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1

Deserts definition

- Areas where there is a substantial deficit of water, because they receive small amounts of precipitation and have high levels of aridity

- Shortage of moisture and temperatures that determines characteristics of the soils, vegetation, the animals, the landscapes and human activities of such areas

- can be defined by the water balance =  The different between water from precipitation and losses due to evapotranspiration and changes in groundwater storage

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Inputs consist of :

- Energy provided by the sun through insolation, wind energy and water energy

- Sediment provided by weathering, mass movement and erosion by both wind and water

- Changes resulting from human activity, such as desertification + climate change

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Outputs consist of :

- Desert landforms both from erosion and deposition

- Losses through evapotranspiration and river flows

- Sand particles blown away by winds

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Stores

- The extensive areas of sand that occupy about 30% of deserts and residual areas of water (rivers and lake), which are small in number.

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Transfers

Rresult from the actions of wind and water e.g water + wind erosion

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Dynamic equilibrium

- Hot desert are dynamic (constantly changing) places

- Therefore the system is in a state of dynamic equilibrium

- Change occurs to upset the balance of the system e.g desertification

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Positive and negative feedback

- System adjusts by a process of feedback

- Can be positive (progressively greater change from the original condition of the system) or negative (system is returned to its original condition)

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Deserts as an open system

- Inputs of energy from the sun, precipitation of wind

- Erosional processes lead to the components of erosional landforms and landscapes

- Wind and water transport leads to competes of depositional landforms and landscapes

- Wind and water transport leads to outputs of water and wind removing sediment from the desert + clear skies which allow large amounts of energy to re-radiate back to space

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Global distribution of deserts

- ⅓ of the earth’s land surface is arid in some form, often referred to as ‘deserts’ of e.g Sahara, Namib, Kalahari, Atacama, Patagonian, Arabian, Thar, Mojave, Sonara and Australian deserts .

- Generally run in parallel belts North and South of the equator in hot arid and semi-arid mid and low latitude locations. Generally lie between the Tropic of Cancer and Capricorn – not on the Equator itself

- Due to high latitude or high altitude, there are cold deserts or coastal deserts such as the Atacama and Namib which have different levels of humidity from those deserts of continental impacts.

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Characteristics of hot deserts : Climate

- Extremes characteristics desert climates

- Wide annual range (35°C summer and 10°C winter

- Wide diurnal range (30°C to 0°C on average).

- Clear skies lead to rapid heat loss at night and high levels of insolation

- Low humidity

- Desert margins have a wide climatic variation as seasonality comes into effect

- Low rainfall (generally <250 mm annually)

- rain comes in the form of unpredictable, intense cloud bursts

- Rainfall can be as low as 15mm

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Characteristics of hot deserts : Soils

- Arid soils (aridisols)

- Two main categories within the aridisols are sierozems (form in desert shrub areas where there is a little more vegetation) and raw mineral soils (where physical and chemical weathering forms a coarse-textured soil)

- Slow rates of weathering and lack of vegetation means soils in hot deserts are shallow

- Unproductive

- limited vegetation

- Saline (evaporating moisture leaves salts behind) and alkaline

- Calcium is concentrated near the surface due to capillary action where moisture in the soil moves upwards

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Water balance

- Compares mean annual precipitation received with the mean annual potential evapotranspiration

- All deserts have a negative water balance, where evapotranspiration is in excess of precipitation

- P. = Q + E+/- S

 

 

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Potential evapotranspiration

- amount of water that could be evaporated or transpired from an area, given sufficient water for this to happen

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Aridity index

- Ratio of mean annual Precipitation and mean annual potential evapotranspiration

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Classification of deserts (aridity index)

- extremely (hyper) arid (<50mm precipitation per year)

- arid (<250mm precipitation per year)

- semi-arid (250-500mm precipitation per year)

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Causes of aridity : Atmospheric circulation

- At the equator there are large amounts of solar radiation

- The sun is directly overhead

- Air in contact with the land is heated

- It rises, cools, condenses

- Precipitation forms

- Rising air is replaced with air from the North and South, creating low pressure areas (inter-tropical convergence zones)

- Rising air tracks polewards and at 20-30° North and south this now cooler air decends

- It warms as it descends, it expands, there is little cloud, clear skies and aridity results in these latitudes

- Cells of circulating air is known as the Hadley cells

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Causes of aridity : Distance from the sea

- Distance from the sea can lead to temperature extremes

- Inland reassume distance from the sea are generally much drier as land heats quickly during the day due to the lack of clouds inland + As air masses move over a continent, they will lose moisture - this causes the air to become very dry

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Causes of aridity : Relief

- Areas on the leeward side mountains lie in rain shadow area

- Reasons for this are : 1. air rising over the mountains cools, condenses and expends much of its rainfall

- 2. Air descending on the downward side warms and the relative humidity is lowered, reducing the rainfall

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Causes of aridity : Cold ocean currents

When winds blow towards shore their lower layers are cooled by contact with the water.

This causes mist or fog to form offshore. This mist may roll inland, but it quickly evaporates again in contact with the warm surface. 

The air has lost some moisture and it becomes drier as it warms making rain even less likely

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Characteristics of hot deserts : Vegetation = Succulents

Succulents = Plants that store water within their tissues eg prickly pear

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Characteristics of hot deserts : Vegetation = Phreatophytic

Phreatophytic = plants have long tap roots, which penetrate downwards until they reach deep sources of groundwater

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Characteristics of hot deserts : Vegetation = Halophytes

Halophytes = salt-tolerant plants, common in the vast plains in inland desert basins

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Characteristics of hot deserts : Vegetation = Ephemeral

Ephemeral =

- have a short life cycle and may form a fairly dense stand of vegetation immediately after rain.

- They evade drought, and when rain falls they develop vigorously and produce large numbers of flowers and fruit.

- The seeds then lie dormant until the next wet spell, when the desert blooms again

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Characteristics of hot deserts : Vegetation = Xerophytic

Xerophytic plants =

- drought-resistant adaptations.

- Transpiration is reduced by means of dense hairs covering waxy leaf surfaces.

- They close their stomata to reduce water loss and either roll up or shed leaves at the beginning of the dry period.

- Some xerophytes - the succulents - store water in their structures.

- Most xerophytes have extensive shallow root networks to search for water.

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