Processes and landforms: weathering, wind and water processes, landforms

Arid Environments – Cambridge International AS & A Level Geography (9696)

1. Climate & Classification

Arid environments are defined by a persistent moisture deficit:

• Potential evapotranspiration (PET) > Precipitation (P) over long periods.
• They cover ~30 % of the Earth’s land surface, mainly between 20° N and 30° S.

Aridity Class (Köppen‑Aridity Index, P/PET)	Mean Annual Precipitation (mm)	Mean Annual PET (mm)	Typical Regions
Hyper‑arid (P/PET < 0.03)	< 50	> 2000	Atacama Desert (Chile), Sahara interior
Arid (Desert) (0.03 ≤ P/PET < 0.20)	50–250	1500–2500	Arabian Peninsula, Central Australia
Semi‑arid (Steppe) (0.20 ≤ P/PET < 0.50)	250–500	1000–1500	Great Plains (USA), Sahel

2. Water Balance in Arid Catchments

The fundamental water‑balance equation is:

$$P = Q + E + \Delta S$$

• P – Precipitation (mm yr⁻¹)
• Q – Runoff (often flash floods)
• E – Evapotranspiration (dominant term in deserts)
• ΔS – Change in soil‑water or groundwater storage

Example: In a typical Kalahari catchment (area 150 km²) annual values are P ≈ 350 mm, E ≈ 900 mm, Q ≈ 30 mm, ΔS ≈ ‑20 mm, illustrating that > 90 % of incoming water is lost to the atmosphere.

3. Weathering & Soil Formation

3.1 Weathering Processes

• Physical (Mechanical) Weathering
  • Thermal expansion & contraction – diurnal temperature swings of 30–50 °C cause rock cracking.
  • Salt crystallisation (haloclasty) – evaporation leaves salts that grow and force grains apart.
  • Exfoliation – pressure release in exposed outcrops creates sheet‑like rock loss.
• Chemical Weathering
  • Oxidation of iron‑bearing minerals during brief wet periods (e.g., formation of reddish soils).
  • Hydrolysis in intense, short‑duration rainfalls – limited but can produce clays in wadis.
• Biological Weathering
  • Root wedging in oasis or wadi margins.
  • Microbial activity in biological soil crusts (cyanobacteria, lichens) that produce organic acids.

3.2 Soil‑Formation Factors (FAOSTAT “CLORPT”) in Arid Zones

• Climate – extreme temperature range, high PET, low P.
• Organisms – sparse vegetation, dominant biocrusts, limited fauna.
• Parent Material – often unconsolidated aeolian sand, alluvial gravels, or weathered basement rock.
• Topography – flat pavements, gentle slopes, or steep wadi walls influencing runoff.
• Time – slow pedogenesis; many soils are shallow and weakly developed.

3.3 Dominant Arid Soil Types

• Regolith (Aeolian) Soils – thin, poorly developed, sand‑ and silt‑rich.
• Calcrete (Caliche) Horizons – carbonate accumulation where evaporation exceeds infiltration.
• Aridisols (Entisols) – shallow, low organic matter, often with a hardpan (duricrust).
• Salinised Soils – surface accumulation of soluble salts due to high evaporation.
• Biological Soil Crusts – cyanobacteria, lichens, mosses; stabilise surface, fix nitrogen, increase albedo.

4. Vegetation & Ecosystems

4.1 Dominant Plant Functional Types

• Xerophytes – deep‑rooted shrubs (e.g., Acacia spp.) with reduced leaf area.
• Succulents – water‑storage tissues (e.g., Aloe, Euphorbia).
• Ephemerals – complete life cycle within a single rain event.
• Halophytes – tolerate saline soils (e.g., Salicornia).

4.2 Physiological Adaptations

• Thick cuticles, sunken stomata, and reduced leaf surface to limit transpiration.
• CAM photosynthesis – nocturnal CO₂ uptake, reducing water loss.
• Extensive root systems (tap‑roots, lateral spread) for deep water extraction.
• C₃ vs C₄ vs CAM – C₃ grasses dominate semi‑arid steppes, C₄ grasses (e.g., Panicum) thrive where temperatures exceed 30 °C and water is slightly more available, while CAM is the main pathway in true desert succulents.

4.3 Ecosystem Services

• Biocrust microbes fix nitrogen, increase soil fertility, and protect against wind erosion.
• Vegetation patches act as “islands of productivity”, enhancing local infiltration and reducing flash‑flood intensity.
• Fauna (reptiles, small mammals, insects) depend on sparse vegetation for food and shelter, maintaining biodiversity.

5. Wind Processes & Landforms

5.1 Wind‑Driven Transport Mechanisms

• Deflation – removal of loose particles, leaving a lag surface (desert pavement).
• Abrasion (Sandblasting) – impact of sand grains that smooth and polish rock surfaces.
• Saltation – hopping movement of sand grains; the dominant sand‑transport mode.
• Suspension – fine silt and dust lifted and carried long distances.

Threshold wind velocity for initiating saltation of fine sand (~0.2 mm) is ≈ 0.2 m s⁻¹; for coarser sand (~0.5 mm) it is ≈ 0.4 m s⁻¹.

5.2 Typical Wind‑Driven Landforms

• Dunes – barchan, transverse, star, linear.
• Yardangs – streamlined ridges aligned with prevailing wind.
• Ventifacts – rock surfaces polished by wind‑borne particles.
• Blowouts (Deflation Hollows) – depressions where sand has been removed.
• Desert Pavement – lag of coarse fragments protecting finer material.

6. Water Processes

Rainfall is scarce but often intense, producing rapid runoff.

• Sheet Flow – thin, uniform flow over bare surfaces; high erosive power.
• Rill & Gully Formation – concentrated flow incising channels; can develop after a single storm.
• Flash Floods – sudden, high‑velocity flows in wadis; capable of transporting boulders and large sediment loads.

Peak discharge can be estimated with the Rational Method:

$$Q = C \, i \, A$$

• Q – peak discharge (m³ s⁻¹)
• C – runoff coefficient (0.7–0.9 on impervious desert surfaces)
• i – rainfall intensity (mm h⁻¹)
• A – catchment area (km²)

Example: A 2 km² wadi receiving a 30 mm h⁻¹ storm (i = 30) with C = 0.85 gives Q ≈ 0.85 × 30 × 2 ≈ 51 m³ s⁻¹, illustrating the potential for destructive flash floods.

7. Landforms of Arid Environments

Landform	Forming Process	Typical Example
Desert Pavement	Deflation leaving a lag of coarse fragments	Namib Desert, Namibia
Yardang	Wind abrasion & differential erosion	Gobi Desert, China
Alluvial Fan	Sudden water discharge from wadis; sediment deposition	Death Valley, USA
Playa (Salt Flat)	Evaporation of shallow, intermittent lakes; salt precipitation	Lake Eyre, Australia
Sand Dune – Barchan	Unidirectional wind transport & deposition	Erg Chebbi, Sahara
Inselberg	Differential weathering exposing resistant rock	Mount Bromo, Indonesia

8. Human‑Environment Interaction & Management

8.1 Pressures on Arid Landscapes

• Over‑grazing → loss of vegetation cover, increased wind erosion.
• Unsustainable groundwater extraction → falling water tables, land subsidence.
• Land‑use change (mining, urban expansion) → habitat fragmentation and increased dust emission.

8.2 Management Strategies (with evaluation)

Strategy	How it Works	Advantages	Limitations / Evaluation
Windbreaks & Sand Fences	Physical barriers reduce wind speed and trap moving sand.	Quick visual impact; relatively low cost; effective on small‑ to medium‑size dunes.	Requires regular maintenance; effectiveness declines if not aligned with prevailing wind; trapped dust can affect nearby crops.
Check‑dams & Contour Bunds	Small barriers across wadis or slopes slow runoff, promote infiltration, and trap sediment.	Enhances groundwater recharge; creates micro‑catchments for agriculture; low‑tech.	Can silt up rapidly during high‑intensity flash floods; may divert flood energy downstream, increasing risk elsewhere.
Drip Irrigation & Deficit‑Water‑Use Crops	Delivers water directly to plant roots, minimising evaporation.	Reduces water consumption by 30–50 %; improves crop yields in arid farms.	High initial capital cost; requires reliable water supply and farmer training.
Biological Soil Crust Restoration	Seeding native cyanobacteria, lichens, and mosses to stabilise the surface.	Improves soil stability, nitrogen fixation, and surface albedo.	Slow establishment (years); vulnerable to trampling and over‑grazing.

9. Detailed Specific Example – Central Sahara (Southern Algeria) 2000‑2020

10. Summary

Arid environments are characterised by extreme moisture deficits, which make physical weathering, intense wind activity, and episodic but powerful water processes the dominant agents of landscape change. The resulting landforms—dunes, yardangs, alluvial fans, playas, and desert pavements—reflect the interplay of wind, water, and limited vegetation. Sparse plant communities and specialised soils (e.g., calcrete, biocrusts) moderate erosion, provide essential ecosystem services, and support limited human livelihoods. Sustainable management must blend engineering solutions (check‑dams, sand fences), water‑saving agriculture (drip irrigation), and biological restoration, while continuously evaluating ecological, social, and economic outcomes.