Key Idea: Most marks in topic 4.1 come from the water cycle as a system and how people change its flows. At HL, also learn stratification, upwellings and thermohaline circulation.
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 and HL ideas
- 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.
- At HL: properties of water, stratification, upwellings and thermohaline circulation.
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 |
| Thermocline | the layer where temperature drops quickly with depth |
| Upwelling | cold, nutrient-rich water rising to the surface |
Last-minute check: answer, then tap to see.
Why does a lake freeze from the top? Water is densest at 4 °C, so colder water floats above it.
What stops warm and cold layers mixing? Their density difference: the thermocline.
Why are upwelling zones so productive? They bring nutrients up into sunlit water.
What drives the ocean conveyor belt? Differences in temperature and salinity.
North-west Europe has milder winters than places at the same latitude in North America. Explain how thermohaline circulation contributes to this.
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.
- For thermohaline circulation, say both temperature and salinity.