Topic 2: Deserts
Hot deserts have inputs, outputs, flows and stores of sediment, water and energy:
Inputs Flows/transfers
- Water enters the system through infrequent - Sediment is moved within a system by:
rainstorms. Weathering
- Sediment can be carried into a desert system Erosion
by wind or water. Transportation
- Energy can come from the Sun, wind or rain. Deposition
- Combined with mobile sediment, deserts are
very active.
Stores Outputs
- Landforms are stores of sediment. - Water evaporates rapidly of may leave
- Water may be stored in the ground or in the system as runoff
rivers and other bodies of water. - Sediment may be carried out of the
system by wind or water.
Deserts are open system so energy can flow in and out of them. They are generally in dynamic
equilibrium – a change in the input or output can cause a negative feedback that restores the
balance. E.g. if temperature decreases, evaporation increases causing ground to dry out. Drier
sediment increases erodibility so dust clouds more likely to form. Dust clouds block some solar
radiation so temperature decreases. However, deserts also experience positive feedback systems,
e.g. when wind hits an obstacle, it slows down and drops load – which forms dune. Young embryo
dune creates larger obstacle, which slows wind speed further and build up sand dune. This
encourages dune growth.
Energy sources in hot deserts
Insolation Insolation is the solar radiation that reaches the Earth – it’s strong in deserts due to:
1) There aren’t usually any clouds to block it.
2) In deserts that are mid to low latitudes, the sun’s rays hit the Earth at high
angles. Higher the sun the stronger the radiation.
High levels of insolation cause high day temps, while cloudless skies mean that nights
are cold. Rapid temperature changes can cause steep pressure gradients that drive
strong winds.
High inputs of energy from insolation cause water to evaporate quickly, leaving
ground very dry which makes sediment more mobile.
Runoff Precipitation is infrequent and unpredictable in deserts. When it rains, its usually in
the form of an intense storm – these cause high inputs of energy.
There are very few plants to intercept rainfall, so there’s a lot of surface runoff.
The ground surface has been baked hard by Sun and there’s often exposed rock
because soil is so easily eroded. These factors decrease infiltration and increase
runoff.
Winds Winds are created by air moving from areas of high to low pressure. Where the
pressure gradient is high, winds can be strong.
In some areas, wind consistently blows in one direction (prevailing winds) which
causes more transportation of sediment and erosion.
Lack of vegetation in deserts mean wind can blow long distances without obstruction,
increasing its power.
, Sources of sediment in deserts
1) Ancient processes: before they became deserts, many areas used to have a different climate
– wet enough for there to be rivers and lakes. Sediment was transported into system in
rivers and deposited on beds of lakes. When they dried up the sediment became available
for building dunes and other depositional landforms. Other areas were underwater, so they
still have marine deposits from millions of years ago.
2) Desert system: some comes from underlying parent material that’s been weathered. Rivers
also bring sediment into deserts – many only flows occasionally, so when they dry up, the
sediment they were carrying is left behind on riverbed or, if they flood, on the surrounding
deserts surface. Wind also transports sediment into deserts and deposits it there.
3) Deserts are sources. Dust clouds form when strong winds blow fine particles out the desert.
4) The sediment budget is the difference between the amount sediment being brought into an
area and the amount leaving. If sediment inputs are higher than outputs, the sediment
budget is positive. If sediment inputs are lower than outputs, the sediment budget is
negative.
5) Sediment budgets determine what type of landscape form. Where the sediment budget is
positive, landscapes are dominated by depositional landforms. Where the sediment budget
is negative, landscapes are dominated by erosional landforms.
Distribution of hot deserts
Aridity index
The aridity index is used to classify deserts. Hyper arid <0.03
P = precipitation Arid 0.03-0.2
Semi-arid 0.2-0.5
PET = potential evapotranspiration
Water balance is the relationship between the mean annual precipitation and potential
evapotranspiration. If P is higher than PET, there is a water surplus. If P is lower than PET, there is a
water deficit.
In deserts, there is a water deficit. The size of the deficit is determined by the aridity index. A low
value means aridity is high – a value below 0.2 means an environment is classified as a desert. Desert
margins are semi-arid environments – they have an aridity index of 0.2-0.5.
Where?
1) Deserts are found around 30o
North and South of the
Equator. Air moves in circular
patterns between equator
and 30o north and south of it
(the Hadley cell).
Hot deserts have inputs, outputs, flows and stores of sediment, water and energy:
Inputs Flows/transfers
- Water enters the system through infrequent - Sediment is moved within a system by:
rainstorms. Weathering
- Sediment can be carried into a desert system Erosion
by wind or water. Transportation
- Energy can come from the Sun, wind or rain. Deposition
- Combined with mobile sediment, deserts are
very active.
Stores Outputs
- Landforms are stores of sediment. - Water evaporates rapidly of may leave
- Water may be stored in the ground or in the system as runoff
rivers and other bodies of water. - Sediment may be carried out of the
system by wind or water.
Deserts are open system so energy can flow in and out of them. They are generally in dynamic
equilibrium – a change in the input or output can cause a negative feedback that restores the
balance. E.g. if temperature decreases, evaporation increases causing ground to dry out. Drier
sediment increases erodibility so dust clouds more likely to form. Dust clouds block some solar
radiation so temperature decreases. However, deserts also experience positive feedback systems,
e.g. when wind hits an obstacle, it slows down and drops load – which forms dune. Young embryo
dune creates larger obstacle, which slows wind speed further and build up sand dune. This
encourages dune growth.
Energy sources in hot deserts
Insolation Insolation is the solar radiation that reaches the Earth – it’s strong in deserts due to:
1) There aren’t usually any clouds to block it.
2) In deserts that are mid to low latitudes, the sun’s rays hit the Earth at high
angles. Higher the sun the stronger the radiation.
High levels of insolation cause high day temps, while cloudless skies mean that nights
are cold. Rapid temperature changes can cause steep pressure gradients that drive
strong winds.
High inputs of energy from insolation cause water to evaporate quickly, leaving
ground very dry which makes sediment more mobile.
Runoff Precipitation is infrequent and unpredictable in deserts. When it rains, its usually in
the form of an intense storm – these cause high inputs of energy.
There are very few plants to intercept rainfall, so there’s a lot of surface runoff.
The ground surface has been baked hard by Sun and there’s often exposed rock
because soil is so easily eroded. These factors decrease infiltration and increase
runoff.
Winds Winds are created by air moving from areas of high to low pressure. Where the
pressure gradient is high, winds can be strong.
In some areas, wind consistently blows in one direction (prevailing winds) which
causes more transportation of sediment and erosion.
Lack of vegetation in deserts mean wind can blow long distances without obstruction,
increasing its power.
, Sources of sediment in deserts
1) Ancient processes: before they became deserts, many areas used to have a different climate
– wet enough for there to be rivers and lakes. Sediment was transported into system in
rivers and deposited on beds of lakes. When they dried up the sediment became available
for building dunes and other depositional landforms. Other areas were underwater, so they
still have marine deposits from millions of years ago.
2) Desert system: some comes from underlying parent material that’s been weathered. Rivers
also bring sediment into deserts – many only flows occasionally, so when they dry up, the
sediment they were carrying is left behind on riverbed or, if they flood, on the surrounding
deserts surface. Wind also transports sediment into deserts and deposits it there.
3) Deserts are sources. Dust clouds form when strong winds blow fine particles out the desert.
4) The sediment budget is the difference between the amount sediment being brought into an
area and the amount leaving. If sediment inputs are higher than outputs, the sediment
budget is positive. If sediment inputs are lower than outputs, the sediment budget is
negative.
5) Sediment budgets determine what type of landscape form. Where the sediment budget is
positive, landscapes are dominated by depositional landforms. Where the sediment budget
is negative, landscapes are dominated by erosional landforms.
Distribution of hot deserts
Aridity index
The aridity index is used to classify deserts. Hyper arid <0.03
P = precipitation Arid 0.03-0.2
Semi-arid 0.2-0.5
PET = potential evapotranspiration
Water balance is the relationship between the mean annual precipitation and potential
evapotranspiration. If P is higher than PET, there is a water surplus. If P is lower than PET, there is a
water deficit.
In deserts, there is a water deficit. The size of the deficit is determined by the aridity index. A low
value means aridity is high – a value below 0.2 means an environment is classified as a desert. Desert
margins are semi-arid environments – they have an aridity index of 0.2-0.5.
Where?
1) Deserts are found around 30o
North and South of the
Equator. Air moves in circular
patterns between equator
and 30o north and south of it
(the Hadley cell).
