The urban system at Higher Level: The same systems ideas as SL, with a different city: Cape Town and its 2018 water crisis. At HL, use the systems view to judge a city: how efficient, how sustainable and how resilient it is.
Practise this as you read
- Draw a city as a systems flow diagram.
- Judge a city's resilience from its inputs and storages.
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A city works like a machine with many parts: An urban system takes things in, changes them, and sends things out.
The points to remember
- A city's components are its storages: people, buildings, transport, power, water and sewage, plants and animals.
- The components are interrelated: change one and the others change.
- Flows of matter and energy link them: inputs, outputs, transfers, transformations.
- A city is an open system: it exchanges matter and energy with its surroundings.
- It has feedback: responses that push it back towards balance, or further away.
- Its microclimate (warmth, wind, shade) is part of the system too.
Storages
- People, buildings
- Roads, power, water
Flows
- Inputs and outputs
- Transfers, transformations
Behaviour
- Open system
- Feedback
Real example: Cape Town, South Africa, has about 4.8 million people. When its dams ran low in 2018, homes, farms, factories and hospitals all had to change: the parts are interrelated.
Remember it as: Stores, flows, open, feedback.
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Everything a city uses comes in across its boundary, and everything it throws away goes out. That is why it is an open system.
Inputs and outputs
- Inputs of matter: food, water, building materials, goods, people moving in.
- Inputs of energy: electricity, gas and fuel; sunlight.
- Outputs of matter: solid waste, sewage, polluted air and CO₂, exported goods, people leaving.
- Outputs of energy: heat lost from buildings, engines and roads.
- A city takes in far more than it can make: it depends on land far beyond its boundary.
Real example: almost all of Cape Town's water comes from six large dams in the mountains outside the city, fed by winter rain. The water is an input; the treated wastewater that flows to the sea is an output.
Inside the boundary, matter and energy keep moving and changing. The papers call these processes.
Transfers and transformations
- A transfer moves something without changing it: commuters by train, water through pipes, goods by lorry.
- A transformation changes it: burning fuel turns chemical energy into heat and CO₂.
- Food becomes waste and sewage; sewage works turn sewage into cleaner water and sludge.
- Parks and trees: photosynthesis, and evapotranspiration that cools the air.
- Say which kind each process is: transfer or transformation.
Transfer (moves)
- Commuters on a train
- Water in pipes
- Rubbish in a lorry
Transformation (changes)
- Fuel burned in an engine
- Sewage treated
- Food into waste
Real example: in Cape Town, water is transferred from the dams through pipes, then transformed at treatment works; parks on the slopes of Table Mountain lose water to the air by evapotranspiration.
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When something changes in a city, the city responds. That response is feedback.
Feedback in a city
- Negative feedback opposes a change and brings the system back towards balance.
- Example pattern: too many people for the resources, so the birth rate falls and the population falls back.
- Positive feedback pushes the change further: growth attracts jobs, which attract more people.
- Feedback is often a human response: a charge, a limit, a new law.
Real example: in 2017-2018 Cape Town's dams fell to about a fifth full. From February 2018 each person was limited to 50 litres a day; use fell by about half, and 'Day Zero', when taps would be turned off, never came.
The guide asks you to draw an urban system as a systems flow diagram. Build it in five steps.
Drawing a city as a systems diagram
- Draw a dashed boundary round the city.
- Draw storages as boxes inside it: people, homes, roads, green space.
- Draw inputs as arrows into the boundary and outputs as arrows out of it.
- Draw transfers as arrows between boxes; label every arrow with what flows.
- Show matter and energy flows differently, for example energy as dashed arrows.
Boundary
- Draw a dashed line round the City of Cape Town.
Storages
- Boxes inside: people, homes and factories, roads, Table Mountain and parks.
Inputs
- Arrows in: water from six dams, food from Western Cape farms, electricity.
Outputs
- Arrows out: wastewater to the sea, waste to landfill, exhaust and heat.
Transfers
- Arrows between boxes: water piped from treatment works to homes.
Label every arrow: An arrow with no label shows nothing. Write what flows ('food', 'sewage', 'heat'), and keep storages in boxes, not floating words.
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Seeing a city as a system helps to judge it: what it wastes, how well it uses its inputs, whether it can last, and whether it can survive a shock.
Waste, efficiency, sustainability, resilience
- Waste and pollution are the city's outputs: rubbish, sewage, fumes, CO₂, heat.
- Efficiency: more done for each unit of input; dense housing and public transport use less per person.
- Sustainability: inputs taken no faster than they can be replaced; outputs no faster than they can be absorbed.
- Resilience: the ability to keep working, and recover, after a shock such as a flood or drought.
- More varied inputs and storages (reservoirs, defences) make a city more resilient.
Efficient
- Less input per person
- Dense, shared services
Sustainable
- Inputs replaced
- Outputs absorbed
Resilient
- Survives shocks
- Many sources, big stores
Real example: after the crisis, Cape Town added groundwater, small desalination plants and water recycling, so it no longer depends on rain-fed dams alone: more varied inputs make it more resilient.
How this comes up: Paper 2, Section B (a): outline how a city can be considered as a system [4] (May 2026).
Cape Town takes in food, water and energy and sends out waste and heat.
Outline how a city such as Cape Town can be considered as a system.
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