Arid Environments (Cambridge AS & A‑Level Geography – Topic 10)
1. Climate of Arid Zones (10.1)
1.1 Definition of Aridity
Aridity occurs when annual precipitation (P) < potential evapotranspiration (PET) .
Aridity Index (AI) = P / PET
AI < 0.2 → hyper‑arid
0.2 – 0.5 → arid
0.5 – 0.65 → semi‑arid
Typical values: P < 250 mm yr⁻¹; PET often 1 500–2 500 mm yr⁻¹.
1.2 Global Distribution
Arid climates are concentrated under the subtropical high‑pressure belts (≈30° N and 30° S) and in the interiors of large continents.
Suggested diagram: world map showing Köppen BWh (hot desert) and BWk (cold desert) zones.
1.3 Temperature & Precipitation Patterns
Temperature
Hot deserts (BWh): mean annual temperature > 20 °C; very large diurnal ranges (up to 30 °C).
Cold deserts (BWk): strong seasonal variation; winter lows often below 0 °C, summer highs > 30 °C.
Continentality intensifies temperature extremes in interior deserts.
Precipitation
Low totals (often < 250 mm yr⁻¹) and highly variable.
Rainfall usually occurs as short, intense convective storms; occasional monsoonal or cyclonic incursions.
PET drivers : high solar radiation, low humidity, clear skies, strong winds.
1.4 Atmospheric Mechanisms
Subtropical high‑pressure cells suppress upward motion → limited cloud formation.
Rain‑shadow effect of adjacent highlands (e.g., Atacama Desert).
Cold‑air drainage from mountains can enhance aridity on leeward slopes.
Occasional tropical cyclones or monsoonal pulses bring episodic rain (e.g., Sahelian fringe).
2. Physical Landforms & Processes (10.2)
2.1 Aeolian Processes
Process Mechanism Typical Landform Result
Deflation Removal of loose particles by wind Regs, desert pavements Saltation Grains bounce in short hops Sand dune migration Creep Rolling of larger particles Surface smoothing, dune lee slopes
2.2 Dune Types (10.2.2)
Transverse dunes – form where wind direction is relatively constant and sand supply is abundant.Linear (seif) dunes – develop under bidirectional wind regimes.Barchan dunes – isolated, crescent‑shaped dunes in areas of limited sand.Star dunes – multi‑arm dunes where winds blow from many directions.
Diagram suggestion: labelled cross‑section of each dune type.
2.3 Other Desert Landforms (10.2.3‑10.2.5)
Playas (dry lake beds) – flat, often salt‑crusted surfaces formed where intermittent runoff evaporates.Wadis (dry riverbeds) – episodic channels that convey flash floods; may host alluvial fans.Reg (desert pavement) – surface layer of closely packed pebbles left after fine particles are removed by deflation.Calcrete / Caliche – hard, calcium‑carbonate‑rich crusts that develop in semi‑arid soils.Halite crusts & salt pans – evaporite mineral accumulations in endorheic basins.Sand sheet – extensive, relatively flat sand‑covered areas with low relief, often the source for dune formation.Yardang – streamlined, wind‑eroded ridges aligned with prevailing wind direction, typical on hard, cohesive substrates.
3. Soils & Vegetation (10.3)
3.1 Soil Profiles in Arid Zones (10.3.1)
Soil Order Key Characteristics Typical Horizon(s) Typical Location
Entisols (Regolith) Thin, weakly developed, high stone content O/A – minimal development Dune surfaces, reg Aridisols Low organic matter, often calcareous or gypsum‑rich Calcic, Gypsic horizons Alluvial fans, piedmonts Saline & Alkaline soils Surface salts, high pH, low hydraulic conductivity Salic, Natric horizons Playas, inland basins
3.2 Vegetation Adaptations (10.3.2)
Xerophytic traits : deep tap‑roots, reduced leaf area, CAM photosynthesis, waxy cuticles, stomatal closure during heat.Halophytic traits : salt excretion glands, succulence, specialised ion transport.Examples
Acacia spp. – nitrogen‑fixing, drought‑tolerant trees.
Halophytes (e.g., Salicornia ) – thrive on saline soils.
Succulents (e.g., Aloe , Euphorbia ) – store water in leaves/stems.
Prosopis juliflora – fast‑growing, deep‑rooted shrub used in re‑vegetation.
Primary productivity is low (typically < 100 g C m⁻² yr⁻¹) and highly pulse‑driven by rare rain events.
4. Population Pressure in Arid Regions (10.4)
Urbanisation – rapid growth of cities on desert margins (e.g., Phoenix, Riyadh, Las Vegas). Water demand often exceeds sustainable abstraction.Rural migration – people move seeking mining, oil, or tourism jobs; leads to settlement in marginal lands.High natural increase – many arid societies have fertility rates > 3 children per woman.Consequences
Over‑extraction of groundwater → falling water tables, salinisation.
Expansion of irrigated agriculture into marginal zones.
Increased waste and pollution in fragile ecosystems.
4.1 Contrasting Country Examples
Country Income level Fertility rate (children / woman) Key pressure on arid land
Saudi Arabia High‑income ≈ 2.5 Large‑scale urban water imports, extensive irrigation projects (e.g., Al‑Qassim), desert‑tourism development. Niger Low‑income ≈ 7.0 Rapid rural population growth, extensive livestock grazing, reliance on shallow wells, frequent over‑grazing.
5. Desertification (10.5)
5.1 Definition (10.5.1)
Desertification is the long‑term degradation of land in arid, semi‑arid and dry sub‑humid areas resulting from a combination of climatic variations and unsustainable human activities.
5.2 Drivers (10.5.2)
Driver Mechanism Typical Impact
Over‑grazing Removal of protective plant cover Soil erosion, reduced infiltration Unsustainable irrigation Excessive water use, poor drainage Salinisation, water‑table decline Deforestation & land clearing Loss of windbreaks, exposure of soil Increased wind erosion, higher surface temperatures Mining & infrastructure Soil disturbance, contamination Habitat loss, altered hydrology Climate variability Prolonged droughts, erratic rainfall Reduced vegetation recovery, amplified stress Population pressure Higher demand for water, food, and land Expansion of marginal agriculture, over‑extraction
5.3 Process Overview – Water‑Balance Approach
Simplified water balance for an arid catchment:
\(P - PET = \Delta S\)
When \(P < PET\) , the soil‑moisture deficit (\(\Delta S\)) grows, causing vegetation loss.
Loss of vegetation reduces surface roughness, increasing wind speed → **positive feedback**: higher wind erosion → further loss of vegetation.
5.4 Indicators of Desertification
Decline in vegetation cover (NDVI trends).
Increase in bare‑ground percentage and reg formation.
Rising groundwater salinity and falling water‑table depths.
Socio‑economic signs: reduced crop yields, out‑migration, increased poverty.
6. Climate‑Change Impacts on Arid Environments (10.6)
Temperature rise – PET increases by ~5 % for every 1 °C warming (IPCC AR6, Table 2.4). Higher PET widens the water‑balance deficit.Precipitation changes – models forecast more intense but less frequent rain events, raising peak runoff and erosion risk.Extreme events – higher frequency of heatwaves and dust storms.Sea‑level rise – inundates low‑lying coastal deserts (e.g., Namib, Arabian Gulf) and can raise salinity of adjacent aquifers.Desert expansion – poleward shift of subtropical highs may enlarge arid zones by 5‑10 % by 2100.
7. Management Strategies – Integrated Approach (10.7)
7.1 Physical Measures (10.7.1)
Windbreaks & sand fences – trap moving sand, reduce wind speed.
Evaluation: Effective locally but require regular maintenance; limited impact on large‑scale sand drift.
Terracing, contour bunds, check‑dams – slow runoff, increase infiltration.
Evaluation: Works well on gentle slopes; costly on steep, extensive terrain.
Drip and subsurface irrigation – minimise evaporative losses.
Evaluation: High water‑use efficiency but capital‑intensive; requires reliable power and skilled operation.
Artificial recharge of aquifers (infiltration basins, recharge wells).
Evaluation: Can raise water tables where geology permits; risk of re‑introducing salts if recharge water is not treated.
7.2 Biological Measures (10.7.2)
Re‑vegetation with native, drought‑tolerant species (e.g., Prosopis juliflora , Acacia tortilis ).
Evaluation: Improves soil stability and micro‑climate; some species become invasive if not carefully selected.
Agroforestry and silvopastoral systems – combine trees, crops, and livestock.
Evaluation: Increases organic matter and shade; may reduce short‑term crop yields during establishment.
Rotational and deferred grazing – allow plant recovery and root development.
Evaluation: Simple to adopt but requires effective community enforcement.
Soil amendments (e.g., gypsum) to reduce salinity.
Evaluation: Can rapidly improve soil structure; cost and logistics limit large‑scale use.
7.3 Policy, Institutional & Community Approaches (10.7.3)
Land‑use planning & zoning – restrict cultivation on steep slopes or highly erodible soils.
Water‑allocation reforms – pricing, metering, and extraction caps to curb over‑abstraction.
Community‑based natural resource management (CBNRM) – local monitoring, participatory decision‑making.
Education & awareness programmes – promote water‑saving techniques and desert‑friendly livelihoods.
International frameworks – UNCCD, SDG 15 (Life on Land), regional desert‑combat initiatives.
Evaluation: Policy measures provide long‑term governance but may be undermined by weak enforcement or lack of funding.
7.4 Case Study Highlights (with brief evaluation)
China’s “Three‑North” Shelterbelt – massive tree‑planting programme to curb sand drift and improve micro‑climate.
Success: reduced wind erosion in many areas. Limitation: high water demand for young trees; some species poorly suited to local soils. Australia’s Murray‑Darling Basin water‑pricing scheme – caps on groundwater extraction in semi‑arid zones.
Success: measurable decline in extraction rates. Limitation: compliance varies among irrigators; economic impacts on small farms. Morocco’s “Green Morocco Plan” – combines drip irrigation, drought‑tolerant crops, and farmer cooperatives.
Success: increased yields and water‑use efficiency. Limitation: high initial capital costs; benefits unevenly distributed.
8. Summary Points (Key Exam Revision)
Aridity is quantified by the precipitation‑PET balance; AI < 0.2 defines hyper‑arid conditions.
Subtropical high‑pressure systems and rain‑shadow effects create the world’s major desert belts.
Cold‑desert climates show strong seasonal temperature extremes and are intensified by continentality.
Core desert landforms – dunes (transverse, linear, barchan, star), sand sheets, yardangs, reg, playas, wadis, calcrete, and salt pans – result from aeolian and evaporative processes.
Soils are generally thin, saline or calcareous; three principal orders are Entisols, Aridisols, and Saline/Alkaline soils, each with characteristic horizons (calcic, gypsic, salic).
Vegetation is highly specialised (xerophytes, halophytes) with low, pulse‑driven primary productivity.
Population growth and urbanisation increase water demand, waste, and marginal land use.
Desertification is driven by a mix of natural climate variability and human activities; a positive feedback loop exists between vegetation loss and wind erosion.
Climate change amplifies aridity: PET rises ~5 % per 1 °C warming, rainfall becomes more erratic, and desert belts are projected to expand.
Effective management requires an integrated mix of physical engineering, biological restoration, and strong policy/community action, each with clear strengths and limitations.
Suggested diagram: Flowchart linking population pressure, climate change, desertification drivers, and the three tiers of management responses (physical, biological, policy) in arid environments.