Alastair Giddings
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Mindmap for AQA AS-Level Geography Hot Deserts Environment Topic

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Deserts
1 Desertificiation
1.1 Case Studies
1.1.1 Southern Spain
1.1.1.1 Causes
1.1.1.1.1 Agriculture
1.1.1.1.2 Development
1.1.1.1.3 Climate Change
1.1.1.2 Impacts
1.1.1.3 Strategies
1.1.1.3.1
1.1.1.3.2
1.1.1.3.3
1.1.2 The Sahel
1.1.2.1 Impacts
1.1.2.2 Strategies
1.1.2.2.1
1.1.2.2.2
1.1.2.2.3
1.1.2.2.4
1.1.2.2.5
1.1.2.3 Causes
1.1.2.3.1 Agriculture
1.1.2.3.2 Deforestation
1.1.2.3.3 Climate Change
1.2 General

Annotations:

  • Desertification is the degradation of semi-arid land by human activities and changes in climate. It leaves land unproductive. Africa and Asia are the worst affected areas but desertification is a world wide problem. 1)A third of land worldwide is at risk of desertification. 110 countries have regions that are threatened. 2) 250million people across the globe are already affected by desertification, the largest proportion of which are in Asia. 3) 46% of Africa is at risk with 25% at high or very high risk.
1.2.1 Management Strategies
1.2.1.1 Agriculture

Annotations:

  • Strategies to make agriculture more sustainable by reducing overgrazing, over-cultivation, and improving the fertility of the soil: 1) Leaving areas of land fallow (not cultivated) - this allows soil to recover from grazing 2) Adopting nomadic farming - nomadic herders constantly move animals on, which stops areas being overgrazed. 3) Rotating crops - different crops need different nutrients, so rotating crops stops the same nutrients being depleted year after year. This improves soil fertility. Adding compost can also improve soil fertility. 4) Planting legumes (e.g. clover) - these improve soil fertility by increasing the amount of nitrogen present.
1.2.1.2 Water Use

Annotations:

  • Strategies to make water use more sustainable by reducing use: 1) Growing crops that need little water (e.g. millet, sorghum and olives) - this can reduce water use. 2) Using drip irrigation - this technique adds small volumes of water to the soil at a time. It reduces wastage of water and also prevents soil ebing eroded by large volumes of water being added in one go.
1.2.1.3 Vegetation

Annotations:

  • Strategies to increase or maintain the level of vegetation to reduce erosion: 1) Planting trees - these act as windbreaks, protecting soil from wind erosion 2) Using alternative energy sources -technology such as solar cookers can help reduce deforestation to provide wood for fuel. These cookers use the sun's energy to heat food and are cheap and easy to make
1.2.1.4 Sustainability

Annotations:

  • Sustainable strategies address the causes of desertification: 1) Most of the strategies mentioned are sustainable because they prevent desertification and once implemented they can be carried on by the local community without external help or money. e.g. once people have been trained on how to make a solar cooker, further external aid isn't needed as that knowledge is now within the country.2) Some strategies just address the problems caused by desertification and so aren't sustainable (as they wont prevent desertification in the future). 3) Examples of unsustainable strategies are transporting water to areas at risk of desertification, and using external aid to provide relief from famine. 4)To address the threat of desertification many countries are drawing up National Action Plans, which aim to prevent desertification using sustainable solutions and involving local communities.
1.2.2 Physical Causes
1.2.2.1 Higher Temperatures

Annotations:

  • 1) Global surface temperature has risen by 0.6 degrees over the last century and is predicted to continue rising. 2) As temperatures increase the rate of evapotranspiration also increases. 3) This dries out soils and lowers surface water levels, leading to soil erosion in the same wat as lower rainfall does (vegetation dies).
1.2.2.2 Lower Rainfall

Annotations:

  • 1) Climate change will probably reduce rainfall in sub-tropical areas (where most semi-arid and arid environments are) 2) If theres less rain then surface water and groundwater will be reduced (as its used up and evaporates but isn't replenished) 3) This means that the volume of water available for vegetation growth is reduced, which leads to the death of vegetation 4) The roots of plants and trees bind the soil together. Fewer plants and trees mean fewer roots , leading to soil erosion
1.2.3 Impacts
1.2.3.1 Social

Annotations:

  • -heat may cause an increase in death rate - food shortages - farmers and local people have to move to find better land - lack of food and water leads to malnutrition, famine, disease and higher death rates - people may become dependant on aid - increased poverty -social tensions in areas that migrants move to -less resources available
1.2.3.2 Environmental

Annotations:

  • - infertile land - Land becomes unusable and useless - increased risk of flooding - loss of biodiversity - sand dunes and sand storms more common - soil erosion/reduced moisture/increased salinity - reduce of native vegetation - reduce in infiltration capacity
1.2.3.3 Economic

Annotations:

  • -poor trade as crops fail and little land is available for cattle - area becomes less attractive to tourists - all food becomes more expensive as less is available - people become dependant on aid -increased poverty -less resources available -reduced income from farming
1.2.4 Human Causes
1.2.4.1 Irrigation

Annotations:

  • can cause desertification in a number of ways: 1) Irrigation depletes surface water and may involve unsustainable pumping of aquifers. As water levels are lowered, water availability for plants decreases leading to soil erosion. 2) Some irrigation techniques can erode soil directly e.g., surface irrigation, where large amounts of water are added to the soil in a short amount of time, washing topsoil away. 3) If too much water is used to irrigate crops the excess can sink into the soil and raise groundwater levels. If the aquifer is saline this may bring high concentrations of salt too close to the surface, increasing the salinity of the soil too much for plants to survive
1.2.4.2 Overgrazing

Annotations:

  • reduces vegetation so leads to soil erosion. Trampling by large numbers of animals compresses and breaks down the structure of the soil, which makes erosion more likely.
1.2.4.3 Population Growth

Annotations:

  • increases pressure on the land as mo0re and mmore food is needed to meet the growing demand. This leads to further overgrazing, cultivation, deforestation and irrigation therefore increasing desertification.
1.2.4.4 Overcultivation

Annotations:

  • reduces soil productivity as the over-exploitation of the soil leaves it without enough nutrients to support plants. Without plants the soil is easily eroded.
1.2.4.5 Deforestation

Annotations:

  • removing trees and therefore tree roots means that (once again) soil is more vulnerable to erosion. Forests are cleared to provide land for farming as well as wood for fuel and building material
2 Desert and Desert Margin Characteristics
2.1 Causes of Aridity
2.1.1 Global Atmospheric Circulation
2.1.2 Continentality

Annotations:

  • 1) The central parts of continents are usually more arid than coastal areas. 2) Moist wind from the sea moves inland and the moisture held is dropped as precipitation. 3) So when the wind reaches the centre of a large continent its carrying very little moisture so very little rain falls. 4) For example, the Turkestan desert exists because its in the central part of Asia.
2.1.3 Cold Ocean Currents

Annotations:

  • 1) In some places cold ocean currents run along the coastline. 2) Wind is cooled as it travels over the cold water and its ability to hold moisture is reduced. 3) Moisture that's stored in the atmosphere is released as precipitation over the ocean before reaching land. 4) So when the wind reaches the land theres very little moisture left, so very little rain falls. 5) For example, the Namib Desert in Africa exists because of the Benguela Current (a cold ocean current) that runs up the West coast of Africa.
2.1.4 Relief

Annotations:

  • 1) Tall mountain ranges force winds upwards. 2) As the air rises it cools and its ability to hold moisture is reduced. 3) Any moisture held is dropped as precipitation over the mountains, so the wind that moves inland has very little moisture, which means that very little rain falls there. 4) This Is called the Rainshadow effect. 5) For example, the Atacama desert in South America exists because of the rain shadow effect of the Andes mountains.
2.2 Rainfall

Annotations:

  • Rainfall is low in deserts and desert margins1) Deserts are arid (dry) environments. Environments are classes as arid if they get less than 250mm of rainfall a year.2) Desert margins (land at the edge of a desert) are semi-arid environments. Environments are classed as semi-arid areas if they receive between 250-500mm of rainfall a year.3) The aridity of an environment depends on rainfall and the rate of evapotranspiration.
2.3 Vegetation

Annotations:

  • The biomass (total amount of living matter) in a desert is low because the lack of water makes it difficult for things to grow. The amount of vegetation within a desert varies - there can be none where there are sand dunes, and a variety of small shrubs, grasses and cacti species in other areas. Plants are specialised to survive in the hot and arid conditions, e.g. cacti have special ways to collect, store and conserve water The vegetation in desert margins is a little different: There's more vegetation in desert margins than in deserts. Vegetation includes shrubs, grasses and trees. The amount of vegetation generally increases the further away from the desert you go, becauses
2.3.1 Xerophytes

Annotations:

  • Plants adapted to withstand drought using adaptations such as Succulence and reducing transpiration. Adaptations:Succulents have modified physical structures in order to resist drought. They store their water in stems, leaves and/or roots. Water absorbed during seasonal rains is stored in specialised tissues of plant cells call vacuoles so that the plant can then use this water during dry times. Reduce transpiration by having spikes instead of leaves to prevent the process but a green stem in order to allow photosynthesis to occur. The use of stomata which close during the day to reduce transpiration but open at night is common to Xerophytes also.Example plant: Saguaro CactiMost commonly found in Southwest USA (Mojave and Sonoran) Description - can grow up to 15m tall and live for roughly 200 years. It is able to absorb up to 8000 litres of water and has extensive shallow roots in all directionsAdaptations - A thick waxy coating keeps water inside from evaporating, Thick cell walls that do not break, green stem, specialised stomata and spikes instead of leaves
2.3.2 Phreatophytes

Annotations:

  • A group of plants with very long roots which allow them to access water from deep below the surface Example Plant: Creosote Bush Most commonly found in USA, Morocco - areas where there is shallow ground water Description: An evergreen shrub, 1 to 3 metres in height with dark green leaves. It is joined at the base and flowers have 5 yellow petails with a smell that is often associated with the smell of rain Adaptions: The long roots mean the plants are able to utilise water sources found deep underground, whilst continuing to photosynthesis throughout the dry summer months. It also has a radial root system. It had a smell and taste which deters animals from eating it and small leaves with stomata that only open at night time. The longer roots enable them to survive in constantly shifting sand dune areas. Their tough roots hold onto the dunes and anchor the plants down.
2.3.3 Halophytes

Annotations:

  • Plants that have adapted to conditions high in salt Example Plant: Saltbush Most commonly found in Australia, America and Eurasia. Description: Its an evergreen shrub that can be up to 10 feet tall but is typically between 2 and 4 feet. Its leaves are thin and 0.5-2 inches long with fruits from four wings at 90 degree angles, densely packed on a stem Adaptations: The plant excretes salt from salt glands onto its leaves when conditions become too saline Succulents such as picklewood have a high water uptake which compensates for the high salt content Some halophytes are kept dormant due to the high salinity but germinate when the rains come and reduce the salinity of the seeds Some root systems of halophytes can exclude salt The sale is compartmentalised in certain tissues of some halophytes, which act as salt storage away from growing cells Some halophytes develop succulence, which dilutes the level of salt in the plant and stores water for use during dry periods
2.3.4 Ephemerals

Annotations:

  • Plants found in arid environments that have seasonal rainfall patterns. they are defined by short life cycles (either annual or perennial) Example plant: Desert Paintbrush Most commonly found in Western USA Description - Perennial shrub that doesn't die after flowering or seeding and grows during spring when conditions are a an optimum. With a gray-green to purple-red herbage and 20-70mm linear leaves this 12-18inch high plant adapts to the desert environment. Adaptations: It grows quickly and then flowers and produces seeds The seeds are hard and are able to resist drought and high temperatures Seeds can survive indefinitely without water and only begin to germinate when the rain washes away anti-sprouting chemicals on their shell Come dormant during periods of drought of extreme temps but do not die Quickly flower into bright colours
2.4 Temperature

Annotations:

  • The mean annual temperature of most deserts is high - usually between 20 and 30 degrees.There are large seasonal variations in temperature - up to 50 degrees in summer and below 0 degrees in winterThere are also large diurnal ranges in temperatures throughout the day in deserts - up to 50 degrees in the day and below 0 degrees at night. This is because of the dry desert air and lack of cloud cover, allowing sunlight in and releasing hot air at night. The temperature in desert margins is a bit lower: 1) The mean annual temperature of most desert margins is lower than in deserts - between 10 and 20 degrees. 2) The temperature variations in desert margins are usually less extreme (between 10 and 35 degrees)
2.5 Soil

Annotations:

  • The soil in deserts is very dry and not very fertile1) Typically deserts are thought to be covered fully in sand however there are areas of bare soil and also soil underneath the sand. 2) Desert soil aren't very fertile because they don't contain very much organic matter because few plants grow there.3) The soils are often sandy (in areas with sand dunes) or stony (in rocky areas).4) The soils are very dry due to the low rainfall and high temperatures. The soil in desert margins is a bit better and it is more fertile due to more vegetation, it contains more water and is less sandy or stony than desert soil as there's more weathering.
2.6 Location

Annotations:

  • Deserts are predominantly found within and just outside the tropics. In the Southern Hemisphere they occur on the Western side of the continents. Major deserts are found in the following locations: North America - the Mexican desert stretching into Southwest USA, where the Sonoran, Moajve and Colorado deserts are found South America: the Atacama desert of coastal Peru and northern Chile Africa: The Namib and Kalahari deserts of the southwest and the Sahara Desert in the North (largest desert in the world) Asi: The Arabian, Iranian and Thar Deserts. The Thar straddles the southern part of the Indo-Pakistani frontier. Australia: The Australian Desert is usually divided into two, The Great Sandy (Western) and Simpson Deserts
3 Desert Landforms
3.1 Wind Erosion Landforms
3.1.1 Zeugens

Annotations:

  • Zeugens are long block-shaped ridges of rock. They're formed in areas where a layer of hard rock sits above layers of softer rock. If cracks form in the hard rock due to weathering processes such as frost shattering, the wind can erode through the cracks and into the softer rock beneath by abrasion. Again, the softer rock is eroded more than the hard rock, and ridges are formed). 
3.1.2 Yardangs

Annotations:

  • Yardangs are narrow, streamlined ridges that are usually three to four times longer than they are wide. Strong winds (blowing in one direcction) carry sand in suspension, which erodes rocks by abrasion. Softer rock is eroded faster than harder rocks and so ridges of hard rock are created (yardangs). These ridges are not always continuous.
3.2 Sand Dunes

Annotations:

  • 1) Sand dunes form when sand grains carried by suspension are deposited as the wind slows down.  2) Vegetation, rocks and other dunes slow wind down causing it to drop its load.  3) The shape and layout of dunes is affected by several factors: Speed, direction and consistency of the wind, the amount of sand being transported and deposited, the nature of the ground surface (e.g.rocky outcrops and uneven ground slow wind down), the amount and type of vegetation (deposition occurs downwind from vegetation and around its base, deforming the shape of sand dunes. Also pants root networks help to stabilise the sand and hold dunes in place). 
3.2.1 Barchan Dunes

Annotations:

  • (cresent shaped) 1) These are isolated dunes that develop from mounds of sand. They form in the direction of the prevailing wind as sand is deposited.  2) When the slope of the dune becomes too steep, sand avalanches occur, depositing sand at the base.  3) Swirling wind currents help to keep the slope steep.  4) The dune slowly moves forward in the direction of the wind. 5) If there's a lot of sand being deposited, many barchan dunes connect with each other to form a barchanoid ridge.  
3.2.2 Star dunes

Annotations:

  • 1) Star dunes have multiple steep faces caused by winds from many different directions. 2) As more sand is added they grow in height and become more defined causing them to appear in more of a star shape.
3.2.3 Seif Dunes

Annotations:

  • Seif dunes are long wiggly ridges of sand. They form from a barchan dune if a change of wind direction occurs. When wind blows from alternate sides, the 'arms' of barchan dunes are elongated and form a long wiggly line.
3.3 Wind Erosion Landforms
3.3.1 Badlands

Annotations:

  • Badlands are so called because they're difficult and dangerous to travel through. Badlands are vast areas of uneven terrain with deep, interlocking canyons, steep ridges, loose sediment and deep sands. Wadis and buttes can be found there. Erosion occurs from flood waters and wind.Softer rock layers and clay soil erode to form canyons, while hard rock layers are eroded less to form ridges.
3.3.2 Wadis and Alluvial Fans

Annotations:

  • A wadi is gully or ravine thats been eroded by seasonal rivers. Depending on the strength of the river, a wadi can have shalow or very steep valley sides. If theres a flat desert at the mouth of a wadis the water spreads out on to the wide plain. This leads to sediment being deposited in a fan shape as energy is dissipated, forming an alluvial fan.
3.3.3 Salt Lakes

Annotations:

  • Salt lakes form when desert rivers, which contain salt, are endoreic (terminate in an inland sea). The water can't leave the lake and evaporation is high resulting in a lake with a high salt content. Some salt lakes are ephemeral - evaporation is so high that they dry up at certain times of the year, leaving the salt behind to form salt pans (ground covered with salt).
3.3.4 Mesas and Buttes

Annotations:

  • A mesa is an isolated, flat-topped, steep-sided landform found only in arid places. They're a type of inselberg composed of a horizontal rock layer. They form in the same way as inselbergs. Buttes are smaller and narrower versions of mesas.
3.3.5 Inselbergs

Annotations:

  • Inselbergs are steep-sided hills that rise up from pediments. They're made of hard rock that is more resistant to erosion than the surrounding rock. The surrounding rock is eroded by water, leaving the hard rock standing out. Inselbergs can also be formed by wind erosion.
3.3.6 Pediments

Annotations:

  • Pediments are desert plains - gently sloping areas of rock (usually covered in a thin layer of debris). They are formed by the erosion of rock by sediment carried in sheet floods or small streams.
4 Desert Processes
4.1 Wind Processes

Annotations:

  • Wind can erode desert rocks in two ways:1) Deflation - the removal of fine, loose particles from the surface of rocks2) Abrasion - small particles being carried by the wind scrape off particles from the rock surface It then transports the eroded material by three processes: 1) Suspension is when very small particles are picked up and carried by the wind 2) Saltation is when small particles are temporarily lifted from the ground and bounce along it. 3) Surface creep is when ;arger particles are hit and pushed along the ground by particles being moved by saltation More particles are transported when the wind is strong and comes from a consistent direction. When theres a reduction in wind speed, the wind drops some of its load - this is deposition
4.2 Rivers
4.2.1 Endoreic

Annotations:

  • Endoreic rivers terminate inland in the form of an inland sea or delta. For example, the River Jordan terminates in the Dead Sea - an inland sea in the Middle East.
4.2.2 Exogenous

Annotations:

  • Exogenous rivers have a source outside of the desert margin. They flow throughout the year despite evaporation reducing their volume. For example, the source of the Colorado River in the USA is in the Rocky Mountains. It flows through the Sonoran Desert and the Grand Canyon to the sea.
4.2.3 Ephemeral

Annotations:

  • Ephemeral river flow intermittently or seasonally after rainstorms. For example, the Todd River in the Simpson Desert (Australia) only flows a few days a year and remains a dry river bed for the rest of the year.
4.3 Weathering
4.3.1 Salt Weathering

Annotations:

  • 1) Salt weathering is caused by saline water, which comes from rainfall or from groundwater thats drawn up to the desert surface by evaporation. 2) This saline water enters pores or cracks in desert rocks.  3) The high temperature in hot deserts causes the water to evaporate, forming salt crystals. As the salt crystals form they expand, exerting pressure on the rocks.  4) As more evaporation occurs the salt crystals expand even more. This increases pressure in the rocks, causing pieces to fall off.
4.3.2 Frost Shattering

Annotations:

  • 1) Frost shattering (freeze-thaw weathering) occurs in areas where there's moisture and temperatures that fluctuate above and below freezing.  2) Water from rainfall enters the joints and crevices in desert rocks. 3) At night, if the temperature drops below 0 degrees the water in the cracks freezes and expands. 4) over time, repeated freeze-thaw actions weakens the rocks and causes pieces to fall off.
4.3.3 Thermal Expansion

Annotations:

  • 1) Thermal Expansion is caused by the extreme dirurnal range in deserts. 2) During the day when its hot rocks expand and at night time when it gets cold again they contract. 3)Sometimes outer layers of the rock flake off because they warm up and cool down at a faster rate than the inner layers. This is called exfoliation.  4) Individual grains may also fall off because these different minerals within the rock expand and contract at different rates. This is called granular disintegration.
4.3.4 Wetting and Drying

Annotations:

  • 1) Some desert rocks contain clay. When clay gets wet it expands and the pressure caused by this breaks off rock fragments during the drying process.
4.4 Flooding

Annotations:

  • Most rainfall in hot deserts is light and infrequent. However, sometimes there may be a sudden, high intensity rainstorm that lats for a short period of time. This can cause two types of flood in a desert - flash floods and sheet floods.
4.4.1 Flash Floods

Annotations:

  • 1) A flash flood is a sudden, strong and rapid dlow of water through a channel. 2)They occur because heavy rainfall cant be absorbed by the dry, hard desert soil - so the runoff collects in channels and flows rapidly downhill. 3) Flash floods have enough energy to transport large pieces of desert rock by traction. They also transport pebbles, gravel and sand by suspension and saltation. The rocks in the water are eroded into smaller fragments by attrition (rocks smashing into each other). 4) The material carried by flash floods erodes the channels by abrasion, making them deeper. 5) At the mouth of a channel, the flash flood waters spread out, slow down and soak into the ground (unless they meet another body of water)
4.4.2 Sheet Floods

Annotations:

  • 1) A sheet flood is a slow-moving, even flow of water over land (i.e. it isn't confined to a channel).2) Like flash floods, they occur after a period of intense rainfall, where water collects across the dry, impermeable desert floor and flows down gentle slopes as a sheet of water. 3) Sheet floods have less energy than flash floods, but can still transport pebbles, gravel and sand by suspension or saltation.4) The material carried by sheet floods erodes the desert surface by abrasion.

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