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

These notes condense the full study guide for this unit.

They cover 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. The topic is one of four Advanced Physical Geography options in Paper 3, which lasts 1 hour 30 minutes and carries 60 marks. For each of your two options, Paper 3 sets a 10-mark structured question and a choice of 20-mark essay questions.

Use the Cambridge A Level Geography hub, tick off points on the 9696 checklist, then try the practice questions. For the AS weathering and slope content this unit builds on, see the physical geography revision notes.

Key terms

Term	Meaning
Aridity	Long-term water deficit: potential evapotranspiration exceeds precipitation
Aridity index (UNEP)	P/PET. Hyper-arid below 0.05; arid 0.05-0.20; semi-arid 0.20-0.50; dry sub-humid 0.50-0.65
Subtropical high	Belt of sinking, warming, drying air near 20-30 degrees latitude (descending limb of the Hadley cell)
Rain shadow	Dry area on the lee side of mountains where descending air warms and dries
Temperature inversion	Warm air above cool air; blocks convection (caused over cold currents)
Diurnal range	Difference between day maximum and night minimum temperature
Thermal fracture	Rock breakdown from repeated heating and cooling stress
Exfoliation	Peeling of outer rock shells; moisture usually needed
Salt weathering	Crystal growth and hydration of salts in pores prise grains apart
Deflation	Wind removal of loose fine particles
Abrasion (corrasion)	Wind-blown sand wearing rock, strongest near the ground
Saltation	Bouncing movement of sand grains
Traction (surface creep)	Rolling and pushing of larger grains along the surface
Suspension	Fine dust carried within the air
Ephemeral stream	Flows only after rain
Episodic rainfall	Rare, irregular, intense storms
Sheet flood	Unconfined water spreading across a gentle surface
Flash flood	Sudden, short, high-energy flow in a channel
Bahada	Merged alluvial fans along a mountain front
Playa	Flat floor of an enclosed basin, briefly flooded, then dries to salt or clay
Pluvial	Wetter period in the past, especially in the Pleistocene
Physiological drought	Water is present but plants cannot absorb it, e.g. in salty soil
Salinisation	Build-up of salts at the surface after capillary rise and evaporation
Desertification	Land degradation in drylands from climatic variations and human activities (UNCCD)

Processes and landforms at a glance

Landform	Main process	Key point to explain
Barchan dune	Aeolian deposition	One wind direction, limited sand; horns point downwind
Seif (linear) dune	Aeolian deposition	Two alternating wind directions
Yardang	Abrasion and deflation	Long ridge parallel to wind in soft rock or sediment
Zeugen	Abrasion undercutting	Hard cap rock over softer rock; jointing helps
Wadi / arroyo	Flash flood erosion	Steep sides, flat floor; may be inherited from pluvials
Alluvial fan	Fluvial deposition	Channel loses gradient and confinement at mountain front
Pediment	Sheet flood erosion and slope retreat	Gentle bedrock slope; piedmont angle
Playa / salt lake	Deposition and evaporation	Lowest point of enclosed basin
Inselberg	Deep weathering and stripping, or scarp retreat	Isolated steep hill on a plain, e.g. Uluru

Studies and examples at a glance

Who or what	What	Key detail	One evaluation point
Atacama and Namib	Cold-current deserts	Humboldt and Benguela currents create inversions and fog	Shows several causes acting together, not ocean currents alone
Griggs (1936)	Laboratory test of insolation weathering	Dry heating and cooling did not break granite; adding water did	Lab samples were small and time was compressed, so results are indicative only
Lake Bonneville, Utah	Pleistocene pluvial lake	About 30,000 to 13,000 years ago; over 300 m deep; shorelines remain	Clear proof of past wetter climate in a now dry basin
Mega-Chad	Holocene humid phase in the Sahara	Lake Chad was once far larger; shorelines visible from satellites	Shows wetter phases occurred after the Pleistocene too
Charney (1975)	Albedo hypothesis	Loss of vegetation raises albedo, which may reduce rainfall	Later research linked Sahel rainfall mainly to ocean temperatures, so vegetation is now seen as an amplifier, not the main cause
FMNR, Niger	Natural regeneration from stumps	Began in Maradi in 1983; about five million hectares by 2004	Depends on secure rights to trees
Zaï pits, Burkina Faso	Water harvesting pits with manure	Yacouba Sawadogo, Yatenga, from the 1980s	Heavy labour; manure in short supply
Great Green Wall	African Union initiative, 2007	Target 100 million hectares by 2030	2020 UN report: about 4% of target reached

Must-know distinctions

• Arid versus semi-arid. Arid: P/PET 0.05-0.20, very sparse plants. Semi-arid: 0.20-0.50, grass and scrub, a short wet season, and rain-fed farming possible in good years.
• Aridity versus drought. Aridity is permanent; drought is a temporary shortfall.
• Thermal fracture versus exfoliation. Thermal fracture breaks rock into grains or blocks; exfoliation peels off shells.
• Saltation versus traction. Saltation bounces; traction rolls or creeps.
• Sheet flood versus flash flood. Sheet flood is unconfined; flash flood is in a channel.
• Alluvial fan versus bahada. A bahada is many fans joined together.
• Pediment versus bahada. A pediment is an erosional bedrock surface; a bahada is a depositional surface of sediment.
• Physical versus physiological drought. No water versus unusable water.
• Natural versus human causes of desertification. Drought starts degradation; land use decides whether it becomes lasting.

Vegetation and soils in brief

• Low biomass and productivity. Water, not heat or light, is the limiting factor. Biodiversity is lower than in humid biomes.
• Limited nutrient cycling. Little litter falls, decomposers work slowly in dry soil, and nutrient stores are small.
• Fragility. Damaged plant cover and soil crusts recover slowly, sometimes over decades.
• Adaptations. Succulents store water and open stomata at night; phreatophytes send roots deep to groundwater; ephemerals survive as dormant seeds; xerophytes have small, waxy or spiny leaves; halophytes tolerate salt. Pale, hairy surfaces and upright leaves limit heating.
• Salinisation. Capillary rise plus evaporation leaves salts at the surface; over-irrigation without drainage raises the water table and speeds this up.

Mini essay plan

Question type: “Assess the extent to which landforms in hot arid environments are the result of past rather than present processes.”

1. Introduction. Define aeolian and fluvial processes and pluvials. State your line: many large landforms are inherited, but present processes still reshape them.
2. Evidence for past processes. Lake Bonneville shorelines and salt flats; Mega-Chad shorelines; oversized wadi networks; vegetated, inactive dunes.
3. Present fluvial processes. Episodic rain, flash floods deepening wadis, sheet floods on pediments, fans still building.
4. Present aeolian processes. Active barchans, yardangs, abrasion near the ground, deflation hollows.
5. Weathering. Salt weathering and thermal fracture supply sediment today.
6. Evaluation. The balance varies by landform: playas and fan size reflect past wetness, while dunes may be active now. Dating evidence is often uncertain.
7. Conclusion. Weigh the evidence and answer “to what extent” directly.

Quick self-test

1. Give the P/PET range for a semi-arid area.
2. Why does sinking air near 30 degrees latitude produce few clouds?
3. Name the cold current off the Namib Desert.
4. What does salt weathering need to be most active?
5. Which wind transport process moves grains by bouncing?
6. Why is abrasion strongest near the ground?
7. What is the difference between a pediment and a bahada?
8. Give one piece of evidence for Pleistocene pluvials.
9. Define physiological drought.
10. Explain how capillary action leads to salinisation.
11. Give one strength and one limitation of farmer-managed natural regeneration.
12. Why does the Great Green Wall now favour a “mosaic” approach?

Answers

1. 0.20 to 0.50.
2. Air warms as it sinks, so its relative humidity falls and condensation cannot happen.
3. The Benguela Current.
4. A supply of salty water (groundwater, dew or spray) and high evaporation.
5. Saltation.
6. Saltating sand travels mostly within a metre or two of the surface.
7. A pediment is a gently sloping bedrock surface formed by erosion; a bahada is a depositional slope of merged alluvial fans.
8. Old lake shorelines around the Bonneville basin in Utah (or the Bonneville Salt Flats).
9. Water is present but plants cannot take it up, because salty soil water reverses osmosis.
10. Evaporation pulls water up through soil pores; when it evaporates, dissolved salts are left at the surface.
11. Strength: cheap and uses existing roots. Limitation: needs secure rights to trees and community rules.
12. Desert margins shift with rainfall, so a fixed tree belt made little sense; the initiative now promotes water harvesting, protecting existing vegetation and improved local land use.

Where marks are usually lost

• Listing causes of aridity without linking each one to a named desert.
• Saying deserts have “no rain” and ignoring flash floods and episodic rainfall.
• Explaining exfoliation by heat alone without noting moisture.
• Giving landform descriptions with no process sequence (for example, not saying why the fan’s channel loses energy).
• Drawing unlabelled diagrams; label the wind direction, slip face and horns on a barchan sketch.
• Confusing pediments with bahadas, or playas with oases.
• Treating desertification as only overgrazing, with no natural factor or interaction.
• Describing the case study but not judging whether solutions are sustainable.
• Writing a one-sided essay when the question asks “to what extent” or “assess”.

Official syllabus

These notes follow the Cambridge International AS & A Level Geography (9696) syllabus for examination in 2025 and 2026, topic 10, sections 10.1 to 10.4. Your teacher will confirm your two Paper 3 options. Check the current syllabus on the Cambridge International website before your exam.