Hot Deserts Content

Inputs

Solar energy, wind, precipitation

Erosional processes

Erosional landforms and landscapes

Water and wind transport

Depositional landforms and landscapes

Outputs

Water and wind remove sediment, clear skies allow reflection of raditaion

Distribution

1/5 surface, mainly N, on tropics lines

Aridity index

Ratio of precipitation to potential evapotranspiration

Hyper-Arid

<0.05

Arid

0.05-0.2

Semi-Arid

0.2-0.5

Water Balance

The relationship between the annual precipitation and water lost to pet

Diurnal Variations

High temperature change between day and night

Low Humidity

Low cloud levels lead to high insolation

Daytime temperatures

Over 30, often highs of 45-55

Cloudless skies

Lead to high diurnal range - heat gets in and out quickly

Night Temperatures

Sub 0, -18 not rare

Typical Diurnal Range

30/40

Soils qualities

Infertile, thin soil profile, saline, alkaline

Impactors on soil

Lack of moisture, High temperatures and evapouration, sparse vegetation

Capillary movement

Leads to accumulation of mineral salts near surface due to solutes moving upwards through spaces between soil particles

Aridsoils

Main desert soil type

sierosems

Aridsoils in semi-arid areas

raw mineral soils

Coarse texture and gravelly, found in arid environments

Leaching

Downward movement of minerals - low in aridsoils

Vegetation Distribution

Little to none in mobile sand sea area

Goals of adaptions

Limit moisture loss, store moisture, procure water through deep root systems, rapidly respond to sporadic rainfall

Xerophytes

Plants with adaptions to survive

Ephemeral plants

Become dormant and loose leaves during droughts - Joshua Tree

Halophytes

Cells adapted to deal with salinated soils - saltbush

Phreatophytic plants

Plants with deep root systems to cover larger area - saguaro Cactus, Creosite Bush

Succulence

Fleshy leaves, stems or roots to store water

Conserving water

Thick waxy cuticles, small spiky or no leaves to limit transpiration

Protection from animals

Inaccessible locations, spiny/bitter/toxic

Drought Avoidance

Short life cycles, rapidly blooming after rainfall, channel all energy into seed production, seeds lay dormant until rainfall

Procuring water

Shallow yet extensive roots as water doesn’t penetrate far

Global Atmospheric Circulation System

Falling cool air between the Hadley and Ferrel cells causes high pressure

Continentality

Coastal locations have higher precipitation and are more stable

Relief

Areas next to mountain ranges can experience a rain shadow effect

Cold Ocean Currents

Global Oceanic Circulation system leads to cool dry air over the coast, unable to hold much moisture

Abrasion

Material carried by moving wind or water hits exposed rock surfaces, wearing them away

Aeolian

Wind action

Chemical Weathering

Breakdown of rocks due to chemical reactions

Deflation

Removal of loose sediment by wind

Endoreic streams

Rivers that flow into lakes and stay inside a drainage basin

Ephemeral streams

Intermittent streams following heavy rain

Erosion

Wearing away of earth’s surface by mechanical action

Exfoliation

Mechanical weathering process, peeling of outer rock layers ‘onion skin weathering’

Exogenous streams

Rivers that originate external to a desert and pass through

Insolation

Solar radiation that reaches the earth’s surface

Mass Movement

Movement of material downhill by gravity and potentially rainfallA

Saltation

A process where sand-sized particles are bounced along a surface

Sediment Budget

Balance between input and output of sediment in hot deserts

Surface creep

Slide/roll of larger particles along the surface - pushed by saltating particles and wind

Suspension

Transportation by wind sub 0.2mm particles are held in air

Thermal Fracture

Freeze Thaw weathering

Transportation

Moving of eroded sediment to deposition site

Weathering

Breakdown/decay of Earth’s surface creating a layer of material that remains in place until further erosional processes - can be biological/mechanical/chemical

Crystal Growth

Water evaporates and dissolved solutes are left to crystalize

Hydration

Rocks absorb available water, causing swelling and vulnerabilities to mechanical weathering

Hydrolysis

Mildly acidic water reacts with minerals causing the creation of salts

Oxidation

Breakdown of rocks by oxygen and water

Cause of orange rocks

Oxidation of Iron

Sheet flooding causes

Rock impermeability and volume of rainfall

Sheet Floods

Shallow yet wide floods which lead to erosional processes

Wadis

Steep sided wide bottomed gorge-like valleys formed by fluvial erosion, home to ephermeral streams

Venifacts

Exposed rocks shaped by wind blown sediment to create a flattened side, sharp edges and ridges

Yardang

Streamlined parallel ridge of rock aligned in the direction of the prevailing wind, seperated from each other by a win-scoured groove

Kharif

Somali desert seasonal wind - Jun-Sep

Irfi

Western Sahara Easterly winds in March

Harmattan

Sahara winter winds - Nov-Mar

Shamal

Arabian gulf winds - May-Jul

Haboobs

Localised winds in the Sudanese Sahara

Zeugen

Rock pillars, pedestals and mushrooms, or yardangs with considerable undercuttings - formed by less resistant rock underlying resistant rock

Sand Dune requirements

Supply of sand, prevailing winds strong enough for transport but weak enough to result in net deposition, steady winds in a direction, obstacle to trap sediment and encourage deposition

Erg

Landscape where there is potential for a dune

Dune qualities depend on

Duration and strength of winds, supply of sediment, morphology of landscape, other winds present

Barchans

Cresent-shaped sand dunes with a gently sloping windward side and a steep leeward side, has horns or wings on the sides causing convex shape, needs unidirectional wind, Wings advance faster than dune, as advance is inversely proportional to dune size, often found in groups

Transverse dunes

Large scale sand ripples, form in large groups, large ridges oif sand with steep downward faces, require large constant supply of sand, can be result of barchans merging due to supply increase

Seif-dunes

Knife edged ridges of sand form parallel ridges of sand separated by wind-scoured depressions, tops and sides of dunes are serrated due to local wind action

Parabolic dunes

horseshoe shaped dune where open end faces upwind, relatively stable and often vegetated, often coastal and need constant sand supply

Star Dunes

Variable slip face directions, abundant supply of sand but variable wind directions, often massive and dome like, creating draa landscape

Draa landscape

A large, elongated dune, potentially measuring hundred of kilometers in length and hundred of meters in height.

Bajadas

Alluvial fans coalesce, causing braided streams as river energy is dissapated

Pediment

Caused when highland meets lowland in hot desert environments, gently sloping areas of bare rock and debris

Alluvial fan

Deposited load from ephemeral streams in wadis meeting pediments and loosing energy

Playa/Salt Lake/Chott

Water evaporates leaving behind the salts (NaCl commonly giving a beach colour) in crystal form on the dry lake bed, humans are known to exploit deposits

Mesa

Caused by the fluvial erosion of horizontal bedding planes over sedimentary bedrock, flat topped isolated plateaus, with steep slopes, lower slopes covered in scree

Buttes

Similar to mesas but much smaller, thought to be eroded and worn down mesas

Inselbergs

Rounded steep-sided hills that rise abruptly from a lowland plane

Speed and nature of weathering

Rocks will be broken down quicker in areas with greater diurnal temperature ranges

Presence of moisture

different landscapes will form due to the varied location volume and frequency of water

Badlands

Formed by water flowing impermeable less-resistant geology, no vegetation due to the erosional and depositional landforms being formed

Features of badlands

Wadis, unstable slopes, mass movement regularly, egullies, alluvial fans and bajadas, pipes and caves leading to natural arches

Desertification

The persistent degradation of dryland ecosystems by human acricities and by climate change

Change in farming to increased livestock numbers

Overgrazing and depletion of soil nutrients, vegetation is unable to re-establish

Climate change leading to less rainfall/increased drought and higher temperatures

Rivers and water holes dry up leading to death of vegetation

Population growth due to hugh birth rates and refugees leads to more land needed to farm more crops and therefore method change, and increased demand for wood

Overcultivation and deforestation leading to the removal of vegetation

Final step before desertification

Decrease in vegetation and exposed soil to wind and rain, increased risk of soil erosion and evaporation from soil

Desertification impacts on landscapes

Soil erosion and topsoil loss, sand dune increase, treeless zone increase, increased sedimentation surrounding water sources, existing landscapes getting encroached on, increased salinity of soils and salt crust developments, sand storm increases, vegetation damaged by sandblasting

Physical Desertification impacts on populations

Reduced soil fertility leads to decrease in crop diversity and yield, soil erosion leads to sedimentation and flooding, dust storms lead to eye diseases and discomfort, breakdown of society leads to young and economically active leaving to find work elsewhere

Human Desertification impacts on populations

Drought and famine lead to malnutrition and starvation, ‘forced’ migration leads to conflict and pressure on neighbouring areas, loss of land leads to loss of homes and cultural ties, loss of culture leads to traditional ways of life being lost