Key Idea: Topic 7.3 is about the two great freshwater problems — how clean the water is, and whether there is enough of it. It pulls together two micros: 7.3.1 — water quality & pollution: how eutrophication (nutrient enrichment), salinisation and other pollution lower water quality, driven by human causes (fertiliser run-off, sewage, industry) and physical ones (shallow, warm, slow water). 7.3.2 — water scarcity & stress: how physical scarcity (the supply itself is too small) and economic scarcity (water exists but is unreachable) leave people short of water, measured against the Falkenmark thresholds. This is Option A (Freshwater), examined on Paper 1. SL answers two options (HL answers three — same questions). Each option gives a structured question (short data-response + Explain/Outline) and a [10] extended answer (Examine / To what extent), marked on markbands.
💧 7.3.1 — Water quality & pollution
Water quality is how clean and usable fresh water is; pollution lowers it. The headline process is eutrophication — nutrients (nitrate, phosphate) feed an algal bloom, the algae decay, dissolved oxygen falls and a dead zone forms. The skill examiners test is reading an eutrophication dataset (nitrate up, oxygen down) and explaining causes as a human strand and a physical strand. Farming also drives salinisation and squeezes wetlands.
[Diagram: geo-line-chart]
Tip: Don't stop at 'fertiliser run-off'. Carry it through: nutrients → algal bloom → algae decay → bacteria use the oxygen → fish die / dead zone. The development marks live in the chain, not the trigger. And remember every cause has a human and a physical version.
🚱 7.3.2 — Water scarcity & stress
Water scarcity is too little fresh water for people's needs, measured per person per year. Physical (absolute) scarcity means the natural supply is too small (dry climate, low water table); economic scarcity means the water exists but is unreachable (no money, pipes or institutions). The skill examiners test is reading availability off a bar graph and classifying a country against the Falkenmark lines, then weighing physical against economic causes in the essay. Over-abstraction and drought deepen scarcity, hitting farming hardest.
Below 1 700 m3 per person per year = stress; below 1 000 = scarcity; below 500 = absolute scarcity. A data question often hands you a country's figure and asks you to classify it — and the essay turns on whether scarcity is mainly physical or economic, an answer that changes with place and scale.
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Exam Tips
- Eutrophication chain: nutrients → algal bloom → decay → oxygen loss → dead zone. The marks are in the chain, not the trigger.
- Every pollution cause has a HUMAN version (run-off, sewage) and a PHYSICAL version (shallow, warm, slow water) — name one of each in a [6] Explain.
- Salinisation = over-irrigation raises the water table, then evaporation concentrates salt at the surface.
- Falkenmark numbers: stress < 1 700, scarcity < 1 000, absolute < 500 m3 per person per year — use them to classify a country from a graph.
- Separate PHYSICAL scarcity (supply too small) from ECONOMIC scarcity (water unreachable); the essay weighs the two by place and scale.
- On the Paper 1 [10] Examine / To what extent essay: develop two+ points, add named examples, weigh them, and finish with a clear judgement.