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.
The signature of a eutrophication dataset — nutrients and algae climb, oxygen falls.
🔒 Interactive diagram
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Causes and impacts of poor water quality
| Strand | Causes | What it does to the water |
|---|---|---|
| Eutrophication — human | Fertiliser run-off, sewage, detergents, industrial discharge | Adds nutrients → algal blooms → oxygen loss → dead zones |
| Eutrophication — physical | Shallow, warm, slow or enclosed water | Warmth speeds algae; weak flushing lets nutrients build up |
| Salinisation | Over-irrigation in dry areas raises the water table | Evaporation leaves salt at the surface, poisoning crops and supply |
| Pressure on wetlands | Agriculture (run-off, drainage) and altered flow (abstraction, dams) | Lowers water quality and biodiversity |
Key terms — water quality
- Eutrophication — nutrient (nitrate/phosphate) enrichment causing algal blooms and oxygen loss.
- Algal bloom — rapid surface algae growth that blocks light and, on decaying, uses up oxygen.
- Dissolved oxygen — the oxygen fish and insects need; it falls as algae decay.
- Dead zone — water so low in oxygen (hypoxic) that aquatic life cannot survive.
- Salinisation — a build-up of salts in soil and water, often from over-irrigation.
- Point vs diffuse source — one outlet (a pipe) versus pollution spread across an area (field run-off).
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.
Why water becomes scarce
| Type | Examples | What it means |
|---|---|---|
| Physical (absolute) | Low/seasonal rainfall, drought, rain shadow, falling water table | The natural supply itself is too small |
| Economic | No money for pipes/pumps/dams, weak governance, poverty | Water exists but people cannot reach it |
| Rising demand | Population growth, irrigation, industry, cities | Demand outpaces a fixed supply, deepening stress |
| Over-abstraction | Pumping aquifers faster than they recharge | Falling water table, subsidence, saltwater intrusion |
Key terms — water scarcity
- Water scarcity — not enough fresh water to meet demand (per person per year).
- Water stress — supply below 1 700 m3 per person per year (Falkenmark stress line).
- Falkenmark thresholds — below 1 700 = stress, below 1 000 = scarcity, below 500 = absolute scarcity.
- Physical (absolute) scarcity — the natural supply is too small (dry climate, low water table).
- Economic scarcity — water exists but money, infrastructure or institutions are lacking.
- Aquifer / over-abstraction — an underground groundwater store, pumped faster than it refills.
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.
✍️ IB-style questions
Parts of a freshwater lake have become heavily eutrophic. Explain one human reason and one physical reason for this.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
To what extent is rising freshwater scarcity caused by physical rather than economic factors?
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick self-check
Tap each card to reveal the answer.
Trace the eutrophication chain. Nutrients feed an algal bloom; the algae block light, then decay; bacteria use up the oxygen, leaving a low-oxygen dead zone where fish die.
Human vs physical causes of eutrophication? Human = fertiliser run-off, sewage, industrial discharge (they add nutrients). Physical = shallow, warm, slow or enclosed water (it concentrates nutrients and speeds algae).
How does irrigation cause salinisation? Over-irrigation in dry areas raises the water table; as that water evaporates it leaves salt at the surface, poisoning crops and the supply.
Physical vs economic water scarcity? Physical = the natural supply is too small (dry climate, low water table). Economic = water exists but a lack of money, pipes or institutions stops people reaching it.
What does a top [10] essay need? Two+ developed points/effects/causes, a named example for each, a weighing of relative importance (by severity or by place/scale), and a clear conclusion.
🎯 Highest-yield exam reminders
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.