Hot Deserts: Deserts as natural systems & Systems and Processes Flashcards
What type of systems are hot deserts?
Open system (have inputs and outputs for energy and matter)
What are some inputs into hot deserts?
- energy from the Sun
- precipitation
- wind
What are the flows in hot deserts?
erosion and weathering -> water and wind transportation -> water and wind deposition
What are the stores in hot deserts?
- erosional landforms and landscapes (e.g. Wadis, ventifacts, yardangs)
- Depositional landforms and landscapes (e.g. sand dunes)
What are the outputs of hot deserts?
- sediment carried out by wind and water
- clear skies allow large amounts of energy to re-radiate back into space
When is a hot desert system in a state of equilibrium?
When the geomorphological processes continually shape the desert landforms
What is the difference between a landscape and a landform?
Landscape is a characteristic
Landform is an element
Q: Explain the role of positive feedback in the development of hot desert landscapes? [4 marks]
- positive feedbacks occur when there is a change to the desert system that triggers further changes to the system
- for example the formation of sand dunes (landform formation in a landscape) wind carrying sediment hits an obstacle -> deposition of material -> sand dune increases in size
- positive feedback can change the system’s equilibrium to make a new dynamic equilibrium
- eventually the sand dune will be so large that wind flow can no longer reach top and deposit sediment, dune stops growing and a new dynamic equilibrium has been reached
What is another example of a positive feedback loop?
Removal of vegetation from deforestation and overgrazing -> reduces moistures emitted into atmosphere -> reduced humidity -> reduces vegetation growth
Q: Explain the role of negative feedback in the development of hot desert landscapes? [4 marks]
- Negative feedback loops restore the balance of the system when there is a change to it
- this can help maintain the landscape in it existing form
- for example the intense weathering of slopes -> build-up of an apron of scree -> this apron extend up the mountainside -> lower slopes are protected from weathering
- keeping the elements of the system the same, less development
What is the distribution of hot deserts?
- linear pattern of hot deserts across the Tropics of Cancer and Capricorn, 20º-30º North and South of the Equator
- Majority of hot deserts found in the Northern Hemisphere
- largest deserts are concentrated in the centre of deserts (e.g. Saharan Desert in Africa)
- Anomalies of this pattern are the Atacama and Namib Deserts as they are in the Southern Hemisphere on the west coasts of large continents
What is an arid climate?
The climate of an area that receives less than 250mm of precipitation per year
What is a semi-arid climate?
The climate of an area that receives between 250-500mm of precipitation every year
What is the aridity index?
- the ratio between mean annual precipitation (P) and mean annual evapotranspiration (PET)
- simplified way of classifying how arid a place is
- varies as ratios are different
hyper arid is <0.03
arid is 0.03-0.2
semi-arid is 0.2-0.5
What are some common characteristics of hot deserts?
- diurnal temperature range due to cloudless skies (highest ever temp recorded was 57.7ºC at Al Aziziyah in Libya in 1922)
- Low humidity due to little precipitation
- Cloudless skies due to little evaporation from little precipitation
- Insolation levels are low due to no clouds
- less than 250mm a year from lack of water
Why are there geographical variations in climate?
Geographical location of the desert
- e.g. if the desert is closer to the coast there are less extremes of temperature (Atacama)
- This can also affect precipitation levels, American deserts = 280mm and Atacama = 15mm
Q: Evaluate the causes of aridity in hot deserts [20 marks]
- The most important cause is Global Atmospheric Circulation…
- The least important cause is relief rainfall…
The most important cause of aridity in hot deserts is global atmospheric circulation…
- process that works at a global scale (largest of all the causes of aridity)
- as Hadley and Ferrel cells descending limbs meet with cool air, causing no cloud formation and this aridity (all the warm air has condensed and released ppt)
- This is the main cause of aridity in Sahara, high pressure from falling limb of Hadley cell (10º-30ºN) is the main cause of aridity there
- However this doesn’t cause hyper-arid deserts, Sahara has an aridity index of 0.18 making it an arid desert
- global atmospheric circulation works with other causes to create aridity in deserts, as the Sahara is in the centre of a continent as is mostly arid due to continentality
What is continentality?
- coastal areas receive higher amounts of precipitation as the moist air is dropped as precipitation
- distance from the sea reduces cloud formation as air reaching the centre of continents has low moisture content
- this can also create greater temperature extremes, no clouds to block insolation and land has a low specific heat capacity to keep warm at night
- work at a large scale but don’t cause hyper-arid deserts
What are cold ocean currents?
- causes hyper-arid deserts but works on a more local scale
- wind is cooled as it travels above cold water
- reduces ability to hold moisture, released as precipitation before reaching land
- insolation burns off any fog as air drifts onland, leaving cool dry air
The least important cause for aridity in hot deserts is relief rainfall…
- mostly work on local levels and is never the main cause of aridity in hot deserts
- it amplifies some of the larger causes already present, like continentality or global atmospheric circulation
- moist air rising over a mountain range cools and condenses, forming clouds and resulting in precipitation on the windward side of the mountain, therefore air on opposite side has no moisture and can’t form clouds
- however the scale of this factor can vary depending on the size of the mountain range
- and does contribute greatly to the hyper-arid Atacama Desert from the rain shadow on the Andes, however, cold currents are still a prevailing cause there
What is the typical soil profile of a an aridisol?
(top going down)
A horizon
B horizon
Bk horizon (thick zone of calcium carbonate accumulation)
C horizon (salts are carried up in the solution)
What are aridisols?
infertile, alkaline and saline soils of desert areas
What is capillary movement?
moisture in the soil moves upwards through tiny spaces between soil particles most effective when evaporation exceeds precipitation
What is leaching?
Downward movement of minerals due to movement soil moisture, when precipitation exceeds evaporation
What are the main 2 types of aridisols?
- Sierozem
- Raw Mineral Soils
(variation due to aridity, precipitation, vegetation and evaporation)
Describe the sierozem soil profile
- calcium rich B horizon can develop underneath thin A horizon
- weathered and solid bedrock
- infertile
- alkaline pH
- depth of less than 100cm
- extremely slow rate of development
- high salinity
- greyish colour!
Describe the raw mineral soil profile
- unproductive (not really infertile)
- accumulation of Ca and Na salts near the surface, limited variation between horizons
- slightly alkaline pH
- depth of less than 100cm
- extremely slow rate of development
- high salinity as salt crusts form at surface
- reddish yellow to grey-brown colour
What are the similarities between sierozems and raw mineral soils?
- No O horizon (organic matter) -> lack of dead organic material or water hinders decomposition
- coarse texture -> physical weathering
- capillary action dominates
What is an erg desert?
a sand sea
the name for desert areas without vegetation
What is Net Primary Productivity?
The amount of carbon uptake after subtracting plant respiration from gross primary productivity
What is Gross Primary Productivity?
The total rate at which ecosystems capture and store carbon as plant biomass for a given length of time
What are xerophytes?
- succulent plants
- Plants with compounds or cells in fleshy leaves, stems or roots where they can store water
- can survive in very dry habitats
What are phreatophytes?
Plants with shallow and very extensive root systems to procure water from damp soils following brief rainstorms.
What are ephemerals?
Plants that cope with drought by becoming dormant or losing leaves during extreme dry spells
What are halophytes?
Plants that can tolerate high levels of salinity
How are plants adapted to increase water storage?
xerophytes contain cells in fleshy leaves/stems/roots where they can store water
How are plants adapted to limit water loss?
- thick waxy cuticles
- closed stomata during the day
- leaves reduced to spikes or needles
How are plants adapted to increase access to soil water?
Having very shallow but very wide root systems (phreatophytes)
How are plants adapted to tolerate drought?
Ephemerals become dormant or lose leaves during dry spells, appearing dead
How are plants adapted to tolerate high salinity?
Halophytes
How are plants adapted to cope with sporadic rainfall?
Some ephemerals have extremely short life cycles, coming into bloom shortly after rainfall
Q: Assess the impact of aridity and soil characteristics on plant adaptations in desert environments?
Arid climates in deserts lead to long period of little precipitation and therefore drought….
A characteristics of desert soils is the alkaline soils…
Arid climates in deserts lead to long period of little precipitation and therefore drought….
- only plants tolerant to drought or dry environments will grow
- for example the Colorado desert has an aridity index of 0.09, making it an arid desert
- the Saguaro cactus is a xerophyte with fleshy leaves and stems so it can store water in drought and it is a phreatophyte as it has a shallow root system to collect water after brief rainstorms
- these adaptations due to the very arid climate allow the plant to collect and store water for long periods and survive
- root system enables plant to absorb any soil moisture before evaporation and fleshy leaves can store water between these periods of rain
A characteristics of desert soils is the alkaline soils…
- aridisols only allow for plants with tolerance to high salinity to grow, raw mineral salts soils and sierozems both have high salinity, in particularseroizems insemi-arid regionshave a high concentration of calcium carbonate in the B horizon.
- Saltbush, a halophyte has salt bladders on its leaves that remove excess salt.
- This adaptation allows the saltbush to draw moisturefrom B horizon of seriozemsin semi-arid areas, despitethe concentration of calcium carbonate (live in high salinity soils)
- HOWEVER the soil profile is greatly affected by climate and rainfall, as rocks and gravels have been physically weathered to create these soils and capillary action is a large process making large salt crusts in raw mineral soils
What are the plant adaptations of the resurrection plant?
Ephemeral
- dormant periods during periods of no rain, rehydrates during rainfall and grows
- folds up tight into a ball in dormant periods creating small SA for water to evaporate
- small roots so can move with wind in search of standing water
How to plan an answer to “Assess the impact of aridity and soil characteristics on plant adaptations in desert environments?”
Characteristic
Example of this characteristic
A plant adaptation
How this helps the plant overcome the problem of this characteristic
Q PLAN: analyse the adaptations of this desert plant in relation to the climate and soils of its environment.
ideas…
xerophyte -> fleshy stems -> store water during long periods of no rainfall
phreatophyte -> shallow but wide root system -> drains any soil moisture -> sierozem soils have a thin, hard baked top layer which encourage surface run-off after heavy desert rains
plant may have thick cuticles and waxy epidermis -> reducing transpiration during the day from the plant
one deep tap root -> access any available groundwater as moisture is quickly evaporated away, or drawn to the surface by capillary action
Must address relation to soil AND climate
What are the adaptations of the saguaro cactus?
Xerophyte
- fleshy leaves, stem and roots -> stores water during long period between rain events
- large amount of parenchyma -> large amounts of water storage to prevent plant temperature rising to lethal levels
- spines -> prevents animals eating them
- thick epidermis waxy cuticle -> prevents water loss and reduces transpiration
one tap root and shallow root system -> collect both groundwater and water from infrequent rain
What are the adaptations of the saltbush?
Halophyte
- sharp spine leaves to drop them during drought
- stem and leaves covered in small white scales helps conserve water
- bloom in summer and spring which often coincides with seasonal rain
- salt bladders on leaves removes and deposits excess salt
Q: Outline the climate of hot desert systems (4 marks)
- Climate of hot desert system are aris due to infrequent and little rain they receive
- Less than 250mm every year and when it does rain it’s in very intense and short-lived storms
- Hot desert systems also experience extreme temperatures in the diurnal range, from 0ºC at night to 50ºC during the day
- Due to no clouds blocking insolation during the day and no clouds for insulation of heat at night
Q: Evaluate the relative importance of different sources of energy in hot desert environments
- Insolation is the most important - largest source, gives characteristics of land but little impact on the land itself
- Wind - impacts the elements in the system and a large process
- Precipitation is the least important
Why is insolation the most important energy source?
- a constant input of energy (all year round)
- effect of intensity is amplified by the high angle of incidence (as deserts are found in mid-low latitudes) and no clouds, creating it as a high energy contributor
- causes the climate characteristic of a diurnal temp range as cloudless skies don’t block any insolation coming into the system
- any water that does evaporate makes the ground dry and makes sediment more mobile
- However, not directly responsible for change in the system other processes of wind and water are. Insolation gives a significant energy source for weathering (thermal fracturing) but drives work of wind and water
Why is wind the second most important energy source?
- constant process (all year-round) as wind moves from high pressure in centre of deserts to margins with low pressure
- large impact on the land as erosion, deposition and transportation of sediment happen constantly (e.g. the formation of dunes)
- influences elements in the system from these processes, contributing to creating an active landscape
- However, rapid temperature changes caused by insolation can cause steep pressure gradients that drive strong winds
Why is precipitation the least important energy source?
- doesn’t contribution but energy as it is a small and rare process (couple of times a year), for example the Atacama desert receives only 15mm of precipitation yearly
- However, when it occurs it is very intense and brings strong erosional processes that help shape the land
- mostly from overland flow as the baked ground and lack of vegetation prevents infiltration
- has disproportionate influence on elements in system through erosion and transportation
What are the 3 sources of sediment in hot desert environments?
- weathering of underlying bedrock
- fluvial, brought into system by rivers
- aeolian, brought into system by the wind
What is loess?
loose material deposited by the wind
What is regolith?
loose material from weathering
What are the 5 geomorphological processes in deserts?
- Weathering
- Mass Movement
- Erosion
- Transport
- Deposition
What is weathering?
- The breakdown or decaying of material in situ that produces regolith
- occurs anywhere with the processes of weathering: mechanical, chemical and biological
What is mass movement?
- movement of material downhill under the influence of gravity
- rocks can be weakened by weathering, vulnerable to mass movement
- deserts have little moisture, vegetation and soil, dominance of rock falls and rock slides
What is erosion?
- the processes that wear away land surface by mechanical action
- by processes of water and wind
What is transport?
- Processes that moves material from the site where erosion took place to the site of deposition
- by wind and water
- traction, saltation, suspension and solution
What is deposition?
- water and wind depositing load of material when they experience a reduction in the capacity it can carry
- happens due to loss of energy or evaporation of water
What are the ancient sources of sediment in hot deserts?
- before the desert existed the areas had different climates, some even wet enough for rivers or lakes
- sediment was transported in by waterways
- deposited into lake beds, lakes dried up as sediment become available for building dunes etc
What are the sources of sediment in hot deserts?
- weathering of underlying parent material
- rivers may bring sediment (fluvial)
- left behind in dried up riverbeds (ephemeral)
- winds (aeolian)
Can deserts be a source of sediment themselves?
yes
e.g. when dust clouds form, strong winds blow fine sediment out of the desert for thousands of miles
What is the sediment budget?
The difference between the amount of sediment being brought into an area and the amount leaving is the sediment budget
What happens if sediment inputs are higher than outputs?
Sediment budget is positive, net sediment gain
- landscape is dominated by depositional landforms and depositional processes/transportational
What happens if the sediment outputs are higher than the inputs?
Sediment budget is negative, net sediment loss
- landscape is dominated by erosional landforms and erosional processes
- becomes a source of sediment
