Impact of climate and human activities

Vegetation and Soils in Arid Environments

Learning objective

Explain how climate and human activities shape the distribution, structure and functioning of vegetation and soils in arid regions, and evaluate management options in the context of sustainable development and climate change.

1. Climate of arid environments

Arid climates are defined by a strong moisture deficit, high temperatures and marked seasonal & inter‑annual variability.

Key climate concepts for the syllabus

• Scale: local (oasis), regional (desert belt), global (climate‑change forcing).
• Systems: interaction of atmospheric circulation, land surface and hydrology.
• Change over time: decadal rainfall trends, seasonal shifts, extreme‑event frequency.
• Cause‑effect: low precipitation → water stress → plant adaptations → soil‑formation pathways.

2. Physical processes in arid environments

• Aeolian erosion – deflation removes fine particles; abrasion shapes rock surfaces; dune forms (barchan, transverse, star) record wind regime and sand supply.
• Mechanical weathering – thermal expansion, salt‑crystallisation, freeze‑thaw cycles dominate over chemical weathering.
• Flash‑flood dynamics – intense, short‑duration storms generate high‑energy runoff, carving alluvial fans, wadis and ephemeral channels.
• Soil‑forming processes (syllabus wording)
  • Limited leaching because water availability is low.
  • Calcification – accumulation of calcium carbonate (caliche) in the B‑horizon.
  • Gypsum precipitation – formation of Gypsisols where CaSO₄·2H₂O accumulates.
  • Salinisation – upward movement of soluble salts and surface crusting when evaporation exceeds precipitation.
  • Hard‑pan development – cementation of surface or sub‑surface layers by carbonates, gypsum or silica, impeding root penetration and water infiltration.

3. Water resources

• Groundwater – deep aquifers (e.g., Nubian Sandstone) recharge very slowly; over‑extraction causes falling water tables and salinisation.
• Surface water – ephemeral streams (wadis) flow only after rain; flash floods provide both hazard and recharge opportunity.
• Oases & artificial water bodies – natural springs or pumped irrigation create micro‑climates that support dense vegetation and agriculture.
• Water‑balance equation (simplified) – P ≈ 0 mm yr⁻¹ vs. ET ≈ 1500 mm yr⁻¹; the surplus is lost as latent heat, leaving little for runoff or infiltration.

Suggested diagram

Figure 1 – Water‑balance schematic for an arid catchment: arrows showing precipitation (P) entering the system, most being lost to evapotranspiration (ET), a small fraction infiltrating to recharge groundwater (G), and occasional flash‑flood runoff (R) to surface channels.

4. Impact of climate on vegetation

• Water limitation – selects for drought‑avoidance (seed dormancy, rapid germination) or drought‑tolerance (CAM photosynthesis, deep tap‑roots) strategies.
• Temperature extremes – favour reduced leaf area, reflective surfaces, thick cuticles or leaf‑roll.
• Rainfall variability – drives “boom‑bust” phenology of annual herbs and opportunistic germination after rare events.

Typical arid‑zone vegetation types

Evaluation of vegetation‑management options

Controlled grazing, assisted regeneration with native drought‑tolerant species and the establishment of wind‑breaks can maintain canopy cover, reduce soil exposure and improve organic‑matter inputs. In the Sahel, community‑led planting of Faidherbia albida (a nitrogen‑fixing tree that drops leaves during the rainy season) has been shown to increase soil organic carbon by ~0.2 % over ten years, enhancing water‑holding capacity while providing fodder – an outcome that aligns with Sustainable Development Goal 15 (life on land). However, the success of such measures is contingent on reliable rainfall; in years of severe deficit, even well‑established seedlings may suffer high mortality, underscoring the need for complementary water‑conservation techniques (e.g., zai pits, mulching).

5. Impact of climate on soils

• Low organic matter – limited litter input → < 0.5 % humus, low aggregate stability.
• Carbonate & gypsum accumulation – calcification and gypsum precipitation form hardpans (caliche, gypsic horizons) that impede root growth and water infiltration.
• Salinisation – high evaporation concentrates soluble salts; electrical conductivity (EC) > 4 dS m⁻¹ commonly defines saline soils.
• Texture – coarse (sand, gravel) → high infiltration, low water‑holding capacity; fine‑grained pockets (e.g., alluvial fan deposits) may retain more moisture.

Spatial variation of soil‑forming processes

In dune settings, wind‑blown sand leads to very young, weakly developed Entisols with little horizon differentiation. On pied‑monts (the foot of a desert plateau) episodic runoff deposits fine silt and clay, allowing modest development of Aridisols with calcic horizons. In interior basins (playas, endorheic depressions) repeated evaporation creates surface salt crusts and thick Gypsisols or saline soils, where gypsum and halite accumulate in the upper horizons.

Typical arid‑zone soil types

6. Human pressures on arid environments

• Desertification – progressive loss of productive land caused by climate variability plus unsustainable land use.
• Over‑grazing – removes protective shrub cover → accelerates wind erosion, reduces organic inputs and promotes hard‑pan formation.
• Fuel‑wood & fire‑wood collection – eliminates surface litter and woody biomass → higher soil temperature, increased susceptibility to aeolian erosion.
• Agricultural expansion & irrigation – creates oases but often leads to water‑logging, rising water tables and salinisation (EC > 4 dS m⁻¹) when drainage is inadequate.
• Mining & industrial activity – disturbs soil profiles, introduces heavy metals and radionuclides, and can alter natural drainage patterns.
• Off‑road vehicle use – compacts surface soils, reduces infiltration, initiates rill formation and destroys biological soil crusts.
• Urbanisation – replaces permeable ground with impervious surfaces, altering runoff patterns and increasing flash‑flood risk downstream.

Land‑use change over time (illustrative timeline)

1. 1970‑80s: Declining rainfall → reduced vegetation → expansion of bare ground.
2. 1980‑90s: Population growth → higher livestock numbers → over‑grazing and hard‑pan development.
3. 2000‑present: Motorised agriculture & large‑scale irrigation → groundwater draw‑down, water‑logging and salinisation.

Policy frameworks

• United Nations Convention to Combat Desertification (UNCCD) – global strategy, National Action Plans.
• Country‑level programmes (e.g., Niger’s “Famine Early Warning System”, USA Bureau of Land Management dune‑stabilisation projects).
• Integrated Water Resources Management (IWRM) policies targeting sustainable groundwater use.

7. Climate‑change impacts on arid environments

• Projected temperature rise of 1.5 °C – 3 °C by 2050, intensifying PET and water‑stress.
• Increased frequency of extreme rainfall events → higher flash‑flood risk but also greater erosion and sediment transport.
• Shift in rainfall patterns may lengthen the dry season, expanding desert margins.
• “Wet‑gets‑wetter, dry‑gets‑drier” feedbacks can accelerate desertification.

Evaluation of significance

Modelling for the Sahel indicates that a 10 % increase in PET could reduce the extent of productive land by ~5 % per decade if rainfall does not rise correspondingly. In the Mojave, climate projections suggest a 20 % increase in the magnitude of summer thunderstorms, which would raise flash‑flood erosion rates by an estimated 30 % and could double the rate of gypsum‑soil formation in alluvial fans. These impacts underline the need for adaptive management that simultaneously addresses water scarcity, soil stability and vegetation resilience.

8. Case studies (contrasting examples)

8.1 Sahelian transition zone (low‑income, semi‑arid)

• Location: West Africa – Mali, Niger, Burkina Faso.
• Climate trend: 30 % decline in mean annual rainfall (1970‑1990).
• Human pressure: Livestock numbers rose 70 % despite falling rains.
• Impact: Reduced shrub cover, hard‑pan formation, loss of ~0.3 % yr⁻¹ of productive land.
• Management response: Community‑based “re‑grazing” and tree‑planting (e.g., Faidherbia albida); modest recovery of soil organic carbon (+ 0.2 % after 10 years).
• Evaluation: Success hinges on sustained community participation and climate‑adapted species; continued rainfall variability limits long‑term stability.

8.2 Southwestern United States – Mojave & Colorado Plateau (high‑income, arid)

• Location: Arizona, Nevada, Utah.
• Climate trend: Slight warming (≈ 0.8 °C) with more intense summer thunderstorms.
• Human pressure: Large‑scale irrigation for alfalfa and cotton; groundwater extraction > 2 km³ yr⁻¹.
• Impact: Water‑table decline > 3 m decade⁻¹, salinisation of irrigated soils (EC ≈ 6 dS m⁻¹), expansion of gypsum‑rich Gypsisols.
• Management response: Adoption of drip‑irrigation, salt‑tolerant cultivars, groundwater‑recharge basins; federal “Desert Conservation Plan” regulates off‑road vehicle use.
• Evaluation: Technological measures have slowed salinisation, but long‑term sustainability depends on capping extraction rates to match natural recharge.

9. Management & sustainable‑land‑use strategies

• Controlled grazing – rotational or deferred grazing to allow vegetation recovery and reduce hard‑pan formation.
• Re‑vegetation with native species – planting drought‑resistant, nitrogen‑fixing trees (e.g., Acacia senegal) and shrubs to increase organic matter and protect soils.
• Soil amendments – gypsum to displace Na⁺ in sodic soils; organic mulches to raise humus content and improve water‑holding capacity.
• Water‑saving irrigation – drip or subsurface irrigation, scheduling with soil‑moisture sensors, and use of zai pits or contour bunds to enhance infiltration.
• Dune stabilisation – straw checkerboards, inoculation of biological soil crusts, and establishment of wind‑breaks.
• Groundwater management – abstraction licensing, artificial recharge (e.g., spreading basins), regular monitoring of water‑table depth and EC.
• Community‑based approaches – participatory land‑use planning, benefit‑sharing from eco‑tourism, and local stewardship of oases.
• Policy integration – align desertification‑control plans with national climate‑adaptation strategies and the Sustainable Development Goals (SDG 15 – Life on Land).

10. Summary

Arid environments are characterised by extreme water deficits, episodic rainfall and intense physical processes that shape distinctive soils and sparse vegetation. Climate variability and human pressures interact to accelerate desertification, soil degradation and loss of biodiversity. Climate change is projected to amplify temperature‑driven evapotranspiration and extreme rainfall, intensifying these challenges. Effective management requires a combination of ecological restoration (controlled grazing, native re‑vegetation), soil‑improvement techniques (gypsum, organic mulches), water‑saving irrigation, and robust institutional frameworks that link local actions to global sustainability goals.