3.1.2.1 deserts as natrual systems

• Desert systems in physical geography


  • Open systems, both energy and matter can leave
  • Change to input/output can cause negative feedback + restores balance
  • Lowest NPP
  • Open systems are self regulating
  • Deserts are considered active, mobile landscapes due to the inputs of wind, water and insolation
• Temp increases -ve feedback loop examples

  Evaporation increases, ground is drier = drier sediment = more easily eroded = dust clouds, block solar insolation = cooling
• Wind hits obstacle +ve feedback loop examples

  Slows down, drops load, drops sediment = start of dune, =young embryo dune = further obstacle , slows windspeed further, encourages further dune growth
• Inputs

  • Water = infrequent rain storms
  • Sediment via wind/water
  • Energy via sun, wind or water
• Insolation

  Solar radiation that hits earth, very strong in deserts, due to higher angle of incidence at mid + low latitudes = more conc
• Insolation in deserts

  • No clouds to block, cause high day time temps + very cold nights = high diurnal range of temps
  • Fluctuation in temps = cause steep pressure gradients driving strong winds
  • Deserts that are at mid and low latitudes, the suns rays hit the earth at higher angle of incidence = more concentrated, higher the sky in the sun = more radiation transferred to the ground
  • Higher insolation = increased evapo/evapotransp = baked land soils = capillary action = sediments =more mobile
  • More energy available to heat up the air in contact with ground due to lack of moisture
• Wind

  Air moving from high pressure > areas of low pressure = strong winds

  subtropical high pressure belt = wind blows out towards edge of desert
• Wind in deserts

  • Mid latitude along subtropical high pressure belt = wind blowing out towards edge of desert
  • Pressure gradient = high = strong winds
  • Wind consistently blowing in the same direction = prevailing wind = more sediment erosion + transport than winds that change frequently
  • Lack of veg in deserts = wind blows long distances without obstruction = increases erosion + increasing power
• Runoff

  • 'Spotty' rainfall = infrequent + unpredictable + often localised, less than 250mm precip annually
  • Hyper arid areas = have almost no precip i.e sahara/atacama
  • Intense storms = high input
  • Few plants + baked ground = lack of interception, ground storage, rapid overland flow/surf run off + reduction in infiltration
• Sources/inputs of sediments

  • Weathering of parent material
  • dominate by loose sediment
  • Fluvial in origin > rivers may bring sediment i.e ephemeral/dry up =sediments may be left behind in dry river beds
  • Episodic rainfall events, flash floods/desert rivers = flood + deposited material on surface
  • Aeolian in origin > sediments originating beyond desert margins transported + deposited via loess
  • Climate was completely different before, i.e green sahara > rivers + lakes, sediment deposited on lake beds/underwater regions = marine deposits
• Sediment budget

  Amount of sediment being brought into an area - amount of sediment being removed from the area
• Significant sources of sediment = large dust clouds blown thousands of miles. Large clouds of loess from the Sahara desert have been transferred and deposited thousands of miles around the world
• Positive sediment budget

  Depositional landforms i.e sand dunes
• Negative sediment budget

  Erosional landforms = yardangs/inselburgs
• Fluxes

  • Weathering
  • Erosion
  • Transportation
  • Deposition
• Stores

  • Landforms = stores of sediment
  • Water = aquifers, underground, rivers = intermittently
• Outputs

  • Water evaporates rapidly
  • Sediment removed by wind/rain
  • Clear skies allow large amounts of solar energy to be reradiated back into space
• Global distribution of mid and low latitude deserts and their margins, semi arid and arid

  • deserts cover 20% of earth's surface
  • Hot desert margins and env generally run parallel to equator, mid + low latitudes of tropic of cancer + capricorn
  • 15 - 30 degrees N+S of equator
  • Semi arid areas = desert margins i.e sahel + sahara
  • Northern hemisphere, generally located towards the west of continent i.e Mojave = north america
  • Sahara = north africa
• Drylands 41% of earth's land surface, found on every continent
• Home to 20% of the population

  18.5% pop growth rate in arid areas = highest pop growth rate that any other ecological zone
• Aridity index

  • Desert = arid env that has very low levels of rainfall, hot deserts receive less than 25 - 250mm annually + have veg along river courses
  • Semi arid receive 250mm- 500mm annually, sparse veg + grassland a few trees
  • Hyper arid - less than 25mm annually, plant growth only with rainfall
  • Water balance = p - pet (precip - potential evapotransp)
  • In deserts p is lower than pet, the size of the difference is the aridity index
  • Low value means aridity = high
  • Below 0.2 = desert
  • Desert margins = semi arid = 0.2-0.5
  • Sahara has the highest aridity
• Arid regions have a P/PET ratio of less than 0.20, meaning that precipitation supplies less than 20% of the amount of water that is needed to support optimum plant growth
• Causes of aridity in hot desert environments

  1. Global atmospheric circulation - 30 degrees N + S of equator, hadley cells and global atmospheric circulation i.e sahara

  2. Relief, rainshadow effect, i.e atacama, andes

  3. Continentality, distance from the sea, i.e turkestan desert central asia

  4. Near cold ocean currents i.e namib africa
• The Atacama lies on the leeward side of the Andes with regard to prevailing winds. At 20 degrees south the southeast trade winds carry moist air from the Atlantic, but as the winds are forced to rise across the Andes, RCCC, causing precip on the windward side. This means that the Atacama lies in a rainshadow.
• Air generally rises at the equator because of the maximum heating of the earth's surface. Once in the upper atmosphere, high-level winds carry the air toward the poles. The air gradually cools and beings to sink to earth between 20 and 25 degrees south (exactly where the Atacama lies), creating high pressure. Descending air begins to warm, and any moisture evaporates into water vapour, so there is no rain.
• The Atacama lies close to cold ocean current flows. The Pacific Ocean is colder at this latitude, therefore, on-shore winds are chilled as they cross the currents and don't have enough warmth to pick up moisture from the ocean surface, keeping the winds dry.
• Characteristics of hot desert environments

  • Climate, characterised by extremes - warm throughout year with hot summer temps, mean annual temp is 20-30 degrees, temps vary largely annually/seasonally, between hottest + coldest months, 0-50 degrees (winter - summer),
  • diurnally, day time + night time, 50 degrees day time, 0 degrees night time = clear skies = significant amounts of insolation + dry air cant trap heat at night or block sun during day, hot grounds, warm air + prevent formation of clouds,
• Types of landscapes

  • Hamada - most of the sand has been removed, leaving behind a landscape of gravel, boulders and bare rock plateaus
  • Reg - are hard impermeable surfaces composed of rock fragments set in sand or clay
  • Erg - sandy deserts/sand seas, common only in about 30% of deserts. Their distribution appears to be climate-linked (i.e. less than 150 mm of rain)
• Semi arid regions

  • Mean annual temp is lower 10-20 degrees, temp variation up to 15 = less extreme, as distance from the tropics increases so does the annual temp range, desert margins have huge variations, seasonal rainfall
• Generally, huge variety in temp patterns around globe due to geographical situation i.e closer to the sea, atacama = cooler less extreme temp then those in continental interiors, rate of evapotransp often exceed precip, desert margins, have slightly lower temps, less moisture is lost due to evapotransp, areas with low night time temps the condensation of dew can equal/exceed amount of rainfall received in some areas.
• Soil in deserts

  • 17% of earth's surface has desert soils, infertile, thin soil profile, alkaline, saline, rates of soil development are extremely slow due to: lack of moisture, extremely high temps + high rates of evap, sparse veg and limited organic material, thick accumulation of basic mineral salts, calcium/sodium compounds due to capillary action/movement, any moisture in soil moves upwards via tiny spaces between soil particles, capillaries, potentially fertile, shown through irrigation schemes
• Types of desert soils

  • Aridisols - infertile alkaline soils of desert areas characterised by accumulations of mineral salts at or near surface
  • Sierozems - semi arid areas
  • Raw mineral soils arid regions - coarse texture due to physical weathering, chemical weathering due to capillary action + salts, lack of moisture, there is little leaching, alkaline, unproductive, minerals + nutrients are available but there is a lack of water + extreme temps, can form hard duricrusts at surface
  • Sierozems, grey dessert soils, semi arid regions, desert margins - darker colour = presence of organic matter, below desert shrub veg, used for cultivation, continued irrigation may lead to rich accumulations of calcium carbonate/gypsum giving it a lighter colour, less sandy and stony dye to there being more weathering
• Desert vegetation

  • Low biomass = lack of water, succulents i.e cacti, xerophytes, Spines, reduce sa for evapo transp, herbivory, shallow tap roots and long tap roots tp reach water table, sunken stomata, thick waxy cuticle, enlarged stem stores water, Adapted to cope with drought and salt with limited nutrient cycling due to lack of water, The amount of vegetation increases with distance from arid regions due to the increase in the availability of water
• Types of desert vegetation

  • Ephemerals - Plants with a short life cycle which flower after rain, seed and die back in the dry season
  • Succulents - Plants which store water in fleshy stems and leaves
  • Xerophytes - Drought-resistant, small leaves reduce the amount of transpiration
  • Pyrophytes - Plants that have adapted to resist fire - margins of semi-arid regions
  • Halophytes - Date palms can grow in saline groundwater
• 20% of world's population lives in deserts
• Deserts are populated by humans seasonally/permanently i.e namibia/cairo
• global atmospheric circ - cause of aridity

  • 30 degrees N + S of equator

  • solar radiation is conc at equator therefore AIR RISES at EQUATOR, air cools as it rises + moisture condenses (RCCC) = rain = LOW PRESSURE ZONE = dry air

  • DRY AIR DESCENDS AT 30 DEGREES N + S OF EQUATOR = HIGH PRESSURE ZONE

  • wind blows OUTWARDS from HIGH PRESSURE ZONES = NO MOISTURE BROUGHT IN BY WIND

  • high atmospheric pressure = sinking, warm, dry air

  • LOW PRECIP = DESERT MARGINS I.E SAHARA
• relief rainshadow effect

  • tall moutains > force air upwards = RCCC precipitation on the windward side

  • lee ward side, dry air descends, slightly warms up and reduces relative humidity = clear skies + no rainfall

  • i.e southeast trade winds meeting the andes = rainshadow effect = atacama desert south america
• continentality - distance from the sea 


  • continental interiors central parts of continents are usually more arid then costal areas

  • moist wind from sea moves inland = precip

  • further inland = little moisture as all water already dropped as rain, very little rain falls = large distance offshore winds cannot carry moisture = most is lost before it reaches inland areas

  • i.e turkestan desert central asia
• cold ocean currents

  • wind is cooled as it travels over cold water = ability to hold moisture is reduced

  • cool dense air that displaces warmer air over the land

  • cool air hits the land it heats up and evaporates any moisture in it, leaving a coastal fog, which brings some moisture

  • Air that travels further inland is dry and warm

  • Proximity to cool ocean currents depresses temperatures and produces sea fog

  • wind reaches land very little moisture is left = little rainfall + atacama + sonoroan deserts

  • i.e namib desert in africa due to benguela current up the west coast