Key Terms: Deserts as Natural Systems (AQA A Level Geography): Revision Note

Global desert systems - key terms

Arid region – A geographical area receiving less than 250mm of precipitation annually, often with little vegetation, water, or soil fertility.

Desertification – The degradation of land in dry regions, often caused by human activity and climatic variations, leading to the expansion of desert conditions.

Drylands – Regions characterised by low rainfall, covering over 40% of Earth’s land surface and including both arid and semi-arid zones.

Dynamic equilibrium – A state in which desert systems balance inputs and outputs, with changes being regulated over time.

Erg – A sandy desert landscape with dunes, covering roughly 30% of deserts and associated with low rainfall levels.

Hamada – A desert landscape where wind erosion has removed fine particles, leaving behind bare rock or gravel plateaus.

Hyper-arid - areas which receive less than 25 mm of precipitation a year.

Open system – A natural system, like a desert, that has inputs (e.g. solar energy, sediment), outputs (e.g. water loss), and internal flows.

Reg – A desert surface made up of angular rock fragments or gravel embedded in clay or sand.

Semi-arid region – A dry region that receives between 250–500mm of rainfall per year, often bordering true desert areas.

Sub-tropical high – A zone of high atmospheric pressure found around 30° north and south of the equator, associated with sinking dry air and minimal rainfall.

Characteristics of hot deserts - key terms

Albedo – The reflectivity of a surface; desert areas with salt pans or dry lake beds reflect more solar energy, often reducing surface temperatures.

Aridisols – A type of soil commonly found in deserts; typically dry, red, shallow, and low in organic content.

Calcrete – A hardened soil layer formed from calcium carbonate, found mainly in semi-arid regions.

Deflocculation - Deflocculation is to reduce or break up from a flocculent (stuck) state : disperse into particles. Clay particles will clump/stick (flocculate) together, in deflocculation, particles will repel and separate.

Diurnal range – The temperature difference between day and night; in deserts, this can exceed 30°C due to low humidity and lack of cloud cover.

Duricrust – A hardened layer within or on top of desert soils, formed by mineral deposits like calcium or gypsum, that inhibits root growth.

Ephemeral plant – A short-lived plant that completes its life cycle quickly after rain before entering dormancy again.

Halophyte – A plant adapted to saline environments, capable of excreting or avoiding salt.

Mesquite – A phreatophyte plant with deep roots that reach groundwater, common in semi-arid environments.

Oxidation - Hot desert soil, is where oxygen reacts chemically with iron-rich rocks to produce the distinctive red colour of the soil.

Phreatophyte – A plant with deep roots that tap into the groundwater table, adapted to arid conditions.

Pyrophyte – A plant adapted to survive or thrive in fire-prone environments, sometimes relying on fire to reproduce.

Succulent – A plant that stores water in its fleshy leaves, stems, or roots to survive extended dry periods.

Xerophyte – A drought-tolerant plant with adaptations such as small leaves, thick waxy cuticles, and extensive root systems to conserve water.

Aridity and water balance - key terms

Aridity index – A numerical ratio comparing precipitation to potential evapotranspiration. Values below 0.2 indicate arid regions.

Cold ocean current – A current that reduces evaporation and rain along coastal deserts by chilling onshore air masses, often leading to fog rather than rain.

Continentality – The climatic effect experienced by inland areas far from oceans, often resulting in extreme temperature variations and dry conditions.

Evapotranspiration – The combined process of evaporation from surfaces and transpiration from plants, typically exceeding rainfall in deserts.

Hadley cell – A global atmospheric circulation cell that drives sinking air in the subtropics, creating high-pressure zones and arid climates.

Intertropical Convergence Zone (ITCZ) – A low-pressure belt near the equator where trade winds converge, affecting seasonal rainfall in semi-arid zones.

Orographic effect – Also called the rain shadow effect; occurs when moist air is forced over mountains, losing its moisture on the windward side and creating dry conditions on the leeward side.

P/PET ratio – The ratio of annual precipitation (P) to potential evapotranspiration (PET); used to define levels of aridity.

Rain shadow – A dry area on the leeward side of a mountain range where descending air loses moisture, contributing to arid conditions.

Runoff – The movement of water across the land surface, often rapid in deserts due to intense rainfall and poor infiltration.

Salinisation – The accumulation of salts in soil, often caused by evaporation, which can make the soil toxic and impermeable.

Thornthwaite index – A method used to classify climates based on the relationship between precipitation and potential evapotranspiration.

Water balance – The relationship between precipitation, evapotranspiration, runoff, and soil moisture storage. Deserts usually have a water deficit.