Cambridge International AS & A Level Geography (9696): Topic 10 – Hot arid and semi-arid environments – Study Guide

This guide covers topic 10, Hot arid and semi-arid environments, sections 10.1 to 10.4 of the Cambridge International AS & A Level Geography (9696) syllabus for examination in 2025 and 2026. It is one of four Advanced Physical Geography options assessed in Paper 3 (1 hour 30 minutes, 60 marks). The syllabus says A Level candidates study two of the four physical options; in Paper 3 you answer on two options, and each has one 10-mark structured question and a choice of 20-mark essay questions.

Use it with the Cambridge A Level Geography hub and the 9696 checklist. For a condensed version, see the revision notes, and test yourself with the practice questions. Weathering builds on the Core Physical Geography guide.

Coverage table

Syllabus reference	What you must know or be able to do (from the syllabus)	Where assessed
10.1	Global distribution and climatic characteristics of hot arid and semi-arid environments	Paper 3
10.1	Definitions and causes of aridity: pressure and wind systems, ocean currents, rain shadow effect	Paper 3
10.1	Key features: high wind energy; diurnal and seasonal variations in precipitation and temperature	Paper 3
10.2	Weathering: thermal fracture, exfoliation, salt weathering, chemical weathering, and their effects	Paper 3
10.2	Wind: corrasion/abrasion, deflation, traction, saltation, suspension	Paper 3
10.2	Water: hydrological regime, episodic rainfall, sheet and flash floods	Paper 3
10.2	Landforms: sand dunes, yardangs, zeugen, wadis, alluvial fans, arroyos, pediments, piedmont zone (bahadas, playas, salt lakes, inselbergs)	Paper 3
10.2	Relative roles of aeolian and fluvial processes; evidence for past climate change (Pleistocene pluvials); role of past processes	Paper 3
10.3	Vegetation: biomass productivity (biodiversity, limited nutrient cycling, fragility); plant adaptations; physical and physiological drought	Paper 3
10.3	Soils: upward capillary movement of water and minerals (salinisation)	Paper 3
10.3	Desertification (natural and human factors) degrading soils and vegetation in semi-arid areas	Paper 3
10.4	Case study: problems of sustainable management in either a hot arid or a semi-arid environment; evaluate attempted or possible solutions	Paper 3

10.1 Distribution and climate

Where they are. Hot deserts lie mainly between about 15 and 30 degrees north and south: the Sahara, Arabian, Thar, Kalahari, Namib, Atacama and Australian deserts, plus the Sonoran and Chihuahuan deserts of North America. Semi-arid zones fringe them, such as the Sahel south of the Sahara.

Defining aridity. Rainfall alone is a weak measure, because a hot place loses far more water to evaporation than a cool one. The UNEP aridity index compares precipitation (P) with potential evapotranspiration (PET):

Class	P/PET
Hyper-arid	below 0.05
Arid	0.05 to 0.20
Semi-arid	0.20 to 0.50
Dry sub-humid	0.50 to 0.65

Causes of aridity.

• Pressure and wind systems. Air rising at the equator in the Hadley cell descends around 20 to 30 degrees latitude. Sinking air warms by compression, its relative humidity falls and clouds cannot form. The result is the subtropical high-pressure belt. The trade winds blowing out of it are dry as they cross land.
• Cold ocean currents. Cold currents on west coasts (the Humboldt off Chile and Peru, the Benguela off Namibia) chill the air above them. Cool air under warmer air forms a temperature inversion, which stops convection. Moisture condenses as coastal fog, not rain. This helps explain why the Atacama and Namib are among the driest places on Earth.
• Rain shadow. Moist air forced over mountains cools and rains on the windward side. Descending on the lee side, it warms and dries. The Andes help create the Patagonian desert; the Sierra Nevada shelters the Great Basin.

Key features. Clear skies allow strong daytime heating and rapid night-time radiation, so the diurnal temperature range is large, often more than 20 °C in the air and greater at rock surfaces. Seasonal range is greater away from the equator. Precipitation is low, highly variable and concentrated in short, intense storms. Semi-arid areas usually have a short wet season and a long dry season. Sparse vegetation and strong surface heating give a high wind energy environment: winds pick up loose, dry sediment easily.

Key terms. Aridity: a long-term water deficit, where PET exceeds P. Drought: a temporary shortfall against normal rainfall. Inversion: a layer where temperature rises with height, which suppresses rising air.

Worked paragraph. Explain why the Atacama is so arid. “The Atacama lies under the subtropical high, where descending air warms and dries, so convection is suppressed. The cold Humboldt Current cools air near the coast, creating an inversion that traps moisture as fog rather than rain. The Andes block moist air from the Amazon to the east, adding a rain shadow. Because the three causes reinforce each other, the Atacama is drier than deserts with only one cause.”

10.2 Weathering and processes

Weathering.

• Thermal fracture. Rocks heat and expand by day and contract at night. Dark minerals heat more than light ones, so stress builds between grains and the rock breaks into grains (granular disintegration) or blocks.
• Exfoliation. The outer layers of a rock heat and cool more than its interior, so shells peel off, rounding boulders and domes. Early laboratory experiments (Griggs, 1936) found that heating and cooling dry granite did not break it, but adding water did. Most geographers now think moisture, from dew or rare rain, is needed for insolation weathering to work.
• Salt weathering. Salt crystals grow in pores as water evaporates, and hydrated salts expand. The pressure prises grains apart. It is most active where salty groundwater or dew is present, such as near playas and at the base of cliffs.
• Chemical weathering. Slow overall, but dew and occasional rain allow hydrolysis and oxidation. Desert varnish, a dark iron and manganese coating, forms on exposed surfaces over long periods.

Wind processes. Wind erodes by deflation (lifting loose particles, forming deflation hollows and leaving stony desert pavements, or reg) and abrasion/corrasion (sand blasting rock). Transport is by suspension (dust, carried long distances), saltation (sand grains bouncing, mostly within a metre or two of the ground) and traction (larger grains rolled or pushed by impact, called surface creep). Because saltating sand stays low, abrasion is strongest near the ground.

Water processes. Most desert rivers are ephemeral, flowing only after rain. Some, like the Nile, are exogenous, fed from wetter areas. Rainfall is episodic: years of drought, then an intense storm. Bare, crusted soils have low infiltration, so overland flow starts quickly. Unconfined water spreads as a sheet flood across gentle slopes; confined water in channels becomes a flash flood carrying very high sediment loads. Water soon seeps into the channel bed (transmission loss).

10.2 Landforms

• Sand dunes. Barchans are crescent shaped, with horns pointing downwind; they form where wind blows from one direction and sand is limited. Seif (linear) dunes form where two wind directions alternate. Star dunes form where winds come from many directions.
• Yardangs. Long, streamlined ridges parallel to the prevailing wind, cut by abrasion and deflation into softer rock or sediment. The Lut Desert in Iran has large examples.
• Zeugen. Tabular ridges where a hard cap rock sits on softer rock. Abrasion undercuts the soft base; joints in the cap let erosion start.
• Wadis and arroyos. Steep-sided, flat-floored channels cut by flash floods. “Arroyo” is used in the Americas. They are often dry for years.
• Alluvial fans. Cone-shaped deposits where a channel leaves a mountain front, loses confinement and gradient, and drops its load. Coarse material lies at the apex, finer material at the edge. Death Valley in California has classic fans.
• Pediments. Gently sloping bedrock surfaces at the foot of a mountain front, with a sharp break of slope (the piedmont angle). Explanations include sheet flood erosion and parallel retreat of the mountain front (Lester King’s pediplanation).
• Piedmont zone. Fans merge into a bahada. In the lowest part of an enclosed basin, floodwater collects as a temporary lake, a playa. Evaporation leaves salt lakes and salt crusts. Inselbergs are isolated steep hills rising from plains, such as Uluru in Australia. They may form by deep weathering and later stripping of the regolith, or by long-term scarp retreat.

Aeolian or fluvial? Wind dominates where sand is plentiful and surfaces are bare, but many landforms, including wadis, fans, pediments and playas, are fluvial. Water does more geomorphic work in a single flood than wind does in years, even in very dry places.

Past climate change. Many features formed under wetter conditions. During the Pleistocene, pluvials (wetter periods) filled basins with lakes. Lake Bonneville in Utah existed from about 30,000 to 13,000 years ago and was over 300 m deep; Great Salt Lake and the Bonneville Salt Flats are its remains, and old shorelines stand as benches on the mountainsides. In the Sahara, a wetter phase in the early to mid Holocene expanded Lake Chad into a much larger “Mega-Chad”, whose shorelines are visible from satellites. Large wadi networks and inactive dunes now covered by vegetation also suggest that past processes, not present ones, built much of today’s landscape.

Worked paragraph. Assess the role of past processes. “Wadis in the central Sahara are often too large for the rare storms that now flow through them. This mismatch suggests they were cut during wetter phases. Lake shorelines, such as those of Lake Bonneville, give firmer evidence of pluvials. However, current flash floods still deepen wadis and build fans, so landforms are best seen as inherited features reworked by present processes, with the balance varying by place.”

10.3 Soils and vegetation

Biomass and biodiversity. Low and unreliable water limits net primary productivity, so biomass is low. Biodiversity is lower than in humid biomes, though many species are endemic. Nutrient cycling is limited: there is little litter, decomposers are slow in dry soil, and stores are small. The system is fragile. Damage from trampling, ploughing or vehicles may take decades to recover.

Plant adaptations.

Strategy	How it works	Example
Succulents	Store water in stems or leaves; waxy cuticle; stomata open at night (CAM)	Saguaro cactus
Phreatophytes	Very deep roots reach groundwater	Mesquite, acacias
Ephemerals	Seeds lie dormant, then germinate, flower and seed within weeks of rain	Desert annuals
Xerophytic leaves	Small, hairy or spiny leaves; sunken stomata; leaves shed in drought	Creosote bush
Halophytes	Tolerate salty soils	Saltbush

Plants face extreme temperatures too. Pale or hairy surfaces reflect heat, and vertical leaves avoid the midday sun.

Physical and physiological drought. Physical drought is a real lack of water in the soil. Physiological drought is when water is present but plants cannot take it up, for example because salty soil water has a higher solute concentration than root cells, so osmosis works against the plant.

Salinisation. High evaporation draws soil water upwards by capillary action. Dissolved salts are left at or near the surface as the water evaporates, forming a crust. Poorly managed irrigation speeds this up by raising the water table. Salinisation causes physiological drought and lower yields.

Desertification. The UN Convention to Combat Desertification defines it as land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities.

• Natural factors: long droughts (the Sahel suffered severe droughts in the 1970s and 1980s), rainfall variability and climate change. Jule Charney’s 1975 albedo hypothesis argued that losing vegetation raises surface reflectivity, which may further reduce rainfall.
• Human factors: overgrazing, overcultivation and shorter fallows, removing trees for fuelwood, and poorly drained irrigation. Population growth and the settling of nomadic herders concentrate pressure around wells and towns.
• Effects on soils and vegetation: loss of cover, soil crusting, wind and sheet erosion, falling organic matter and fertility.

Evaluation. Satellite studies show “regreening” in parts of the Sahel as rain partly returned, so early accounts overstated a steady human-driven advance of the Sahara. Human factors still matter locally, so most geographers now see interaction rather than a single cause.

Worked paragraph. “In the Sahel, drought reduces plant cover, but people decide how land is used during drought. When herds stay large around boreholes, overgrazing strips vegetation that would otherwise recover. Natural factors therefore start the degradation, while human factors decide whether it becomes lasting desertification.”

10.4 Case study: the semi-arid Sahel (Niger and Burkina Faso)

You must study either a hot arid or a semi-arid environment. This guide uses the Sahel, the semi-arid belt south of the Sahara. Your teacher will confirm which case study your class uses.

Problems for sustainable management. Rainfall is low and highly variable, with a short wet season. Population growth increases demand for food and fuelwood. Shorter fallows reduce soil fertility. Bare soils crust, so heavy storms run off rather than soak in. Herders and farmers compete for land and water.

Attempted solutions.

1. Farmer-managed natural regeneration (FMNR), Niger. Tony Rinaudo began working with farmers in the Maradi region in 1983. Farmers protect and prune shoots growing from living tree stumps and roots instead of planting new trees. By 2004, FMNR was reported on about five million hectares, roughly half of Niger’s farmland (Reij and colleagues documented this regreening). Strengths: cheap, uses local knowledge, gives fodder, fuelwood and shade, and improves soil. Limitations: depends on secure rights to trees and on community rules to stop cutting; it spreads unevenly.
2. Zaï pits and stone lines, Burkina Faso. Zaï are small planting pits that catch runoff, filled with manure or compost. The farmer Yacouba Sawadogo improved the method in the Yatenga region from the 1980s. Lines of stones along contours slow runoff and trap soil. Strengths: low-cost and restores crusted land. Limitations: digging pits is heavy labour, and manure is often scarce.
3. The Great Green Wall. Endorsed by the African Union in 2007, it now aims to restore 100 million hectares of degraded land by 2030. A 2020 UN status report found only around 4% of the target had been reached. It has shifted from a single tree belt to a mosaic of restored landscapes using water harvesting and local land-use methods. Limitations: funding gaps, conflict, and doubts over how many planted trees survive.

Evaluation. Bottom-up methods (FMNR, zaï) have changed large areas because farmers see direct benefits. Top-down planting has struggled. Success depends on land rights, labour, rainfall and security as much as on technique.

Common errors

• Writing “deserts have no rain”. Deserts have low, highly variable rainfall; flash floods are important.
• Explaining exfoliation by heating alone, ignoring the role of moisture.
• Mixing up saltation (bouncing) and traction (rolling or creep).
• Calling every desert landform aeolian; wadis, fans, pediments and playas are fluvial.
• Defining physiological drought as “no water”; it is water the plant cannot use.
• Treating desertification as only natural or only human.
• Describing the case study without evaluating solutions, which loses AO4 credit.

For exam technique across all papers, see exam preparation for 9696 and the Coastal environments option guide.

Official syllabus

This guide follows the Cambridge International AS & A Level Geography (9696) syllabus for examination in 2025 and 2026, topic 10, sections 10.1 to 10.4. Check the current document on the Cambridge International website for the full wording.