Impact of climate, vegetation and human activities

Impact of Climate, Vegetation and Human Activities on Soils in Hot Arid and Hot Semi‑Arid Environments

Learning Objective

Explain how climate, vegetation and human activities influence soil formation, properties and distribution in hot arid and hot semi‑arid regions, and evaluate management options in the context of the Cambridge AS & A‑Level Geography (9696) syllabus.

1. Overview of Hot Arid & Semi‑Arid Zones

• Definition (Cambridge 9696)
  • Arid: mean annual precipitation < 250 mm yr⁻¹
  • Semi‑arid: 250–500 mm yr⁻¹
• Global distribution – roughly 30 % of Earth’s land surface; includes the Sahara, Arabian Peninsula, Australian Outback, parts of the Sahel, the Atacama (Chile) and interior of the United Arab Emirates.
• Typical climate
  • Mean annual temperature 20–35 °C; daytime maxima often > 35 °C.
  • Very high potential evapotranspiration (PET) – 1 200–2 500 mm yr⁻¹.
  • High aridity index (AI) values < 0.2 (arid) or 0.2–0.5 (semi‑arid).

Aridity Index (AI) – classification

Worked example 1 (arid): P = 200 mm, PET = 1 200 mm → AI = 0.17 (arid).
Worked example 2 (semi‑arid): P = 400 mm, PET = 1 200 mm → AI = 0.33 (semi‑arid).

2. Soil‑Forming Factors in Arid & Semi‑Arid Environments

All five classical factors operate, but their relative importance differs from humid regions.

1. Climate – controls temperature‑driven physical weathering, limits leaching, drives upward capillary rise of water and salts.
2. Organisms (vegetation & soil biota) – sparse, drought‑adapted plants provide the only significant organic input; termites, ants and occasional burrowing mammals mix the upper few centimetres.
3. Relief – flat desert plains promote wind erosion; gentle slopes concentrate runoff in episodic flash floods, forming alluvial fans.
4. Parent material – often carbonate‑rich (limestone, dolomite) or siliceous sand; influences the type of salts that accumulate.
5. Time – many arid soils are relatively young (Entisols) but long‑term stability of calcrete hardpans can take > 10 000 yr.

3. Climate Impact on Soil Processes

1. Temperature – thermal expansion/contraction fractures rock, creating coarse fragments that dominate surface layers.
2. Precipitation – low totals restrict leaching, limit horizon development, and concentrate soluble salts near the surface.
3. Evaporation & PET – when evaporation > precipitation, capillary rise brings dissolved ions upward, leading to:
   • Calcrete (calcic) hardpan formation.
   • Salinisation (solonchaks) and sodification.

Landforms Directly Linked to Climate

• Calcrete (hardpan) – precipitation of calcium carbonate in the B‑horizon when upward‑moving moisture evaporates.
• Desert pavement – removal of fine particles by wind leaves a surface of tightly packed gravel.
• Salt crusts (solonchaks) – surface accumulation of Na‑rich salts in areas of high evaporation.

4. Vegetation–Soil Interactions

• Organic matter input – typically 0.5 %–3 % (very low). Seasonal grasses form short‑lived crusts; deep‑rooted shrubs (e.g., Acacia) concentrate litter and bioturbation locally.
• Nutrient cycling – limited leaching means most nutrients stay near the surface; microbial activity is constrained by moisture, so mineralisation is slow.
• Feedbacks – sparse vegetation → higher surface temperature → increased evaporation → more salt accumulation → further reduction in plant growth.

5. Human Activities, Population Pressure & Climate Change

Human interventions can either accelerate degradation or improve soil stability. The table below summarises the main pressures and their dual impacts.

6. Detailed Soil Formation Processes (linked to factors)

• Physical weathering – thermal stress, wind abrasion, occasional freeze‑thaw on high‑altitude deserts (climate & relief).
• Chemical weathering – dissolution of CaCO₃, gypsum, Na‑salts (climate + parent material); limited oxidation/hydrolysis due to moisture scarcity.
• Salinisation & Sodification – upward capillary rise of water concentrates Na⁺, Cl⁻, Ca²⁺, SO₄²⁻ at the surface (climate + parent material).
• Biological activity – surface microbes, occasional termites and ants that mix the top 5–10 cm (organisms).
• Landform development – calcrete hardpans, desert pavements, salt crusts (climate + relief).

7. Soil Classification – WRB vs. USDA

Both systems are examined in the A‑Level syllabus. The table below gives the main orders, typical sub‑orders, and the diagnostic criteria required for an exam answer.

8. Typical Soil Types in Hot Arid & Semi‑Arid Regions

1. Aridisols / Calcisols (desert soils)
   • Shallow, often with a calcrete or gypsum hardpan.
   • Organic matter < 1 %.
   • Alkaline (pH > 8) due to carbonate accumulation.
2. Entisols / Regosols (young semi‑arid soils)
   • Weak horizon development; occasional weak B horizon.
   • Organic matter 1 %–3 %.
   • Variable salinity; can support limited agriculture with proper management.
3. Solonchaks (sodium‑rich saline soils)
   • Surface crusts of Na‑carbonate or Na‑sulfate.
   • High electrical conductivity; vegetation limited to halophytes unless ameliorated.

9. Comparative Table: Arid vs. Semi‑Arid Soils

10. Case Studies

10.1 Sahel (Hot Semi‑Arid, Africa)

• Climate trend – increasingly erratic rainfall; multi‑year droughts more common.
• Soil response – shallower B horizons, exposure of calcrete, rising surface salinity.
• Human pressure – population growth ≈ 2 % yr⁻¹, intensified grazing and marginal cultivation.
• Land‑use interventions
  • Traditional zai pits – improve water infiltration, concentrate organic matter; low cost, community‑managed.
  • Modern drip‑irrigation – high water‑use efficiency, enables higher‑value crops; capital intensive, salinity risk if drainage poor.

10.2 Australian Outback (Hot Arid, Low‑income pastoral region)

• Climate – mean annual rainfall 150–250 mm; PET > 2 000 mm.
• Soil type – predominantly Aridisols with calcrete hardpans; occasional Solonchaks in low‑lying depressions.
• Management practices
  • Rotational grazing with paddock fencing – reduces over‑grazing, allows vegetation recovery.
  • Strategic mulching (straw, wood chips) – protects surface crust, reduces wind erosion.
  • Gypsum amendment on sodic soils – displaces Na⁺, improves structure.
• Evaluation
  • Successes: measurable increase in vegetative cover, reduced dust emissions.
  • Limitations: high labor and fence maintenance costs; effectiveness depends on rainfall variability.

10.3 United Arab Emirates / Atacama Desert (High‑income arid regions)

• Climate – extreme aridity (AI < 0.05 in parts of the UAE; AI ≈ 0.03 in Atacama).
• Soil type – Calcisols with thick calcrete horizons; extensive Solonchaks in reclaimed coastal plains.
• Management innovations
  • Large‑scale drip‑irrigation combined with fertigation for date‑palm orchards – yields up to 12 t ha⁻¹.
  • Artificial groundwater recharge (infiltration basins) to mitigate falling water tables.
  • Use of bio‑char and compost to increase organic matter and water‑holding capacity.
• Evaluation
  • Successes: high productivity, export‑oriented agriculture, reduced surface salinity through controlled leaching.
  • Risks: heavy reliance on energy‑intensive pumps, potential for long‑term groundwater depletion, high capital costs.

11. Management Strategies (Evaluation Framework)

1. Maintain ground cover – windbreaks, mulch, or low‑growth native species; reduces wind erosion and surface crust disruption.
2. Control irrigation – drip or subsurface systems, regular EC monitoring, drainage channels to prevent salinisation.
3. Manage grazing intensity – rotational grazing, rest periods, silvopasture (livestock‑tree integration).
4. Re‑vegetate degraded areas – plant deep‑rooted natives such as Acacia senegal, Prosopis juliflora, or halophytes on saline patches.
5. Soil amendment – gypsum to displace Na⁺ in Solonchaks; organic compost or bio‑char to raise humus and water‑holding capacity.

12. Mapping the Syllabus Key Concepts

13. End‑of‑Section Exercise (AO2 – Application)

1. Calculate the Aridity Index for a location with mean annual precipitation 320 mm and PET 1 400 mm. Classify the climate (arid, semi‑arid, etc.).
2. Using the table of soil‑forming factors, match each factor (climate, organisms, relief, parent material, time) with the dominant soil‑forming process it controls in hot arid environments.
3. Briefly evaluate the suitability of drip‑irrigation for a semi‑arid farm on Entisol soils with a shallow water table, considering both benefits and salinity risks.

14. Summary

Hot arid and hot semi‑arid soils are shaped by a combination of low precipitation, high evaporation, sparse vegetation and intense human pressure. Climate drives dominant physical weathering, limited leaching, and upward movement of salts, producing distinctive landforms such as calcrete hardpans, desert pavements and solonchak crusts. Vegetation provides the modest organic inputs that sustain nutrient cycling, while human activities can either exacerbate degradation (over‑grazing, poorly managed irrigation) or enhance stability (zai pits, drip irrigation, gypsum amendment). A clear understanding of the soil‑forming factors, classification criteria, and the evaluation of management options is essential for addressing sustainability challenges in these marginal environments.