Soil Erosion

Question

Step-by-step solution

1. Step 1

   Deforestation — trees intercept rainfall (reducing its impact force), their roots bind soil particles together, and leaf litter adds organic matter that improves soil structure. When trees are removed, rain hits bare soil directly, dislodging particles; without roots, soil is no longer anchored and washes away easily.

2. Step 2

   Overgrazing — too many animals on a pasture strip vegetation faster than it can regrow. Bare soil is exposed to rain and wind. Livestock hooves also compact soil, reducing infiltration and increasing surface runoff, which accelerates erosion.

3. Step 3

   Ploughing up and down slopes — ploughing along the contour of a hill channels runoff horizontally across the hill, slowing it. Ploughing vertically (up-down) creates channels that funnel rainwater rapidly downhill, increasing erosion significantly.

4. Step 4

   Monoculture farming / leaving soil bare — after harvest, fields left bare (no ground cover) expose soil to wind and rain. Monocultures have shallow root systems that leave gaps between plants, reducing soil binding compared to diverse vegetation.

Question

Step-by-step solution

1. Step 1

   Loss of topsoil / reduced soil fertility (environmental + economic) — topsoil contains most of the nutrients and organic matter needed for crop growth. Once removed, subsoil exposed beneath is nutrient-poor, stony and difficult to farm. Crop yields decline, threatening food security and farmers' incomes.

2. Step 2

   Desertification (environmental) — in semi-arid regions, loss of vegetation and topsoil leads to the spread of desert-like conditions. Bare, dry ground absorbs less water and reflects more heat, further reducing rainfall and making recovery almost impossible. The Sahel region of Africa is a major example.

3. Step 3

   Sedimentation and water pollution (environmental + economic) — eroded soil particles are carried by rivers and deposited as silt in reservoirs, riverbeds and coastal areas. Reservoirs lose capacity; rivers become shallow and flood more easily; coral reefs and coastal fisheries are smothered by sediment, harming livelihoods.

Question

Step-by-step solution

1. Step 1

   Contour ploughing — ploughing along the contour (horizontal) lines of a slope rather than up and down. This creates ridges that trap water and slow runoff, reducing the kinetic energy of water flowing over the field and giving it time to infiltrate rather than erode soil.

2. Step 2

   Terracing — cutting the slope into a series of flat steps (terraces). Each terrace retains soil and water. Traditional terracing is used in rice farming in Southeast Asia (e.g. Philippines Banaue Rice Terraces) and prevents soil from washing down steep slopes.

3. Step 3

   Afforestation and shelter belts — planting trees and hedgerows along field edges breaks wind speed, reducing wind erosion. Tree roots bind soil; canopies intercept rain. Shelter belts of trees planted on windward sides of fields are especially effective in semi-arid and prairie regions.

Question

Step-by-step solution

1. Step 1

   Three causes of soil erosion in the Sahel:

   (1) Population growth and overgrazing: rapid population growth in Sahel nations has increased livestock numbers beyond the carrying capacity of grasslands. Overgrazing strips vegetation faster than it can regrow, exposing bare soil to wind and seasonal rainfall. The exposed soil, with no root binding, is easily eroded in the brief but intense wet season rains.

   (2) Drought and reduced rainfall: the Sahel experienced severe multi-year droughts in the 1970s and 1980s (and again in recent decades), reducing vegetation cover dramatically. Without plant cover, the albedo (reflectivity) of the land surface increases — reflecting more solar energy back rather than evaporating moisture, further reducing rainfall in a positive feedback loop. This physical driver amplifies the human pressures.

   (3) Deforestation for fuelwood and agriculture: growing populations require firewood for cooking and land for crop production. Trees are cleared, removing root binding and canopy protection. Without canopies, the intense raindrop impact directly dislodges soil particles (splash erosion), and without roots, soil washes away in surface runoff during rainstorms.

2. Step 2

   The Great Green Wall initiative: Launched in 2007 by the African Union, the Great Green Wall aims to restore 100 million hectares of degraded land across 11 Sahel countries by 2030, primarily through tree planting and vegetation restoration along an 8,000 km band from Senegal in the west to Djibouti in the east.

3. Step 3

   Evaluation — Advantages: (1) Trees and restored vegetation provide root binding, reducing erosion — and their canopies intercept rainfall, reducing splash erosion impact. (2) Restored vegetation increases local evapotranspiration, potentially increasing rainfall and reducing the positive feedback loop of bare soil → reduced rainfall → more bare soil. (3) The initiative incorporates traditional land management practices (Farmer Managed Natural Regeneration, FMNR) — allowing existing tree stumps to regrow rather than planting new seedlings — which has proven highly effective and low-cost in Niger. (4) Restoration creates new livelihoods (agroforestry, grazing under trees) and carbon sequestration that can be sold as carbon credits, providing income.

4. Step 4

   Evaluation — Disadvantages and limitations: (1) Progress is far behind target — by 2020, only ~15% of the 100 million hectare target had been achieved. Funding pledges ($14 billion from COP15) have been slow to materialise. (2) Many tree planting projects fail — seedlings planted without local community ownership die in the dry season, wasting resources. (3) The underlying drivers (population growth, poverty, climate change) are not addressed by tree planting alone — without reducing overgrazing pressure and improving livelihoods, restored land may be cleared again. (4) Climate change continues to reduce Sahel rainfall, threatening the survival of restored vegetation.

5. Step 5

   Conclusion: The Great Green Wall concept is sound and the FMNR approach has proven genuinely effective in local contexts (e.g. Niger). However, the overall initiative's effectiveness is undermined by insufficient funding, political challenges across 11 countries, and failure to address root causes. It is most effective when combined with poverty reduction programmes, community governance of land, and livestock management regulation.

Question

Step-by-step solution

1. Step 1

   Comparing contour ploughing and terracing for steep slopes:

   Contour ploughing works by creating horizontal ridges across the slope that slow and trap surface runoff. It is most effective on gentle to moderate slopes (5–15°). The ridges created by a plough are relatively small and can be overwhelmed by the volume and velocity of runoff on very steep slopes (>20°). On a 30°+ slope, the water energy is too great for shallow contour ridges alone to prevent erosion.

   Terracing physically restructures the slope by cutting it into a series of wide, flat steps supported by retaining walls. Each terrace is nearly level, so water accumulates on the flat surface and infiltrates slowly — very little surface runoff occurs. Terraces can function effectively even on very steep slopes (rice terraces in the Philippines exist on slopes of 40°+). The retaining walls (stone or compacted earth) permanently prevent downslope movement of soil.

2. Step 2

   Recommendation: For this scenario (>30° gradient, rice farming), terracing is more appropriate. Contour ploughing is insufficient for slopes this steep — the runoff volume would overwhelm the ridges and erosion would still be severe. Rice cultivation requires near-level ground for flooded paddy conditions, which only terracing provides. The Banaue Rice Terraces in the Philippines (>2,000 years old, slope ~45°) demonstrate that terracing is the proven solution for exactly this scenario.

3. Step 3

   Limitation of terracing: Terracing requires enormous initial investment — both in labour (cutting steps into rock and soil, building retaining walls) and time. For a small-scale farmer with 5 hectares on very steep slopes, the construction cost may be prohibitive without government subsidy or community labour-sharing. The retaining walls must be maintained regularly — if walls collapse during heavy rainfall, catastrophic erosion of multiple terraces can occur rapidly, worse than if no terracing had been done.

Question

Step-by-step solution

1. Step 1

   Percentage reduction calculation: Reduction = (45 – 2) / 45 × 100 = 43/45 × 100 = 95.6% reduction. This shows that the combination of terracing with vegetation cover reduces soil loss to near the sustainable threshold of <5 t/ha/yr.

2. Step 2

   Cost-effectiveness evaluation:

   Contour ploughing (Plot 2) reduces erosion by 60% ((45–18)/45 × 100) with relatively low cost — it requires only a plough, which many farmers already own, and can be performed seasonally. However, at 18 t/ha/yr, it is still far above the sustainable threshold of 5 t/ha/yr, so while cost-effective in reducing erosion, it does not achieve sustainability.

   Terracing alone (Plot 3) achieves 82% reduction to 8 t/ha/yr — close to but not quite at the sustainable threshold. The cost of terracing construction is substantial (retaining walls, earthworks), but once built, it requires minimal ongoing input.

   Terracing + vegetation (Plot 4) achieves 95.6% reduction to 2 t/ha/yr, the only plot below the sustainable threshold. Adding vegetation (ground cover plants, hedgerows) adds modest ongoing cost to the terracing investment.

   Best trade-off: terracing alone (Plot 3) is near-sustainable at moderate cost — the addition of vegetation (Plot 4) to achieve full sustainability is a relatively small additional investment that closes the gap below 5 t/ha/yr. The combination is the most justified solution.

3. Step 3

   Why Plot 4 may not be fully sustainable long-term: Even at 2 t/ha/yr, soil is still being lost. Over 50 years, this accumulates to 100 t/ha of topsoil lost. If topsoil depth is limited (common on steep slopes where soil formation rates are very low — typically 0.1–0.5 t/ha/yr of new soil formation), net soil loss continues. Additionally: (1) Climate change may increase rainfall intensity in the region, exceeding the design capacity of terraces. (2) If vegetation is removed or poorly maintained, erosion rates will rise rapidly. (3) The sustainability threshold of 5 t/ha/yr is itself an average estimate — local conditions (topsoil depth, soil formation rate) determine the true sustainable limit, which may be even lower.