GEOS2100 Arid Environments Notes

Introduction

  • Definition of arid landscapes:
    • Hyper-arid
    • Arid
    • Semi-arid
  • Climatic zonation of arid lands
  • Geographic distribution of arid lands
  • Common landforms of arid zones
    • Sand Dunes
    • Fluvial systems

Aridity

  • Aridity is scientifically defined with regard to some measure of water shortage.
  • One common index is P/ETP. Where P is annual precipitation and ETP is mean annual potential evapotranspiration.
  • Aridity increases as the value of the ratio declines:
    • Semi-arid (0.2 < P/ETP < 0.5)
    • Arid (0.03 < P/ETP < 0.2)
    • Hyper-arid (P/ETP < 0.03)

Causes of Aridity

There are five major climatic causes of aridity:

  • Continentality: Increasing distance from maritime or other moisture sources. Commonly extends to the western margins of subtropical landmasses as trade winds lose their moisture as they blow across the continent (Australia).
  • Anticyclonic subsidence associated with the anticyclones of the subtropical high-pressure belt. The subsidence produces adiabatic warming and low humidity. Rain-bearing weather systems may penetrate these regions associated with equatorward-moving fronts and troughs in the easterlies.
  • Orographic influences (rainshadow effects) accentuate aridity in the lee of major mountain ranges such as the Andes, Rockies, and the Southern Alps, which is often linked to the occurrence of foehn winds. Many occur over relatively small areas, i.e., the Island of Hawaii, alpine basins in New Zealand.
  • Coastal upwelling in proximity to the coastline where the cold ocean waters enhance local atmospheric stability and suppress rainfall, i.e., Namibia, Peru.
  • Albedo: Desert regions with high surface albedo may reinforce aridity by reflecting a high percentage of solar radiation, thereby leading to slightly lower temperatures. This process may reduce convection and possibly enhance subsidence, resulting in less rainfall.
  • Cold Polar Regions (cold airmasses, continentality). Antarctic, Greenland Ice Sheet

Geographic distribution of major deserts

  • Dryland Areas Using Koppen’s Climate Classification. In these regions, potential evaporation and transpiration exceed precipitation
  • The most “Developed” arid continent

Rainfall and Aridity Agents of Geomorphic Change in Arid Landscapes

  • The Aeolian Domain:
    • Derived from the Creek word aeolus, aeolian processes result from the action of the wind, i.e., the entrainment-transport-deposition of sediment.
  • The Fluvial Domain:
    • Here the work of water is dominant. Typically, in semi-arid regions streams (often ephemeral) erode valleys and deposit the material in the form of alluvial fans and valley trains.
  • NOTE: seldom do desert landforms result exclusively from either aeolian or fluvial processes but instead often result from a combination of both.

The Wet and The Dry

Channel Country of far western QLD, where mean annual precipitation is typically less than 250 mm.

  • Flooding of monsoon rains
  • Dry Channel Country playa
  • Summer flood waters in the Queensland Channel Country
  • Queensland's Channel Country Rivers

Classifying Arid Landscapes

  • A) Shield and Platform Deserts:
    • Located in areas of relative tectonic stability such as Australia.
    • Are often associated with vast stable plain lands.
    • Dominated by erosional surfaces cut across basement rocks and/or huge shallow basins such as the Lake Eyre Basin.
    • These regions whose presence may owe little to contemporary aridity are often surmounted by remnants of mountain ranges – a feature common to much of inland Australia.

Inselberg and Pedimont Landscape (Australia)

  • Inselberg
  • Dry channel
  • Alluvial apron
  • Joint-guided gullies
  • Pediment
  • Fault
  • Granite bedrock
  • Alluvial apron

Inselberg

  • A residual hill which usually surmounts an eroded plain. They often form prominent landforms as a result of their superior resistance to weathering.
  • Their origin is much debated but may arise from scarp retreat across bedrock, scarp retreat across deeply weathered rock, or differential weathering followed by stripping of regolith.

Pediment

  • A smooth planoconcave upward erosion surface, typically sloping down from the foot of a highland area and graded to either a local or more general base level.
  • A pediment belt may include a combination of playas and fans.
  • They have low slope angles < 10° and a sharp break in slope between the pediment surface and the hill mass behind.
  • Thought to result from the lateral planation of rivers or the backwearing of mountain fronts. Another hypothesis is that they were formed under more humid conditions and have subsequently been exhumed.
  • They are often overlain by a veneer of alluvium.

Alluvial fans

  • Cone-shaped deposits of fluvial sediment.
  • Typically radiate downslope from points where drainage leaves the mountains.
  • In desert environs fans are commonly dry and subject to ephemeral flow.
  • Their occurrence is broadly influenced by a lack of vegetation, long time periods between floods, where the ratio of depositional area to catchment area is small.

Dry Channel (Wadi or Wash)

  • Ephemeral river or stream common to desert regions.
  • Hydrographs of these systems are often very step reflecting the short but intense rains typical of desert regions.
  • This produces the flash flood.
  • The descending limb of the hydrograph may also be a step as transmission losses result in rapidly decreasing flow.
  • High rates of sediment transport are common due to the abundance of sediment – may result in debris flows

Canyon and Escarpment Country

  • Cuesta
  • Mesa
  • Butte
  • Foot slope
  • Stripped plain
  • Resistant caprock

Butte

  • A resistant flat top outlier that rises above the surrounding landscape, which has been eroded away to a new base level.
  • Commonly associated with old volcanic plugs

Mesa

  • A flat topped plateau rising above the surrounding landscape.
  • Commonly consists of horizontally bedded sediments, which are more resistant to weathering and erosional processes, i.e., basalt.

Canyons

  • Typically controlled by structural weaknesses in the earth’s crust (faults), joints, and bedding plains.
  • Down cutting tries to reach local baselevel.
  • Meanders in canyons may form in response to geologic controls or mass movement deposits as canyon walls collapse.

Playa

  • A closed depression in a dryland region that is periodically inundated by water.
  • Also known as chott, pan, and kavir.
  • Can be determined by:
    • Occupy regional or local topographic lows
    • Ephemeral
    • Usually have flat surfaces
    • Usually vegetation-free
    • Have a hydrologic budget where evaporation greatly exceeds inputs

Clay Pans

  • A degradational feature of arid landscapes.
  • Develop in response to vegetation clearance (overgrazing).
  • Seasonally recharged with fine sediment by overbank flow.
  • Occasionally topographically confined by dunes

Salt pan

  • Salt may be blown in from coastal regions often contributing to sabkhas.
  • Salt may also be washed in or derived from groundwater discharge.
  • Evaporation of water and concentration of salts on the playa floor will eventually produce a salt lake/pan.
  • Lake Eyre is one of the largest salt lakes with a surface area of approx 9300km2

Principal Dune Forms

  • Parabolic
  • Barchan
  • Linear
  • Star
  • Other forms

Terminology for dune morphology

  • The morphology of dunes is controlled by:
    • The supply of sand and its physical characteristics such as size, shape, and density.
    • Surface type.
    • Wind regime.
    • Moisture.
    • Vegetation.
    • Human activities.

Classification of Aeolian Dunes

Relationship between dune type, sand supply and wind direction

  • DUNES
    • FREE Transverse Barchan Dome Reversing Linear Sandridge Seif Star Network (Sheets) Zibar Streaks
    • ANCHORED Vegetation Nbkha Parabolic Coastal Blowout Topography Echo Climbing Falling Lee Lunette

Parabolic Dunes

  • Are characterized by a ‘U’ shape with trailing wings and an active unvegetated dune front that advances by avalanching.
  • Commonly found in coastal and semi-arid regions.
  • Sand is eroded from upwind of the dune head, over which it is transported before being deposited onto the slip face.
  • The trailing wings may be partly vegetated or even anchored.
  • Parabolic dune activity is often a good indicator of climate variability or impact of land management policy as dunes may become reactivated when vegetated cover is reduced.

Barchan Dunes

  • Occur in two main areas: 1) on the periphery of sand seas, and 2) in sand transport corridors that link sources to depositional areas.
  • They tend to be associated with unidirectional wind regimes and are nearly symmetrical, with a single slip face located between two horns.
  • Characterized by high rates of erosion on the stoss slope and deposition on the lee slope.
  • In regions of slightly greater sand supply barchan dunes may coalesce to form barchanoid ridges – at even higher rates of sand supply transverse ridges may form.

Linear Dunes

  • Linear dunes are ridges of sand aligned parallel to the dominant wind direction.
  • Are the most common of all dune forms and cover half to two-thirds of all sand seas.
  • Range in size from 0.2 km wide and 5 to 20 km long to 0.3 to 2.7 km wide and 190 km long.
  • Usually form multiple asymmetric parallel ridges with wide interdune areas (swale) which may be partly vegetated.
  • Associated with regions that experience wide-angle unidirectional or bi-directional wind regimes.
  • May have rounded or sharp ridge crests.

Star Dunes

  • Characterized by 3 or 4 four arms that radiate from a central peak.
  • May reach heights of 300 m and widths of 3000 m.
  • They are found in regions of significant sand supply and complex wind regimes.
  • Star dunes may form singly, in chains, or in clusters often along the poleward margins of sand seas where there are marked changes in the seasonality of the regional wind regime.

Other common dune forms

  • Climbing dunes:
    • Typically found on the margins of sand seas and dune fields where hills, cliffs, etc. cause migrating dunes to ‘climb’ their windward slopes.
  • Lee dunes:
    • Form in the lee of obstacles such as hummocks or vegetation, where reduced wind speed promotes the accumulation of sand.
  • Lunettes:
    • Low relief crescentic dune forms are typically associated with the margins of pans and alluvial outwash surfaces.
    • Composed mainly of clay and silt-size material which aggregates to form small pallets. These disaggregate and bond during precipitation forming lunettes.
  • Nebkha:
    • The most widespread type of anchored dune (to vegetation).
    • The vegetation traps the sand and shelters it from re-entrainment.
    • The vegetation must be able to grow at a rate exceeding sand supply to avoid becoming buried and/or to allow the continued growth of the nebkha.
    • Form in deserts, wadi, along coasts, and even in cold humid environments.

Desert Pavements

  • Defined as ‘armoured surfaces comprising intricate mosaics of coarse angular or rounded particles, usually only one or two stones thick set in deposits of sand, silt, or clay’ (Cooke 1970, p560).
  • Also known as gibber plains and stony mantles.
  • Caused by the selective deflation of fines from the surface, leaving a lag of coarse-grained sediments.
  • The surface lag armours the surface and protects it from further deflation by the airstream.
  • Other mechanisms may also contribute to the formation of pavements including freeze-thaw, wetting and drying, or solution and recrystallization of salts.

Other types of arid land processes

e.g., sand storm

  • Dust – a driver of climogeomorphic, biogeophysical, and biogeochemical processes

Summary

  • Arid landforms reflect the weathering processes due to extremes of temperature and rainfall.
  • Dominant erosional processes are commonly the result of aeolian and/or fluvial processes.
  • Fluvial systems are typically ephemeral.
  • Aeolian systems are characterized by high-frequency low-magnitude events which are periodically punctuated by extreme events.
  • Arid land systems are typically “fragile” and require careful land management.