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Option 7: Soils and Biome
Biome: Desert Biome
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Biomes of the World
Biomes
• A biome is a major world region that is
characterised by similarities in climate,
soil, vegetation and animal life
• All these aspects of a biome are interrelated
What You Need to Know!!
• The factors that influence the
development of biomes
• The characteristics of the desert biome
• How humans alter biomes
Influencing Factors
• Climate: perhaps most important
• Determines vegetation and animal life
that can live in a region
• Main influences are temperature and
precipitation
Influencing Factors
• Soils: influence type of vegetation
• Soils are in turn influenced by climate as well
as parent rock
• Vegetation: depends on soil and climate,
influences plant and animal species in region
• Animal Life: must adapt to conditions of
climate and vegetation
• Human impacts have modified biomes on a
huge scale
Desert Biome
Desert Biome
• Desert: ‘an arid region that is
characterised by little or no rainfall, in
which vegetation is scarce or absent,
unless it is specially adapted’
Desert Biome
• Availability of water: determines
plant and animal life in the desert
• Annual precip depends on geographical
location 1cm-35cm
• Precip in short intervals, unpredictable
year by year
• Strong tendency for evaporation
Desert Biome
• Temperature: deserts can be hot like the
Sahara where summer temps hit 50 degrees
• Or they can be cold like the Gobi Desert
where winter temps hit –40 degrees
• Location: Coastal eg. Sonora Desert
• Interior of continents eg. Great Basin Desert
and Australian Desert
Desert Biome
• Make-up: contrary to belief, deserts are not
entirely covered in sand
• Mainly regions of boulders, gravel or bare
rock
• Only about 10% of deserts are sandy
• Productivity: least productive of the 9
biomes
• Potentially fertile soils in some, irrigation
Desert Biome
• Adaptation: all deserts have some plant and
animal life that have adapted
• Population: some deserts are populated
either seasonally by nomadic tribes or
permanently
• 3 categories: extremely arid, arid & semiarid
Desert Biome
• Extremely arid: at least 12
consecutive months with no precip
• Arid: annual precip less than 250mm
• Semi-arid: annual precip between
250-500mm
Distribution of Deserts
Distribution of Deserts
• Majority on western margins or in the
interiors of continental masses
• Low-latitude deserts: 15-30degrees N and
S of the equator. Sahara, Arabian and
Kalahari deserts, California’s Sonora desert
• Mid-latitude deserts: 30-40degrees N and
S of the equator, continental interiors. The
Great Basin and Mojave deserts of USA
Distribution of Deserts
• The dry regions of the world cover
nearly 30% of the earth’s surface
• No other biome covers so large an area
Origin of Deserts
• Most located in high pressure belts 1530degrees N and S equator
• Air masses are heated by equator
• Angle of the sun is almost perpendicular
• Air rises, convectional rain occurs
• Equatorial air masses travel away from
equator, begin to descend at about
30degrees N and S
Origin of Deserts
• Compression warms the descending air to
form belts of high pressure
• On reaching the earth’s surface, the air is
returned towards the equator as part of the
trade winds system
• Moisture is sucked from the surface because
the air masses are increasing in temperature
and able to hold more water
Origin of Deserts
• These conditions and an absence of surface
water rule out cloud formation and
precipitation
• Clear skies, maximum sunshine and drought
result
• Rain-shadow effect often involved also
• Coastal mountain ranges create a barrier to
the movement of moist sea air, forcing it to
rise
Origin of Deserts
• As it rises it expands and cools
• Water it contains falls as rain on the
windward side of the mountains
• When air reaches inland or leeward side
of the mountains, it has lost all
moisture and can not provide rain
• Rain-shadow effect, land beneath rainshadow becomes rain-shadow desert
Rain Shadow Effect
Origin of Deserts
• Coastal deserts, western margins of
continents
• Cold ocean currents play role
• Moisture laden winds blow in from
ocean, must pass over cold currents
• Currents absorb heat from air, reduce
ability to hold water
• Air masses lose water over ocean
Origin of Deserts
• Precipitation and fog
• Contact with warmer land made, air is
heated again and is able to retain what
moisture it has left
• Sucks up more moisture from land
• Coastal region becomes desert
• Atacama desert, only true rainless
desert (Humboldt Current)
Origin of Deserts
• Continental interiors
• As air masses move across a continent , lose
moisture by precip, even in absence of
mountains
• When these masses reach interior, have
become quite dry, land beneath receives little
rain
• Continentality (central Asia, Gobi desert)
North American Deserts
•
•
•
•
Chihuahua Desert
Sonora Desert
Mojave Desert
Great Basin
• Sandwiched between 2 mountain ranges:
Rockies to east and Sierra Nevada to west
• Cover an area 12 times size of Ireland
North American Deserts
North American Deserts
• Chihuahua, Sonora and Mojave are hot
deserts
• Great Basin considered a cold desert,
due to its more northerly latitude as
well as its higher altitude
North American Deserts
• Climate: rainfall often low and unpredictable,
comes in short bursts between long rainless
periods
• When rain falls there is a rapid run-off
• Evaporation rates regularly exceed rainfall
rates
• Sometimes rain starts falling and evaporates
before hitting the ground
• Low level of infiltration and high rate of
evaporation minimise its effectiveness for
vegetation growth
North American Deserts
• Climate: rainfall mainly seasonal
• 3 hot desert regions receive either
winter or summer precip
• Their annual rainfall varies between
150mm and 300mm
• Cold, Great Basin, receives 300mm of
precip annually, occurs throughout the
year, winter precip comes as snow
North American Deserts
• Temperatures: characterised by their
extremes
• Annual range between 20 and
30degrees
• Diurnal (day/night) range is often
greater than 30degrees, leading to the
description that ‘night is the winter of
the desert’
Geofacts
• The greater part of the Sonora Desert is
in Arizona, Arizona means ‘arid zone’
• The all-time maximum temperature in
the US was 57degrees, recorded in
Death Valley in the Mojave Desert
North American Deserts
• Temperatures: daytime temps high often
reaching 45degrees
• In low latitudes the sun is high in the sky,
shining almost vertically on the ground
• Rays are concentrated over small areas and
give great heat
• Lack of cloud cover and absence of
vegetation means very little heat is lost and
most is absorbed by the bare rock or stony
surface
North American Deserts
• In contrast, night-time temps are much
lower, often falling below freezing in
winter
• Heat built up during the day lost after
nightfall as there is no cloud cover or
vegetation to help retain heat
• Also, dry air cools very quickly
Desert Soils
Desert Soils
• Aridisols: Latin word for ‘dry’ are the
dominant soils of desert and semidesert regions
• Range from sandy and fine-textured to
gravelly and coarse-textured
• Soils were eroded and washed down
from mountain areas during occasional
torrential downpours over 1000s years
Desert Soils
• Coarse textured soils are found on the
lower slopes of mountains, well-drained
due to coarse texture
• Fine soils have been washed into the
basin areas, giving them deep soil cover
• If winds are strong, these fine particles
are blown elsewhere leaving coarser
particles of soil behind
Desert Soils
• Aridisols tend to be poorly developed
• Low level of precip means that there is little
chemical weathering
• Although they have a high mineral matter the
surface has little organic matter
• This limits the soil building properties of
micro-organisms that might convert organic
matter into humus
• Aridisols have poorly developed horizons and
may be without a noticeable A horizon
• Light grey colour
Desert Soils
• Abundant nutrients, only need water to
be productive
• Rapid growth of plants after a
downpour
• But, with so little organic matter, they
are unable to retain all the moisture
that falls, and there is intense
evaporation
Desert Soil
• In arid conditions, moisture moves up
through the soil by capillary action
• This brings dissolved minerals to the
surface
• Salinisation
• Calcification