General Studies - GEOGRAPHY - DESERTS

Overview

• A desert is a region that receives almost no rainfall. In general deserts are areas with a moisture deficit i.e. lose more moisture than receive
• Deserts are defined as areas with average precipitation less than 250 mm per year or where more water is lost by evaporation and transpiration than falls by precipitation
  
• Deserts are located where vegetation is sparse or nonexistent
  
• Deserts constitute about one third (33%) of the Earth’s land surface
  
• The largest desert on Earth is Antarctica
  

Classification of deserts

The world's largest deserts (excluding polar deserts)

• Hot deserts
  
  • This is the most common form of desert
    
  • They have large diurnal (daily) and seasonal temperature variation, with daytime temperatures reaching more than 45 C in the summer and dipping to 0 C at night in the winter
    
  • Water acts to trap IR radiation from both the sun and the ground, and dry desert air is incapable of blocking sunlight during the day or trapping heat at night
    
  • The largest hot desert is the Sahara Desert
    
• Cold deserts
  
  • Cold deserts (aka polar deserts) are deserts which occur in extremely cold regions. In cold deserts, the mean temperature during the warmest month is less than 10 C
    
  • Cold deserts form due to extreme lack of precipitation (in the form)
    
  • Cold deserts are covered in snow and ice. Due to lack of liquid water, cold deserts cannot support life
    
  • Instead of sand dunes, polar deserts have snow dunes (in areas where precipitation is locally available)
  • The largest cold desert is the continent of Antarctica
    
• Montane deserts
  
  • Montane deserts are deserts that occur at very high altitudes
    
  • Example: Ladakh, Tibet
    
  • These places are profoundly arid (low humidity) due to their large distance from the nearest available source of moisture
    
• Rain shadow deserts
  
  • Rain shadow deserts form when tall mountain ranges block clouds from reaching areas in the direction of the wind
    
  • As air moves over the mountains, air cools and moisture condenses, causing precipitation on the windward side of the mountain. When the air reaches the leeward side, it is dry since it has already lost all its moisture, resulting in a desert
    
  • Example: Tirunelveli area in southern Tamil Nadu
    

Flora and Fauna in deserts

The snow surface at Dome C in Antarctica is representative of most of the continent's surface

• Although deserts are generally thought to support little life, in reality deserts do have high biodiversity
• Animals in the desert include kangaroo rat, coyote, jackal, jack rabbit and lizards
• Most desert animals remain hidden during the daytime to control body temperature and limit moisture needs
  
• Animals that have adapted to live in deserts are called xerocoles. A particularly well-studied adaptation is the specialisation of mammalian kidneys shown by desert-inhabiting species
• Deserts typically have plant cover that is sparse but diverse
• Most desert plants are salt and drought tolerant, such as xerophytes
• Some desert plants store water in their leaves, stems and roots. Others have long taproots that penetrate deep into the ground to reach the water table, or have roots that spread over a wider area in order absorb moisture from the ground
• Another desert adaptation is the development of long spiny needle-like leaves that lose less moisture to transpiration
  
• The giant Saguaro cacti, which grow to about 15 m height, are commonly found in the Sonora desert in Arizona (USA). The Saguaro cacti grow slowly but live up to 200 years, provide nests for desert birds and serve as desert trees
  

Water in deserts

• Rain does fall occasionally on deserts, and when they do, desert storms are often violent
• Large storms in the Sahara deliver up to 1mm of rain per minute
• Normally dry streams, called arroyos or wadis, can quickly fill up following rain and cause dangerous flash floods
  
• A few deserts are also crossed by ‘exotic’ rivers – rivers that originate elsewhere but run through desert areas. These rivers lose enormous quantities of water to evaporation while journeying through the desert, but have sufficient volume to ensure continuous flow. Examples: Nile, Colorado and Yellow rivers
• Desert lakes can form where rainwater or meltwater in interior drainage basins is sufficient. Desert lakes are usually salty, shallow and temporary.
  
• Since they are shallow, wind stress can make the lake waters move over several sq km.
  
• When desert lakes dry up, they leave a salt crust or hardpan. This flat area of clay, silt and sand encrusted by sand is called a playa or sink. The flat terrains of playas and hardpans makes them excellent speedways and natural runways for aircraft
• Examples of desert lakes: Great Salt Lake (Utah, USA)
  
• The Atacama Desert in Chile is the driest place on Earth. Blocked from moisture on both sides by the Andes and the Chilean coastal range, the Atacama is virtually sterile and devoid of all life. The average rainfall in the region is 1 mm per year. Some weather stations in the desert have never received rain.
  

Mineral resources in deserts

The Saguaro Cactus tree in the Sonoran Desert (USA) can grow up to 15 m and live up to 200 years

• Deserts may contain a great amount of mineral resources over their entire surface
• The red colour of many sand deserts is due to the occurrence of laterite. Laterite, rich in iron and aluminium, is commonly used in making bricks
• Evaporation enriches mineral accumulation in desert lakes, including gypsum, sodium salts and borates
• The Great Basin Desert (USA) has been extensively used to mine borates, which are used in the manufacture of glass
• The Atacama Desert (Chile) is abundant in saline minerals. Sodium nitrate for fertilisers and explosives has been mined from the Atacama since the middle of the 19^th century
• Significant petroleum deposits are found in desert regions. However, these oil fields were originally formed when the areas were shallow marine environments. Subsequent climate change has rendered these regions arid
• Deserts are also increasingly seen as sources of solar energy. It is estimated that all the world’s electricity needs could be met by 10% of the solar energy tapped from the Sahara Desert

Oasis

• An oasis is an isolated area of vegetation in a desert, usually surrounding a spring or similar water source
• Oases provide natural habitats for animals, plants and even humans
• Oases are formed from underground rivers or aquifers, where water reaches the surface by natural pressure

List of important deserts

S. No.	Desert	Location	Notes
1	Antarctica	Antarctica	Largest desert on earth
2	Arctic	Arctic	Second largest desert
3	Sahara	Northern Africa (Egypt, Libya, Sudan, Morocoo, Algeria)	Largest hot desert Third largest desert
4	Arabian desert	Arabia (Saudi Arabia, UAE, Yemen)
5	Gobi desert	Mongolia, China
6	Kalahari desert	Southern Africa (Botswana, parts of Namibia, South Africa)	Supports plants and animals since much of it is not a true desert Receives about 75-200 mm of rainfall per year
7	Patagonian desert	Argentina	Cold weather desert
8	Great Victoria Desert	Australia

DESERTIFICATION

Overview

• Desertification is the extreme deterioration of land in arid and dry areas due to loss of vegetation and soil moisture
  
• Desertification results mainly from human activities but is influenced by climatic variations
• Desertification directly results in biodiversity loss and loss of productive capacity

Causes of desertification

• The primary reasons for desertification are
  
  • overgrazing
  • over-cultivation
  • increased fire frequency
  • water impoundment
  • deforestation
  • overdraft of ground water
  • increased soil salinity
  • climate change
• Droughts by themselves do not cause desertification. However, continued land abuse during droughts increases land degradation leading to desertification.
• Nomadic lifestyles with slash and burn agriculture can directly lead to desertification

Historical and current desertification

The Atacama Desert (Chile), the driest place on Earth, is almost completely sterile and devoid of all life. The only such place on Earth, it has often been compared to planet Mars.

• Desertification is a historic phenomenon: the world’s largest deserts were formed by natural processes over long intervals of time.
  
• Dated fossil pollen indicate that the Sahara has been changing between desert and fertile savanna. The Sahara is currently expanding southward at a rate of 48 km per year
  
• Drought and overgrazing in the 1930s transformed parts of the Great Plains in the US into the Dust Bowl
• Slash and burn agriculture in Madagascar has caused almost 10% of the country to become barren, sterile land

Countering desertification

• Counter-desertification techniques usually focus on two major aspects
  • Provisioning of water
    
  • Fixating and hyper-fertilising soils
    
• Fixating of soils is done by means of shelter belts, woodlots and windbreaks. Made from trees and bushes, these reduce soil erosion and evapotranspiration
• Soil fertilisation and enrichment is often achieved using leguminous plants (which extract nitrogen from air and fix into soil). Grains, barley, beans and dates are used for this purpose
• Stacking stones around the base of trees and artificial groove digging can also help plant survival by collecting morning dew and retaining soil moisture
• Desertification can also be temporarily forestalled by using sand fences (using bushes and trees), which decrease wind velocity and hence soil erosion and moisture loss
• The Green Wall project in Africa aims to plant trees in a 15 km strip from Senegal in the west to Djibouti in the east. The project aims to counter desert progression while also providing economic opportunities to the local populations
  

United Nations Convention to Combat Desertification

• The UN Convention to Combat Desertification (UNCCD) aims to combat desertification and mitigate the effects of drought
  
• The Convention was adopted in Paris in 1994 and came into effect in 1996. The UNCCD has 193 member nations including India
• The Convention seeks to achieve its goals through national-level action programmes that incorporate long term strategies supported by international cooperation
• It is the first and only legally binding framework to address the problem of desertification
• The nodal agency for implementing the UNCCD in India is the Ministry of Environment and Forests

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