Spadefoot toads sleep in underground burrows for months or years covered with a jellylike substance that keep them moist. The heavy raindrops that cause ground vibrations wake them up. Aestivating animals include, some species of reptiles and the desert snails. They can remain dormant, i.e. the snails can remain in their shells as long as five long years.
b) Resistant eggs: Some desert animals are able to complete their life cycle in short periods. They produce many tough and resistant eggs which tolerate years of heat. The eggs hatch after heavy rainfall. Examples are the crustaceans, Triops called as Shield shrimps that are found in the Australian deserts.
c) Migration: Some animals migrate to different areas. Birds inhabiting deserts migrate to avoid the harsh They come back when the season is favourable for them. They breed after rainfall. In the Some animals migrate to different areas. Melopsittacus undulates travels thousands of kilometres during extremely dry seasons and return later.
d) Nocturnal activity: Animals also rest in shade during day time and carry out fewer activities. Many desert animals avoid the day time heat and live in cool deep burrows underground.
e) Soaring: Some birds soar high above where temperatures are much cooler. Examples include Hawks, Vultures and Eagles.
f) Big ears and long legs: Some desert animals have bog oversized ears that help to cool the body by radiating In these animals the blood vessels in the ears are located just under the skin, and as air flows around them it cools the blood. The cooled blood helps to cool the body. Examples are Jackrabbits, Desert Foxes, Camel and Gazelle.
g) Adaptations of the camel: The camel is known as the ‘ship of the desert’. It can survive extreme heat and dry It has a number of adaptation strategies to survive in the hot deserts. It conserves water by excreting concentrated urine and dry faeces. It stores fat in their hump for food so they can survive long periods without food. The camel has long eyelashes that protect the eyes and nostrils which can be closed during sand storms. It has a thick coat of fur and underwool that keep them warm during cold desert nights and also insulate them against the scorching day heat. It can lose up to 25 % of its body weight when there is no water availability and rehydrate when there is water availability. A camel can drink up to 150 litres of water in ten minutes when there is water availability. It can also survive when its body temperature increases up to 12°C.

9.9 Anthropogenic factors leading to desertification

We have so far learnt how deserts are formed naturally. Human beings can induce or accelerate the desertification process in many ways. Overgrazing, deforestation, poor irrigation techniques etc. can lead to desertification. Approximately more than 110 countries have drylands which are potentially at risk from desertification. This can affect almost 900 million people.

Overgrazing: This is unsustainable grazing. It occurs when the number of cattle, goat that can graze an area exceeds the carrying capacity of the land. When this capacity is exceeded changes in vegetation occur. For example, drought-resistant species replace edible species. Also the soil quality is reduced. The animals break down the soil structure thus increasing its vulnerability to erosion.
Overcultivation: This occurs when more food production is required to feed large populations.
Deforestation: This occurs when forest are cleared for agricultural purposes and also for fuelwood. It leads to desiccation of the soil and lowered water tables.
Inappropriate irrigation practices: Poor irrigation techniques can result in reduced soil fertility. This is due to salinisation and waterlogging.

9.10 Combating Desertification

The United Nations Convention to Combat Desertification (UNCCD) 2006 is an international treaty that aims to promote effective action through innovative local programs and international partnerships. More than 1 billion people have little access to water, and by 2030 the demand will increase by 30%. This can displace 700 million people by 2030. So land degrading practices should be discouraged and sustainable land use practices should be followed. The World Day to Combat Desertification and Drought is observed on 17th of June annually. This date is the day the General Assembly adopted the Convention to Combat Desertification, which is the sole legally binding international agreement linking environment and development to sustainable land management. The importance of observing this day is to make the public aware of land degradation, work for improving the conditions for people living in the drylands, to restore soil productivity, and to address drought mitigation. This can be achieved by involving the local people as a participatory approach in addressing the problems related to desertification and land degradation.

9.11 SAQ

What are deserts?
Describe the different types of deserts with suitable examples.
Briefly explain the adaptations of animals in deserts.
Explain the fluvial processes in deserts.
Write short notes on wind processes in deserts.

9.12 Summary

In this unit we have studied about deserts, types of deserts various fluvial and wind processes that occur in deserts. We have also understood the different life forms both plants and animals and their adaptations in desert regions. We have further learnt that human activities can also cause desertification and we should follow sustainable land practices to combat anthropogenic desertification.