Key Idea: Most marks in topic 4.1 come from what drives the water cycle and how people change its flows. Learn to draw the cycle as stores and flows.
1. What drives the cycle
- Solar radiation heats water so it evaporates; gravity pulls it back down and downhill.
- Evaporation absorbs heat; condensation releases it (latent heat).
- Most water is in the oceans (about 96.5%); very little is fresh surface water.
2. The cycle as a system
- Draw stores as boxes and flows as arrows.
- Transfers move water (run-off, infiltration); transformations change its state (evaporation).
- A water body is in steady state when inputs equal outputs.
3. Human impacts on flows
- Urbanization: hard surfaces cut infiltration, so run-off is faster and floods are flashier.
- Deforestation: less interception and transpiration, more run-off and soil erosion.
- Harvesting a lake or aquifer faster than it refills is unsustainable.
How it is examined:
| Question | How to score |
|---|---|
| [1] State a store or flow from a diagram | Use the exact name: infiltration, not 'soaking in'. |
| [2] to [4] Draw a systems diagram of the cycle | Boxes for stores, labelled arrows for flows. |
| [2] to [3] Explain how land use changes flows | Name the change, the flow it alters and the result. |
| [7] Explain how the water cycle affects climate | Latent heat, ocean heat transport, clouds and albedo. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Infiltration | water entering the soil |
| Percolation | water moving down through the soil |
| Advection | wind carrying water vapour or clouds |
| Sublimation | ice turning straight into water vapour |
| Steady state | inputs and outputs balance, so the store stays the same size |
Last-minute check: answer, then tap to see.
Which heats water to drive evaporation? Solar radiation.
Does condensation absorb or release heat? It releases latent heat.
How does a car park change run-off? Less infiltration, so faster, larger surface run-off.
How are stores drawn in a systems diagram? As boxes; flows are arrows.
IB-style questionExplain[4 marks]
A town has replaced fields with houses, roads and car parks. Explain how this may increase the risk of flash flooding.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- In a systems diagram, label every arrow with the flow's name.
- Say whether a flow is a transfer or a transformation.
- For land-use questions, name the flow that changes: infiltration, interception or run-off.
- Learn the order of stores by size: oceans, ice, groundwater, surface water, atmosphere.