Dryland systems: water, sediment and degradation
Examine limited moisture, episodic flows and the evidence needed to explain land degradation.
A-level study: check your qualification and chosen options below.
Aridity is a water-balance problem
Low rainfall alone does not describe the moisture available to a landscape. Evaporative demand, seasonal timing, infiltration and stores also matter. A short intense storm can produce runoff while leaving a dry soil profile elsewhere.
Distinguish a climatic description from the condition of a particular field. The same annual rain can have different effects when concentrated into fewer storms or falling outside a crop’s growing period.
Worked example
Fictional annual precipitation is 180 mm and potential evapotranspiration 1,200 mm. P/PET = 0.15. The ratio summarises climatic dryness; it is not the actual amount of water evaporated or a measure of a household’s water access.
Common mistake: Potential evapotranspiration describes demand under stated conditions, not observed water loss from a dry surface.
Episodic runoff and sediment pathways
Dry channels can carry powerful flows after intense rainfall. Infiltration capacity, surface crusting, slope and rainfall intensity affect whether runoff develops. An infrequent event can transfer substantial sediment even where annual rainfall is low.
Wind transport also depends on available loose sediment and conditions at the surface. Saltation, suspension and surface creep move different particle sizes. Vegetation and moisture can limit movement; wind speed alone does not explain a landform.
Worked example
A fictional channel carries 36,000 m³ in two hours: average discharge is 5 m³/s. A brief peak could be much larger. This average cannot establish the competence of the flow or the size of sediment transported.
Common mistake: Low annual rainfall does not imply weak fluvial erosion during every storm.
Desertification and a response that can be evaluated
Desertification concerns land degradation in drylands. It should not be reduced to a desert edge steadily advancing. Climate variability and land use can interact, and the relevant evidence concerns changes in land condition and function.
A response needs a plausible mechanism and a measured outcome. Vegetation cover may improve after reduced grazing, but rainfall and displacement of grazing elsewhere can affect interpretation. Ask who bears the restriction and whether benefits persist.
Worked example
Vegetation cover in a fictional managed plot rises from 20% to 32%; a comparable untreated plot rises from 18% to 26%. The gains are 12 and 8 percentage points. The 4-point difference is useful evidence only if the plots and their prior trends are sufficiently comparable.
Common mistake: A greener satellite image after a wet year does not alone establish successful restoration.
Try a different resource
Climate graphs
Use the accompanying exact monthly values for calculations. Both axes begin at zero.
Scroll the diagram sideways to see the full resource.
| Month | Rainfall (mm) | Temperature (°C) |
|---|---|---|
| Jan | 83 | 7 |
| Feb | 79 | 8 |
| Mar | 113 | 11 |
| Apr | 89 | 14 |
| May | 29 | 18 |
| Jun | 21 | 21 |
| Jul | 32 | 23 |
| Aug | 35 | 22 |
| Sep | 22 | 19 |
| Oct | 21 | 15 |
| Nov | 85 | 11 |
| Dec | 107 | 8 |
Resource description
Jan: 83 mm, 7°C; Feb: 79 mm, 8°C; Mar: 113 mm, 11°C; Apr: 89 mm, 14°C; May: 29 mm, 18°C; Jun: 21 mm, 21°C; Jul: 32 mm, 23°C; Aug: 35 mm, 22°C; Sep: 22 mm, 19°C; Oct: 21 mm, 15°C; Nov: 85 mm, 11°C; Dec: 107 mm, 8°C
Write an explanation
Check your answer
- Develop interacting mechanisms rather than two disconnected lists.
- Compare time scales and spatial variation.
- Use a locally specified taught example with before-and-after evidence.
- Judge the strength of a response against its costs, durability and counterfactual.
Practise and review a written answer
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Course links
Next topics
Hot deserts: water, adaptation and development · Water cycles: budgets, residence time and feedback
Selected teaching for this option. Detailed named landscapes, field evidence and the full specification still need study with your teacher.