Hot deserts

Roughly 1/3

How much of Earth’s land surface is made up of deserts

(Generally run in) parallel belts North and South of the Equator

Global distribution of hot deserts

Global prevailing winds and cold ocean currents

Why the majority of hot deserts form on the Western side of continents

Less than 250mm

Annual rainfall of an arid area

Aridity index

The ratio between precipitation and potential evapotranspiration

Continentality

The impact of increasing distance from the coast on the climate of an area

250mm-500mm

Annual rainfall of a semi-arid area

Water balance

The relationship between precipitation received and water lost to potential evapotranspiration in a hot desert

Desert

An arid environment receiving very low levels of rainfall

Potential evapotranspiration

The amount of water that could be lost from soil, plants, and the ground through evapotranspiration if there was an unlimited supply of water

Atmospheric demand for water based on its climate rather than actual evapotranspiration rates

What does potential evapotranspiration represent

AI<0.05

Hyper-arid aridity index

0.05<AI<0.20

Arid aridity index

0.20<AI<0.50

Semi-arid aridity index

Decreases

As aridity increases, frequency

Very low causing very frequent clear skies and extreme insolation

Hot desert humidity characteristics

Huge variety (e.g. coastal Atacama = far cooler than Sahara)

Temperature characteristics between deserts depending on individual physical geography such as continentality

(Usually) short downpours following long dry periods (total annual rainfall sometimes falls within hours)

Characteristics of rare desert rainfall events

Often exceeds rate of precipitation

Hot desert evaporation rate characteristic

Infertile, alkaline, (fairly) saline, and have a thin soil profile

When soils do develop in hot deserts they are often

Lack of moisture, extremely high temperatures and evaporation rates, sparse vegetation, and limited organic material

Causes of extremely slow soil development rates in hot deserts

(Often have) a thick accumulation of basic mineral soils at or near the surface

Characteristic of desert soils due to capillary action

Capillary movement

Where any moisture in the soil/subsoil moves upwards through the tiny spaces between soil particles (capillaries)

When evaporation exceeds precipitation

When capillary action occurs, making it prevalent in hot desert environments

Aridisols

Main soil type/order in hot deserts

Sierozems

Type of aridisol dominant in semi arid areas

Raw mineral soils

Type of aridisol dominant in arid areas

Generally very thin (less than 1m deep)

Main characteristic of aridisols

Coarse texture, often rocky due to physical weathering, heavy salt presence at the surface (due to capillary action)

Raw mineral soils characteristics

Slightly alkaline and unproductive rather than infertile as extreme temperatures and lack of water limit vegetation growth

Raw mineral soils typical acidity

Hard salt crusts

If salt concentrations are high enough, capillary action can cause the surface of raw mineral soils to form

Darker colour, more productive (than raw mineral soils) due to some organic material presence

Sierozems characteristics

Very low growing (in height), very diverse

Typical hot desert vegetation characteristics

Xerophytes

Most plants in hot deserts are classed as plants that have adapted to water scarcity

Succulence

Hot desert plant adaptation/characteristic causing specialised, fleshy leaves/stems/roots that can store large amounts of water

Shallow, wide, extensive root systems

Hot desert plant adaptation/characteristic causing plants to react quickly to rainfall as water does not penetrate deep into the soil or remain there for very long by procuring water rapidly

Thick waxy cuticles and closed stomates

Hot desert plant adaptation/characteristic enabling them to effectively store water and limit transpiration

Spiky/bitter/toxic (or living in inaccessible areas for animals)

Hot desert plant adaptation/characteristic that deters thirsty animals

Phreatophytes

Hot desert plant adaptation/characteristic causing drought tolerance through specialised deep roots that can reach the water table

Drought avoidance

Hot desert plant adaptation/characteristic causing ephemeral plants to go dormant during drought events and complete very short growth/life cycles within weeks of rainfall events

Halophytes

Hot desert plant adaptation/characteristic causing salt tolerance

Atmospheric circulation, continentality, relief (rain shadows), cold ocean currents, wind

Most common causes of aridity

High angle of incidence

Characteristic of the equator causing significant insolation and a net gain in energy

Heats and begins to rise

Impact of net gain in energy at the equator on the air in contact with it

Begins to cool and condense(to dew point and release water through precipitation hence becoming drier) and track polewards

Rising air from the equator after being heated by the high angle of incidence then

20-30 degrees N/S

Area where the cool, dense air of the Hadley Cell sinks back down towards the surface and combines with descending air from the neighbouring Ferrel cell

Little to no cloud formation occurs

Falling cool, dense air from the Hadley Cell warms and expands as it descends meaning that

Falling limb of the Hadley and Ferrel Cells

The cause of high pressure in the region 20-30 degrees N/S

Less moisture available for cloud formation

Increasing continentality (distance from seas) causes

Lower specific heat capacity

Reason why land heats up quicker and to a higher temperature than water bodies, but then cools more significantly and rapidly at night too, illustrating how continentality causes aridity

Oceanic evaporation (causing cloud formation to drift inland)

What causes far more cloud formation in coastal areas than those inland that see far more insolation

Leeward side (downwind side)

Side that is sheltered from prevailing winds and often experiences a rain shadow when a mountain is involved

Rain shadow

When moist air brought inland by prevailing winds is forced to rise over mountains leading to cooling, condensing, and cloud formation on the windward side causing relief rainfall and hence loss of moisture. Once over the peaks the now less moist air sinks and expands as it is warmed leading to lower humidity and less cloud cover

Global oceanic circulation

Cold ocean currents occur because of

Fog/mist (offshore)

Any wind moving over cold ocean currents is cooled, causing moisture to condense and create

Is burned away (by insolation), leaving cool air that cannot hold much moisture

What happens to fog/mist caused by cold ocean currents when they reach land due to the fact that land heats up quicker than the sea due to intense insolation

Cold ocean currents

Some vegetation has adapted to utilising water that condenses on it as dew from fog/mist as it may be the only water it ever receives in response to aridity caused by

Wind, water, energy (from insolation), and sediment

Hot desert inputs

Characteristic erosional and depositional desert landforms

Hot desert stores/components

Water and wind removing sediment, insolation energy reradiated back to space

Hot desert outputs

Abrasion

When material carried by moving wind or water hits exposed rock surfaces, thus wearing them away, often referred to as sandblasting or sandpapering

Aeolian processes

Processes relating to the action of the wind

Chemical weathering

The processes leading to the breakdown of rocks due to chemical reactions, typically requires water and exposure to air

Deflation

When wind removes dry, unconsolidated (loose) sand, silt, and clay particles from the surface and transports them away

Deposition

When the velocity of the wind/water decreases to a point where it can no longer transport the material it is carrying

Endoreic streams

Rivers that are closed and do not flow out to sea or other rivers, instead end inland in lakes or swamps

Ephemeral streams

Rivers that only flow after precipitation and remain dry for most of the year

Erosion

The wearing away of the Earth’s surface by the mechanical action of aeolian and fluvial processes

Exfoliation (onion skin weathering)

A process of mechanical weathering that results in the breaking, splitting, or peeling off of the outer rock layers

Exogenous streams

Rivers that originate outside the hot desert environment in more humid locations and flow through the hot desert environment - usually perennial

Insolation

The incoming solar radiation that reaches the surface

Mass movement

The movement of material downhill under the influence of gravity, may also be assisted by water

Saltation

Process of transportation where particles are transported by bouncing and hopping along the surface

Sediment

Any naturally occurring material that has been broken down by the process of erosion and weathering and has then been transported and deposited by wind or water

Sediment budget

The balance between the input and output of sediment in hot deserts

Surface creep

Process of transportation where saltating particles hit larger particles/rocks that are too heavy to hop, causing them to slowly slide/roll along the surface from a combination of the push of the saltating grain and and the movement of wind

Suspension

Process of transportation where the smallest sediment particles are held in the air

Thermal fracture

A type of mechanical weathering of rock resulting from its rapid and repeated heating and cooling caused by high diurnal range. This causes the exposed rock to expand during the day and contract at night, causing repeated stress over time and hence fracture, often leading to granular disintegration

Transportation

The processes that move material from the site where erosion took place to the site of deposition

Weathering

The breakdown/decay of rock at or near the surface creating regolith that remains in situ until it is moved by erosional processes. Can be mechanical, biological/organic, or chemical

Insolation, wind, and runoff

What provides the energy to drive most geomorphological processes in hot deserts

High angle of incidence

When insolation is concentrated on a smaller area of land than at higher latitudes and hence warms the surface far quicker

Surface heats up faster

Impact of the lack of water in hot deserts meaning less insolation is used for evaporation which transports heat back into the atmosphere as latent heat

Rainfall in hot deserts

Described as ‘spotty’, typically very localised and spatially and temporally unpredictable

Significant geomorphological processes

Impact of rare rainstorms in hot deserts as baked ground and limited vegetation cover combine to cause rapid overland flow

Sediment cell

Deserts are a store of loose sediment and much of it is transported and deposited within the desert boundary itself, forming a

Weathering (of underlying parent material), fluvial or aeolian transportation into the desert

Sources/origins/inputs of sediment in hot deserts

When ephemeral rivers dry up or when rivers flood during intense rainfall

How rivers tend to deposit imported sediment into hot deserts

Net sediment loss

Areas of hot deserts that are dominated by erosional processes are a source of sediment and the subsystem is said to have a

Net sediment gain

Areas of hot deserts that are dominated by depositional processes are said to have a

Source of sediment

Major role of large hot deserts on a global scale

Geomorphological processes

Mass movement, weathering, erosion, transportation, and deposition

Broken down material remains in situ as regolith

What happens after weathering that makes it different from erosion

Rock falls

Form of mass movement where small blocks of rock become detached from an exposed cliff and fall freely to the base of the cliff

Rock slides

Form of mass movement where there is failure throughout the rock as a whole and the material collapses en masse rather than as individual blocks

Steeper slopes and accumulation of coarse material at the base (steep, solid rock cliffs are hence common in hot deserts)

Impacts of mass movement (rock slides and rock falls) on the landscape

Vertically jointed rocks

Common cause of mass movement where underlying, less resistant rock is more easily eroded than that above it, meaning that as it erodes, mass movement and cliff retreat of the rock above it occurs

Load

The material being transported (only agents of transportation in hot deserts = wind and water)

Traction, saltation, suspension, and solution

Water transports its load through

Traction

When larger stones and boulders roll along the riverbed by the movement of water downstream due to high levels of discharge and hence energy. Fluvial equivalent of surface creep