hot desert systems and landscapes

inputs to a desert system

* solar energy
* wind energy
* water energy
* geology/relief
* human activity
* climate change
* temperatures

processes in a desert landscape

* animal/plant respiration
* erosional and depositional landform formation
* weathering (thermal fracture, granular disintegration, exfoliation)
* wind erosion (deflation, abrasion)
* water erosion (hydraulic action, solution)

outputs from a desert system

* evapotranspiration
* sand
* landforms
* water and sediment loss

positive feedback in deserts example (desertification)

1. desertification takes place
2. there is a decreased yield of agricultural products and livestock
3. local population increase cultivate of land and livestock density
4. deforestation and overgrazing occur
5. soil quality is decreased, vegetation reduces, soil erosion occurs
6. more desertification

negative feedback in deserts example (prey)

1. there is a lack of food/prey
2. decrease in top/apex predators
3. there is an increase in prey
4. provides more food for predators
5. there are more top/apex predators so prey decreases again

deserts are…

open systems with inputs and outputs

hamada

bare, rocky surfaces

reg

stony deserts with rock fragments scattered over large plain

erg

mainly sand (‘sand sea’)

arid areas classification (+characteristics)

receives less than 250mm of rainfall annually

* very dry climates
* low levels of precipitation, high rates of evapotranspiration
* high diurnal temperature ranges due to lack of cloud cover

distribution of arid areas

generally located between the tropics, 15-30 degrees North and South of the equator

* they are found in areas of high pressure (which is between the tropics due to global atmospheric circulation)

climate characteristics

* less than 250mm of rainfall annually
* have a high diurnal temperature range: during the day there are high levels of insolation as solar radiation is not reduced by cloud cover, during the night clear skies allow ground temperatures to fall rapidly
* can be as high as 37 degree diurnal temperature ranges
* the highest temperature ever recorded was 56.7 degrees in Death Valley, California 1913

soil characterises

* thin, highly saline and lack organic content
* capillary action draws water and minerals from the water table to higher levels of soil, when the water evaporates only salt is left behind
* there is not enough water to support plants and animals so there is a lack of organic content
* small surface particles have low water retention

vegetation characteristics

* have thick waxy cuticles and small, spiky leaves to prevent moisture loss
* fleshy stems and bulbous roots increase ability to store moisture
* deep and wide root systems increase water gathering ability
* flowering quickly forms a short life cycle to follow erratic storms
* fleshy stems, bulbous roots and thick protective bark increase salt tolerance
* adapted to deserts: xerophytes e.g. cacti and giant agave can grow in deserts

water balance in hot deserts

* there are high temperatures and dry air leading to high evapotranspiration
* this means there is little storage and runoff
* plants and animals must adapt to the scarcity of water

aridity index

the relationship between precipitation and potential evapotranspiration

* aridity index = annual precipitation/annual potential evapotranspiration
* for arid areas this is between 0.05 and 0.2

causes of aridity

1. atmospheric processes (pressure)
2. winds
3. continentality
4. relief
5. cold ocean currents

atmospheric processes

deserts are found between 15-30 degrees north and south of the equator corresponding with patterns of atmospheric circulation

* deserts are found in areas of persistent dry descending air and in stable high pressure systems
* anticyclones are areas of high pressure where winds are light, sinking air is warmed, as it warms its ability to hold moisture increases making precipitation unlikely
* characterised by clear skies, low precipitation, high insolation, high evapotranspiration and high diurnal temperature ranges

cells that form high pressure in the model of global atmospheric circulation

* the descending limb of the Hadley cell forces cold, dense, high pressure air down towards the surface of the earth
* subsidence prevents convection leading to characteristic high pressure conditions that lead to aridity

winds

many arid areas are located where the prevailing wind blows from the land to the sea, thus carrying moisture

* e.g. the Sahara Desert, although it is right next to the Atlantic Ocean, the trade winds are northeasterly trades which blow across the large continent of Africa towards the Atlantic Ocean

continentality

many arid areas are located within large land masses in continental areas

* as an air mass moves from an ocean over a continent it will lose moisture as precipitation
* areas with a maritime location receive more rainfall than inland areas
* e.g. the Taklamakan desert is a continental isolated desert at over 4000km from the nearest ocean

relief

the rain shadow effect:

* as the air rises up the windward side of the mountain it cools and condenses into clouds
* as the air mass passes over the mountain it sinks and warms up on the leeward side, the clouds dissipate and there is less rainfall
* e.g. Death Valley is in the rain shadow of the Sierra Nevada mountain range and the Pacific Coast Range

cold ocean currents

driven by frictional stress at the interface between the ocean and the wind causing it to move in the direction of the prevailing wind

* as the air masses come into contact with cold ocean currents the air is cooled and its capacity to store moisture is decreased
* it becomes more dense and sinks
* as the air is blown onto land rain formation is suppressed due to a lack of moisture and it’s tendency to sink
* overall precipitation is low so aridity is high

sources of energy in hot deserts

1. insolation
2. winds
3. runoff/rain

insolation

the amount of heat (shortwave radiation) that reaches the Earth’s surface

* the angle of incidence is high so radiation is concentrated on the tropics
* insolation is high due to a lack of cloud cover
* the hot air in deserts is responsible for high levels of evapotranspiration
* in more moist areas, the sun’s energy evaporates water on the surface taking heat energy back into the atmosphere as latent heat so cooling the environments
* in dry areas, the lack of moisture means the ground becomes very warm, making more energy available to heat the air above it

winds

deserts are situated along subtropical high pressure belts so are subject to localised strong winds driven by global atmospheric circulation

* the strong seasonal trade winds allow for the transportation of sediment
* e.g. the southeast trade winds in the Atacama or the harmattan in the Sahara

runoff

intense and infrequent storms lead to rapid overland flow and cause significant erosion and transportation of sediment

sources of loose sediment

1. weathering of ground rocks
2. erosion of rocks
3. fluvial origin
4. sediment transported and deposited by aeolian processes
5. mass movement

sediment cells

describe the movement of sediment

* e.g. sediment is inputted, it is transferred by wind or water, some is placed in a sediment sink/area of deposition, some is transported away by wind or water

sediment budget

considers the relative amounts of sediment in each part of a hot desert

* areas dominated by erosion have a net sediment loss
* areas dominated by transportation have a net sediment gain

weathering

the breakdown of rock in situ

* rates affected by climate, geology and stress-strain behaviour of materials exposed to weathering
* it produces regolith which can be eroded and transported easily
* mainly physical and chemical in arid areas

erosion

degradation of the surface of the earth caused by a moving force that carries material away

insolation weathering/thermal fracture (physical)

air temperatures can reach over 40 degrees and surface temps over 80 during the day in summer months

* this means rock surface expand and contract daily
* it occurs in different ways due to rock type, structure, chemical composition and colour

granular disintegration (physical)

occurs as a result of a large temperature range causing minerals to expand and contract at different rates

* lights and dark minerals heat and cool at different rates, leading to stresses within the rocks and eventual disintegration

block disintegration (physical)

caused by heating and cooling of well jointed rocks e.g. limestone

* rocks break down joints and bedding planes as they are lines of weakness

exfoliation (physical)

peeling of surface rocks

* high temperatures in the day means surface of rocks heat up and expand, at night they cool and contract
* repeated expansion and contracting causes surface to peel or flake off

salt weathering/crystal growth (physical)

high temperatures draw saline groundwater to the surface, water evaporates to leave behind salt crystals

* the growth of salt crystals between pores and joints creates stresses and eventually disintegration of rocks

freeze-thaw weathering (physical)

water trapped in rock joints expands when it freezes

* the pressure exerted inside rocks causes it to shatter and form scree

wetting and drying (chemical)

water saturates rock after flash flood or seasonal rainfall events and causes it to swell

* as water evaporates it contracts as it dries
* repeated expansion and contracting causes the rock to disintegrate

hydration (chemical)

minerals absorb water and cause rocks to swell and expand

* increase in volume of the rock causes stresses
* zones of weakness develop and lead to granular disintegration and flaking of the rock

oxidation (chemical)

oxygen dissolved in water reacts with some minerals to form oxides and hydroxides

* this can lead to red staining
* oxidised minerals increase in volume which weakens the rock

solution (chemical)

some minerals can be dissolved in rainwater which can make it slightly acidic

* this causes flaking of surface layers of rock

biological weathering

limited in deserts due to lack of vegetation

* some plants have long root systems that can grow into cracks and widen them
* respiration can also release CO2 that can be used in carbonation

mass movement

the movement of material downhill under the influence of gravity

rock falls

small blocks of rock become detached and fall to the surface of the cliff

rock slides

a failure throughout the whole rock causes it to collapse en masse

deflation

loose and fine regolith is removed by wind

* aided by dry and loose soil or sparse vegetation
* operates through progressive removal pf smaller matter
* larger matter is left behind to form a desert pavement

abrasion/corrasion

abrasive action of wind-borne particles against rocks (acts like sandpaper)

* the aeolian particles carve rock into different shapes overtime
* larger particles are more erosive because they are heavy, but are confined to a couple of metres above the surface
* can form pedestal rocks

attrition

takes place as grains of sand carried by the wind collide with each other and become smaller and smaller as they do so

transportation

the processes that move material from the site of erosion to the site of deposition

* occurs when wind velocity exceeds the critical threshold
* the movement of sediment is induced by drag and lift

suspension

wind carries fine dust

* particles are

saltation

the movement of particle in a series of small bounces across the ground during periods of strong wind

* particles are 0.15-0.25mm in diameter
* the weight of the particles means that the process occurs only a couple of meters above the ground
* falling grains hit other stationary grains to set them in motion

surface creep

heavier grains are rolled or pushed along the ground

* particles are >0.25mm in diameter
* they are too heavy to by lifted so are rolled

deposition

the laying down of material or sediment

* as wind velocity decreases it reaches a point where it can no longer transport the sediment it is carrying
* reduction in capacity due to fall in energy
* obstacles may slow down the wind to form sand shadows

exogenous rivers

rivers that originate outside of deserts

* they have enough water to flow continuously despite high rates of evapotranspiration
* e.g. the River Nile or Colorado River

endoreic rivers

rivers that flow into deserts and then terminate in a lake or inland sea

* e.g. the River Jordan terminates in the Dead Sea

ephemeral rivers

rivers that flow intermittently after a storm event or after being fed by snowmelt from neighbouring mountains

* e.g. the Ugab River (Namibia)

episodic flash flooding

infrequent, intense rainfall events lead to high rates of surface runoff as soils are baked dry and infiltration capacities are low

sheet flooding

water flows over the landscape as a sheet

channel flash flooding

water is confined within a channel

deflation hollows (aeolian)

depressions caused by the removal of fine sediment by deflation and the prevailing wind

desert pavements (aeolian)

rocky desert floor covered by small rock fragments, formed by removal of loose fine sediment by deflation to leave behind larger rocks

ventifacts (aeolian)

individually isolated rocks with one obviously weathered side, caused by abrasion from wind-borne particles eroding the rock face facing the prevailing wind over time

yardang (aeolian)

parellel ridges of rock, weaker rocks are eroded by abrasion to form deep troughs whilst strong rocks remain

zeugen (aeolian)

similar to pedestal rocks, weaker rocks underlying stronger rocks are eroded by processes of saltation and surface creep (as well as abrasion) whilst the stronger rock remains to form a mushroom type shape

barchan dunes (aeolian)

present shaped dunes with two horns, formed as the prevailing wind from a singular direction shapes deposited sediment on both side of the dune

seif dunes (aeolian)

elongated parallel dunes, formed as wind direction changes causing one horn of a barchan dune to elongate into a straighter dune

wadi (water)

a dry gorge-like desert landform, formed overtime by water erosion (hydraulic action, abrasion) following intense storm events as water carves through the rock

bahadas (water)

formed as streams flow out of parallel wadis, they decrease in velocity enough to deposit the sediment they carried in a series of alluvial fans that join together to form bahadas

pediments (water)

gently sloping rock surfaces formed by parallel retreat of rock faces

* parallel retreat occurs as rocks are removed from the base of a cliff at the same rate as weathering occurs

playas (water)

following rainfall events, water evaporates from hollows to leave behind salts and minerals forming a salt lake

inselbergs (water)

relict hills formed when rain was more prominent in the past, the landscape was carved by water erosion initially and continues to be weathered by aeolian processes turning mesas to buttes to spires

relationships between processes, time and landforms

according to King’s Arid Cycle of Erosion

* each cycle beings with a rapid rate of tectonic uplift followed by a long period of crustal stability
* the cycle passes through youth, mature and old
* it passes from very mountainous, to a landscape covered by pediments to a flat pediplain

desertification def

land degradation in arid, semi-arid and sub-human areas resulting from various factors including climatic variation and human activity

could eventually effect…

over one third of the Earth’s and surface and over a third of the population

how has distribution of hot deserts changed over the last 10,000 years

about 8000 years ago

* in the interglacial period there were much warmer and more humid conditions
* forests were widespread and thriving in wet conditions
* aridity fell with many present-day deserts becoming grasslands

\
about 5000 years ago

* monsoon rains over Africa began to diminish and conditions became more arid
* by around 3000 years ago conditions were similar to what they are today

areas most at risk of desertification

* the Sahel
* areas of Western Asia such as Iran and Afghanistan
* places that have had more people move to them, a greater population leads to increased pressure on resources which encourages harmful human behaviours that lead to desertification
* the most high risk areas tend to develop around the margins of deserts due to the expanding exploitation of such margins due to an increased demand for resources as a result of the growing population

climate change (natural cause)

* the rapid increase of greenhouse gases in the atmosphere is warming the climate
* higher temperatures reduce evapotranspiration and rainfall leading to more frequent periods of drought
* without sufficient water, vulnerable drylands are converted into deserts

specific climate events (natural cause)

* e.g. El Niño leads to more frequent and increase droughts in some areas which increases desertification (e.g. Ethiopia in 2015)

overcultivation (human cause)

intense farming characterised by growing crops constantly in the same areas of marginal land, nutrients are quickly removed and soil fertility is reduced

* soils also become drier and thinner due to lack of organic matter so are more likely to be washed or blown away

overgrazing (human cause)

areas of marginal grassland are overstocked with livestock leading the vegetation to be stripped, soils become dry and loose so can be more easily eroded

overirrigation (human cause)

certain methods of irrigation can lead to build up of salt on the surface of land forming impermeable, infertile soils

population increase (human cause)

adds additional pressure on land to be productive as it must be cleared for farming, housing and providing fuelwood for cooking

firewood demand (human cause)

land must be stripped of trees leaving it exposed to wind and rain

tourism (human cause)

high concentration of safari minibuses can cause damage to vegetation leading to soil erosion and desertification

threats to ecosystems

• loss of nutrients in soils leave soils infertile to reduce biodiversity

• as vegetation is destroyed, animals are forced to migrate in order to find food

• water sources become depleted, the water table falls and plants and animals die

• habitats are destroyed disrupting food chains and ecosystems

threats to landscapes

* sandstorms can lead to increased abrasion
* increased formation of sand dunes as loose material is no longer trapped by vegetation
* landslides may occur on steep slopes where vegetation is removed
* increase salinity is irrigation is used

threats to people

* dust clouds effect air quality causes health problems even for people living far away from desert regions
* lack of water causes hygiene issues and spread of diseases
* forced migration due to food insecurity (potential social tension in areas migrants move to)
* food insecurity
* loss of vegetation reduces food productivity forcing people to abandon settlements
* increased poverty in rural areas
* greater dependence on food imports and food aid

risk of climate change

leads to extreme weather events, results in greater erosion of soils

* by the mid 21st century temps in arid areas could rise by 2.5 degrees
* evapotranspiration will decrease and droughts will become common and more severe