Learn this topic properly. Interactive diagrams, worked examples, check questions and exam-style practice with mark schemes. Open “4 · Water” in FortyFive →
The continuous movement of water between storages.
Storage
Oceans, ice caps, groundwater, lakes, soil, atmosphere, living things.
Residence time
The average time water stays in a storage.
Aquifer
Rock that stores groundwater and allows it to flow.
About 97% of water is in the oceans; most fresh water is in ice caps and groundwater. Only a tiny fraction is in lakes and rivers.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Use the correct term for each flow (infiltration vs. percolation vs. throughflow): precision is rewarded.
4.2Water access, use and security
Key terms
Water security
Reliable access to enough safe water for health, livelihoods and production.
Physical water scarcity
Not enough water to meet demand.
Economic water scarcity
Water exists but there is no money or infrastructure to access it.
Water footprint
Total volume of fresh water used to produce goods and services.
Demand is rising due to population growth, irrigation (about 70% of global freshwater use), industry and urbanisation; supply is affected by climate change, pollution and overuse.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Distinguish physical from economic water scarcity: the solutions are very different.
4.3Aquatic food production systems
Key terms
Maximum sustainable yield (MSY)
The largest catch that can be taken each year without reducing the stock in the long term.
Aquaculture
Farming of fish and other aquatic organisms.
Bycatch
Unwanted species caught by fishing gear.
Fishing quota
A legal limit on the amount of fish that can be caught.
Overfishing is driven by demand, technology (sonar, huge trawlers) and the tragedy of the commons. Example: the collapse of the Grand Banks cod fishery (1992).
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: MSY assumes we know the stock size and growth rate exactly; real estimates are uncertain, so managers usually set catches below MSY.
4.4Water pollution
Key terms
Point / non-point source
Pollution from a single identifiable source / from many diffuse sources.
Eutrophication
Excess nutrients (nitrates, phosphates) causing algal blooms and oxygen depletion.
Biochemical oxygen demand (BOD)
The amount of oxygen used by microorganisms to break down organic matter.
Indicator species
Species whose presence or absence shows the level of pollution.
Dead zone
An area of water with too little oxygen to support most life.