Hot desert systems and landscapes

Inputs 

- water enters the system due to infrequent rainfall 

- sediment is able to enter the sytem carried in wind or water 

- energy in the system is sourced from sun, wind or rain

Flows and transfers

- sediment is moved through the system by: weathering, erosion, transportation, and deposition

Stores 

- Landforms are stores of sediment, water may be stored in the groundwater, or in rivers (although overland stores of water are often intermittent)

Outputs 

- Water rapidly evaporates, or leaves the system as runoff

- sediment may be carried out of the system by wind or rain

Deserts are open systems; meaning energy and matter both leave and enter. When the system is in a dynamic equilibrium it means that inputs and outputs are balanced, however if this is disrupted feedback loops will be triggered. This could be:

Negative (balance is restored) Eg. temp increases so evaporation increases and the ground around bodies of water becomes drier. this dry sediment is more easily eroded and may form dust clouds, this dust block solar radiation, cooling the area. 

Positive (change is amplified) Eg. wind hits an obstacle, slows down and drops the load. this dropped sediment begins the start of a dune. the small dune forms a larger obstacle to the wind, causing more sediment to be dropped, so the dune grows. 

Positive feedback loops can create new equilibriums. Eg with the example of the dune, the growing dune may change the wind flow until it can no longer lift enough sediment to the top of the dune. At this point the dune will stop growing. 

Hot desert environments typically run in parallel belts north and south of the equator in arid mid to low latitude locations along the tropics. 

Deserts which are not located along the tropics are usually found in continential interiors, adjacent to mountain range, or near cold ocean currents (as shown in causes of aridity). 

Around the edges of hot deserts semi arid regions can also usually be found. 

Climate 

The mean annual temp of arid deserts is usually high- between 20-30 degrees C. However there are large seasonal variations in tempurature- above 50 in summer and below freezing in winter. There is also a large diurnal range (daily)- up to 50 during the day and 0 at night. this is a result of the dry desert air being unable to block sunlight in the day or trap heat at night. 

In semi arid deserts on desert margins the mean annual tempurature is usually lower- between 10 and 20, and tempurature ranges seasonally and daily are less extreme- between 10 and 35 degrees. 

Water balance and aridity index 

In hot deserts mean annual precipitation is lower than potential evapotranspiration- meaning that there is a water deficit. The difference between these values (the size of the water deficit) gives the aridity index, used to indicate a deserts level of aridity. An arid desert recieves less than 250mm of rain a year and has a value below 0.2 on the aridity index. Semi arid areas recieve up to 500mm of precipitation a year with up to 0.5 on the aridity index. 

Vegetation 

The biomass (total amount of living matter) in deserts is low because the lack of water makes it difficult to adapt to the conditions to survive. The amount of vegetation with deserts varies- sometimes with none where sand dunes are present, whilst small shrubs, grasses and cacti can be found in other areas. 

Plants become adapted to live in the arid environments eg Cacti have become specialised with enlarged stems to store water, a waxy coating to reduce transpiration and protect from strong winds, small spiny leaves protect the plants from being eaten, and long and shallow roots absorb water from a large area, long taproots are able to reach deeper water. 

Generally the further away from the centre of the desert you travel the more vegetation can be found as there is a large supply of water. 

Soil

In deserts areas of bare soil can be found, if not soil can be found beneath sand. soils are not very fertile as they do not contain much organic matter (which returns nutrients to the soil) this means that few plants can find the nutrients to grow. 

Soils usually develop to be infertile with a thin soil profile which tends to be alkaline and quite saline- this is due to the capillary action bringing mieral salts to the surface of the soil. 

Soil development is slow in desert environments due to the lack of moisture, high tempurature, high evaporation, sparse vegetation, limited organic material. 

In desert margins soil is more fertile as there is more vegetation, and soil contains more moisture. Soil is also less sandy and stony because there is more weathering. 

Climate affects soil: Infrequant rain and high evapotranspirationv (due to high temp) means that soil is dry. Evaporation draws salt to the surface with water through capilary action in the soil- however salt cannot evaporate and so is left behind on the surface soil. 

Climate affects vegetation: the lack of rain means vegetation cover is sparse. 

Soil affects climate: soil erosion leads to dust clouds- particals can prevent water molecules condensing into clouds which reduces rainfall. 

Vegetation affects soil: Lack of plant cover and decomposition means soil lacks nutrients. sparse vegetation cover increases the risk of soil erosion, as there the soil becomes less stable. 

Vegetation affects climate: a lack of plant cover means there is little transpiration- inhibiting cloud production. With few plants there is little evaporative cooling near the ground, adding to high tempuratures. 

Soil affects vegetation: Desert soils are dry, salty and low in nutrients so plants have to be highly adapted to grow. soils are thin and prone to erosion so seeds transported to the area cannot start growing or conolise the area. 

Global Atmospheric Circulation 

Hot deserts are most commonly found at 20-30 degrees north and south of the Equator, this is where the downward circulation of air from the Ferrel and Hadley cells form a band of high pressure. This means that little air is able to rise, cool and condense to form clouds, and so these areas have low levels of rainfall. 

Trade winds from the tropics to the equator curve west (due to the Coriolis effect- the middle of the earth spins faster than at the tropics as the circumference is highest at this point) and winds flow from high to low pressures. For example in Saharan africa north easterly trade winds begin over land and so do not carry any moisture. 

Continentality 

Coastal areas have a more moderate climate with little annual variation in tempurature, and recieve higher levels of rainfall, however further into a continent temps become more extreme and drier. This is because as an air mass moves from the ocean over a continent it looses moisture as rain, but cannot pick it up over land (few significant bodies of water for evaporation) so inland areas are more likely to become arid. EG Turestan desert 

Relief 

Around the mid-latitude areas there are several very dry areas on the leeward side of mountain ranges. these often experiance the rainshadow effect where moist air is brought inland be prevailing winds, and mountain ranges force the air upwards so it cools and condenses to form relief rainfall on the windward slopes. When the air passes the summit it begins to drop and clouds begin to evaporate. this leads to a drop in humidity with clear skys and low rainfall- to form an arid area. EG the atacama desert on the pacific coast of Chile from prevailing south earst trade winds over the andes. 

Cold ocean currents 

Wind moving over cold water is cooled, causing humidity to increase, this moisture in the air forms fog and an off shore mist. the land heats up faster than the sea, creating a gentle onshore breeze moving mist onland. intense heating from the sun over land causes fog to evaporate, and as the air was cool it is unable to hold much moisture and so cloud formation and rainfall is rare. EG atacamba desert and namib (southern africa). 

Insolation

Insolation is the solar radiation that reaches earth, this is strong in deserts where, there are few clouds to block its path, in deserts in the mid to low latitudes the suns rays hit the earth at high angles and the higher the sun is in the sky the more raditation that is transfered to the ground. 

High levels of insolation cause high daytime tempuratures and the cloudless skies means that nights are cold. These rapid Diurnal tempurature changes can cause steep pressure gradients which drive strong winds. High inputs of energy from the sun can cause water to evaporate quickly, leaving ground dry and sediment more mobile. 

Runoff 

Rain is infrequent and unpredictable, and when it does rain it often comes with intense storms causing a high input of energy into a local area. There are few plants to intercept rain so surface runoff is high, the ground surface is also baked by the sun, making it difficult for water to infiltrate into the soil as soil mosture, increasing runoff and the chances of fast flooding. 

Winds 

Winds are created by air moving from high to low pressure areas. where the pressure gradient is high (big difference between high and low pressures) winds are most likely to be strong. In some areas wind constantly blows in the same direction (prevailing wind) this causes more sediment erosion and transport than winds which change direction frequently. 

A lack of vegetation means that wind can blow for long distances without obstructions and so increases in power. 

Sediment sources 

Much of the sediment in deserts comes from acient processes. Before they came deserts many of these areas had completely different climates- where it was wet enough for there to be rivers and lakes. This meant sediment could be transported into the system via rivers and depositited on lake beds. When these dried up later, this sediment became available for dunes and other depositional landforms. Some areas which were once underwater have marine deposits from many years ago. 

Sediment is still brought into desert systems today. This may be from a parent rock which has been weathered. Rivers can also bring sediment into deserts, some only flow occasionally (emphemeral) so when they dry up the load is left on the riverbed. If rivers food load is depositied on flood plains. Wind can also carry and deposit sediment.. 

Deserts themselves can also be sources of sediment themselves. Dust clouds form when strong winds blow particals out of the desert. 

Sediment Cells the movement of cells is known as a sediment cell. Each of these cells have a series of inputs (sources of sediment), transfers (movement of sediment by wind and water), sediment sinks (areas of deposition) outputs (sediment transported out of the desert) 

Sediment budgets 

This considers the relative amount of sediment in each component of the desert system, in order to discover the difference between sediment going into and out of the area. if sediment inputs are higher than outputs the sediment budget is positive, but if sediment inputs are lower than outputs the budget is negative. 

The sediment budget of an area determines what type of landscape can be formed. When the budget is positive- meaning there is more deposition than erosion, the landscape will be dominated with depositional landforms. This is the same for a negative budget with erosional landscapes being dominant. 

Weathering- the decay or break down of a material where it is. 

Mass movement- the movement of material downhill under the influence of gravity, may also be assisted by rainfall. This may be through rock slides or rock falls. 

Erosion-refers to processes which wear away the land surface by mechanical action, the material is then removed from the site via transportation processes. 

Transportation- the processes whcih move material from the site where erosion takes place to where it is eventually deposited, this occurs via wind and water in deserts. 

Deposition- occurs when the transporting process no longer has enough energy to transport its load, so it is dropped. 

Mechanical:

Thermal fracture- this type of weathering results from the rapid heating and cooling of rocks i desert environments- due to the large diurnal range. This heating and cooling cycle, with some rocks heating to 80 degrees in the day and dropping to 10 degrees at night, this allows the rock to expand during the day and then contract while it cools at night. This causes a weakening of the rock and it begins to fracture. 

Granular disintigration- the light and dark colours within rocks heat and cool at different rates and so contract and expand at different extremes. This causes stresses to be made in the rock and it breaks down in individual grain. 

Block disintegration- thermal fracture of well jointed rocks leads to rocks breaking down along joints which are the main points of weakness. 

Exfoliation- the peeling of a surface layer of rock, due to rock layers at the surface expanding and then contracting. This causes the surface layer of the rock to crack and peel off from the main rock, and layers begin to become separated. 

Chemical:

Crystal weathering- when the salt in rocks crystalises out of a solution, due to high temps drawing saline groundwater to the surface, and then this is evaporated leaving salt crystals on the surface. When these crystals grow and accumilate between pores in the rock it creates stress and the rock begins to disintegrate. 

Hydration- some rocks are able to absorb available moisture, which causes rocks to physically swell after combining with other minerals in the rock. This swelling causes stresses in the rock and makes it more vulnerable to other mechanical weathering. 

Oxidation- the break down of rocks contianing iron by oxygen and water which leads to a red staining on rocks, the iron also reacts with oxygen to form iron oxide which is weaker so the rock is more partial to crumbling. 

Carbonation- occurs when carbon dioxide in the atmosphere dissolves in rainwater forming a weak acid. This reacts with the rocks contianing calcium carbonate - such as limestone, so rocks slowly dissolve. 

Erosion:

Deflation- wind is able to pick up and move fine particals from the surface, however it doesnt have the energy to move large heavy particals, so only large particals are left on the surface. 

Abrasion- refered to as sand blasting or the sandpaper effect where the material carried by the wind hits the exposed rock face and creates a range of erosional features. 

Transportation:

Suspension- the smallest particals (less than 0.15mm) are picked up and carried in the moving air

Saltation- smallish particals (0.25 mm) are picked up by wind, before being dropped again in a bouncing motion, when they hit the ground they may hit other particals causing them to also jump up and begin bouncing. 

Surface creep- larger particals are hit by saltating particals, causing them to slowly roll or creep across the surface. 

Deposition- the velocity of the wind decreases and it no longer has the energy to transport. 

Exogenous- rivers originate outside of the desert in highlands or more humid environments, and flow into the desert. Often the flow of warter is large enough to maintain a flow all year, but they also respond to seasonal rainfall. This accounts for the largest volumn of water to the environment. 

Endoreic- rivers which flow into deserts but terminate inland in a lake or inland sea. Such as the river Jordan terminating at the Dead Sea. 

Ephemeral- rivers flow intermittently or seasonally flowing a rainfall event, which are generally particularly heavy in desert environments. 

Sheet flooding- a sheet flood is a slow moving even flow of water over land- meaning that it isnt confined to a channel. Similar to a flash flood they occur after an intense rainfall period when water flows overland due to the baked impermiable desert floor. 

Have less energy than flash floods, but are still able to transport small amounts of sediment, which erodes the desert surface by abrasion. 

Channel flash flooding- a sudden strong flow of water through a channel. These occur in the desert because water is not absorbed by the baked soil, so instead the runoff collects in channels. 

These floods have enough energy to transport large pieces of desert rock by traction, and smaller sedements by suspension. The rock in the water are also made smaller by abrasing against each other, and channels become significantly abraded. When they reach the mouth of the channel the waters are able to spread out and soak in to the ground. 

Deflation hollows- depressions in the ground. Created by the removal of large amounts of loose particals by the wind. These can be large and several metres deep. If the hollow becomes deep enugh it may reach the water table (preventing further deepening), and can then form a salt lake or salt pan. 

Eg the Quattara Depression in Egypt is 134m below sea level and covers over 18 000 km2. 

Desert pavements- surfaces of interlocking stones, packed together tightly, and resting on top of a finer desert surface such as sand. They form when the wind blows away the silt and sand from the desert surface- leaving gravel and rocks which are too heavy to be removed by deflation. 

Ventifacts- individual stones wih one or more smooth sides which have been abraded by small particals carried in the wind, the side of the stone facing the most common prevailing wind side is abraded the most which leaves a flattened face. If the rock moves or the direction of prevailing wind changes then another side of the rock will be abraded, leaving a combination of smooth faces and sharp edges on the rock. 

Yardangs- narrow streamlined ridges which are usually three times longer than they are wide. Strong prevailing winds carry sand in suspension which abrades the rocks. Softer bands of rock are eroded faster than harder bands so rdges of hard rock (the yardangs) are left. The end of the yardang facing the wind is usually widest and tapers down towards the end. 

Zeugen- long blocked shaped ridges of rock formed where bands of hard rock are sitting above a layer of softer rock. When cracks are eroded in the hard rock due to weathering, the wind is able to abrade through the cracks to the softer rock, which is eroded faster forming ridges or top heavy mushroom pedistals. 

Sand dunes- form where sand grains are carried by suspension and are depositied when the wind speed slows down, this could be due to vegetation, other dunes or rocks. The formation of the dunes are affected by: the speed, direction and consistency of the wind; amount of load carried; nature of the ground surface; the amount and type of vegetation. 

Barchan dunes- isolated dunes which develop from mounds of sand, forming in the direction of prevailing wind as sand is deposited. When the slope of the dune becomes too steep avalanches deposit sand at the base of the dune downwind. The dune moves forward in the direction of the wind. 

Seif dunes- long wiggly ridges of sand. They are formed from barchan dunes if a change of wind direction occurs. When wind blows from alternate sides, the 'arms' of the barchan dunes are elongated and there ridges become a wiggly line. 

Wadis- steep sided, wide bottomed gorge like valleys, which has been eroded by seasonal rivers, or by rivers in the past. Depending on the strength of the river the wadi may have very steep or shallow valley sides. Because of its form it is able to fill up with rain water very quickly. 

Eg Wadi Rum, Jordan. 

Alluvial fans- if wadis contain an empemeral or perennial stream, depositional material may build up on pediments due to the loss of energy when the water reaches a gentler slope. This creates alluvial fans as wadis run to pediments. The material is graded with the heaviest closest to the wadi mouth. 

Bahadas- in areas where several wadis are close together, alluvial fans may join to form one bahada which is a uniform slope of sediment. 

Pediments- where there is a distinct break in gradient as highlands meet gentle lowlands a pediment is found, wich is a gently sloping area of rock and debris. They are formed by the deposition of material washed down from the uplands, which are depositied when the flow of water is slowed by the decline in its angle. 

Playas- shallow extremely flat depressions at the low point of a pediment. Temporary lakes periodically drain into them following a rainfall event. This water quickly evaporates and leaves a layer of salt and silt. 

Inselbergs- rounded steepsided hills which rise abruptly from a lowland plain. They are made of hard rock which is more resistant to erosion. The surronding rock is eroded by water which leaves the hard rock standing tall. Eg. Ayers rock. 

Mesas- isolated flat topped plateaus with steep slopes and cliffs on at least one face, which fall away into Wadis. 

Buttes- smaller than mesas, beieved to be the remains of disected plateaus where water has eroded all but a thin pillar. 

Eg, monument valley. 

No desert landscape is the same; due to the unique interaction of processes, time and landforms. Some desert landscapes may be dominated by a partcular type of landform, whilst others are made up of a combination. Landscapes may be dominaed by erosional landforms, depositional landforms and aolian or fluvial. 

The processes which operate in desert systems have the power to create or change landforms, meaning that deserts change over time. 

Changes in inputs changes a landscape, and may result in a shift from one dominant landscape to another- eg erosional to despositional due to changes to the sediment budget. Landforms created in the past also experiance change. Eg a Wadi may get wider every time it is flooded. 

This means that landscapes are made up of mixtures of landforms representing different periods of change. Eg some landforms may have been created when the landscape had a more humid climate (eg Wadis). Changes also occur across a range of time scales. Eg abrasion is slow and gradual, wheras episodic water erosion is relativly quick. 

Landscapes in hot deserts often change slower than in other climates, as many processes require water, so processes operate slow and episodically. 

18 000 years ago (last glacial maximum)- aridity was very widespread, in the north were cold deserts, south desert distribution was similar to today but were far more extensive.

8000 years ago (interglacial period)- warmer conditions and more humid, forests were were warm and aridity fell dramitically with many currently arid areas becoming grasslands. 

5000 years ago (present)- conditions much as they are today, with conditions becoming more arid. 

reasons for change:

• current extent of deserts has remaind unchanged for 5000 years- showing a stable climate with few majr shifts or changes. The atmospheric circulatory system has also remained unchanged- so there has been no singnificant changes in the location of deserts. 
• this contrasts the maximum of the last glacial period where there was sigificant climatic functions leading to a large change in extent and distribution of deserts. 

Climate change- causes desertification by reducing rainfall and increasing tempuratures. 

Reduced rainfall- if there is less rain then surface water and groundwater will be reduced (isn't replenished after evaporaing), this means that less water is available for the uptake by vegentation, this causes a reduction in plant growth. The roots of plants and trees are which bind soil together, without these roots soil becomes eroded more easily and land degration leads to desertification. 

Higher tempurature- as tempuratures increase so does the rate of evapotranspiration, this leads to soils drying out and lowers surface water levels which causes soil erosion along with loss of vegetation. 

Drought- desertification is ultimatly the result of land degration, and so although drought is only a short term condition it leads to soil degration and desertification in the long term. This is because drought leads t a decline in agricultural production and areas may become over grazed by animals, with farmers having to extend to more marginal areas. When this land is mis managed it may lead to lad abuse and soil degration. 

Overgrazing- more animals grazing on smaller patches of land as a cause of increased demand for food. This reduces vegetation and so leads to soil erosion from a lack of plant roots. Trampling from a large number of animals compresses and breaks down the soil and makes erosion more likely. 

Overcultivation- as a result of population pressure, reduces the soil productivity as the over expliotation of the soil leaves it without enough nutrients to support plants, however without plants the soil is easily eroded. 

Deforestation- the removal of trees and roots means soil is again more vulnerable to erosion. Forests may be cleared for farmland or to use wood as fuel. 

Irrigation- depletes surface water and may overexploit aquifiers, this lowers the water availability for plants- and so plant cover decreases. If too much water is used to irrigate crops the excess water sinks into soil and raises groundwater levels- however if groundwater is saline it may bring high concentrations of salt to the surface, meaning it is harder for plnts to survive in saline soil. 

Population pressure- increases the need for food, water and fuel which leads to the above activities which further increases desertification. 

Ecosystems:

• As land becomes less fertile, less plants can grow reducing the amount of organic matter going into the soil- which reduces the nutrients available for further plant growth. Less animals can be supported so biodiversity decreases. 
• Desertification changes the distribution of species, with some species dying out or migrating to less degraded areas. In places where desertifiction is new, species which are previously adapted to the environment and can outcompete the origional species. 
• Through desetification carbon stored in the soil is released to the atmosphere, increasing climate change. 

Landscapes:

• Spread of desert conditions may cause increased formation of sand dunes as vegetation can no longer trap loose material (making sandstorms more likely). Moisture (unsheltered by plants) evaporates more quickly.
• Sediment is no longer binded by vegetation making erosion faster and landslides more likely. 

Populations:

• A reduction in the fertility of land causes a decline in agricultural productivity. Meaning farmers struggle to make a living or food for the family. This causes migration, and pressure is placed on the lands they migrate to and could lead to overcrowding in urban areas. 
• When people are unable to migrate it may lead to famines, and loss of lifes. 
• Dust produced from soil erosion may cause respiratory issues. 
• Lower rainfall levels may lead to a reduction in clean water, and cause the spread of water borne diseases. 

By 2050 tempuratures in drylands are predicted to have risen between 2-5 degrees. This will accelerate the risk of desertification. As rates of evapotranspiration increase, soil water will decrease and drought will become more common. Extreme weather events such as drought and intense rainfall will increase. 

This will lead to knock on impacts such as:

• soils less productive, settlement and agriculture less viable causing out-migration. 
• population pressure in ciies and increased desertification. 
• changes and adaptions made to conserve soil or change agricultural practices. 

The future of desert populations depends on the the actions taken to mitigate and adapt to the impacts of desertification and prevent even more land being degraded. 

If action isnt taken to mitigate desertification or if action isn't funded or managed properly the rate of desertification may increase. The continued reduction in agricultural productivity would increase malnutrition and famine, and a lack of food security would be a barrier to development. Conflict may also be experinced between countries who suffer from food and water scarcity. 

However further desertification could be prevented and some areas could be returned to their previous state if action is taken soon. Locals can plant vegetation to bind soil and act as a windbreak to loose sediment. Farmers may utilise crop rotation and improve irrigation. 

Globally response to climate change has the power to make a large effect, as if GHG emmisions can be limited tempurature rises and rainfall declines can be limited. 

Local can change lifestyles in order to adapt- with farmers diversifying to keep livestock which is less sensitive to desertification than crops, and so produce could remain relitivly reliable. Improvements to agricultural tech can help to increase productiity, and increase produce per acre. Eg by using fertilisers.