How human activities change water flows at Higher Level: The same ideas as SL with different examples: Mumbai's 2005 flood, Haiti, the Murray-Darling and the Three Gorges Dam. At HL, expect to name the factors behind a flash flood from a fact file.
Practise this as you read
- Link every activity to a flow and a direction.
- Use the numbers: percentages, peaks, lag times.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Concrete changes the flows: Cities replace soil and plants with hard surfaces. Rain that once soaked in now runs straight into drains and rivers, so rivers rise faster and higher, and a flash flood becomes more likely.
What urbanisation does to the flows
- Roofs and roads are impermeable: less infiltration, so more surface run-off.
- Drains rush run-off to rivers: higher, faster peaks and flash floods.
- Less infiltration means less groundwater recharge: the water table falls.
- Fewer plants means less evapotranspiration, so less water vapour.
- City heat raises evaporation and can bring heavier rain downwind.
- Cities pump water from rivers and aquifers; exhaust gases make acid rain.
Real example: on 26 July 2005, 944 mm of rain fell on Mumbai in 24 hours. Much of the city's mangrove swamp and open ground had been built on, and blocked drains could not cope, so the city flooded and over 1,000 people died across the state.
Memorize terms 3x faster
Smart flashcards show you cards right before you forget them. Perfect for definitions and key concepts.
Trees catch rain, hold the soil and return water to the air. Cutting them down changes each of these flows.
The points to remember
- Fewer leaves: less interception, so more rain reaches the ground at once: more run-off.
- Fewer roots and less leaf litter: less infiltration, less groundwater storage.
- Fewer trees: less transpiration, drier air and less rain locally.
- Bare soil is eroded: more sediment and nutrients wash into rivers.
- Bare soil also loses water by evaporation and dries out.
Remember it as: No trees: less caught, less soaked, less breathed out, more run off.
Goes down
- Interception
- Infiltration
- Transpiration
- Groundwater storage
- Local rainfall
Goes up
- Surface run-off
- Flood risk
- Soil erosion
- Sediment in rivers
- Evaporation from bare soil
Real example: Haiti has lost almost all its original forest. In 2004 Tropical Storm Jeanne dropped heavy rain on the bare hills above Gonaïves; with little to hold it, flash floods and mudslides killed about 3,000 people.
Say up or down: Every point needs a direction: 'less transpiration', 'more run-off'. 'It affects run-off' is not enough.
Farming changes the water cycle through what it takes out (irrigation) and how it treats the soil.
The points to remember
- Irrigation abstracts water: rivers, lakes and aquifers shrink.
- Ploughed, bare fields: more run-off and soil erosion, muddier rivers.
- Machines and overgrazing compact the soil: less infiltration.
- Fertiliser run-off causes eutrophication; pesticides poison aquatic life.
- Draining wetlands removes a natural water store.
Taking water
- Irrigation lowers rivers, lakes and aquifers.
- Fish and wetlands lose their water.
Bare soil
- Ploughing and compaction raise run-off.
- Eroded soil clouds the rivers.
Chemicals
- Fertilisers wash in: algal blooms.
- Pesticides kill non-target species.
Real example: irrigation in Australia's Murray-Darling basin takes so much water that, in the drought years after 2002, the Murray no longer reached the sea and its mouth had to be kept open by dredging.
Cause, then effect: Link each farming practice to what it does to the water: 'fertiliser washes into the lake and causes algal blooms', not just 'fertiliser'.
Know your predicted grade
Take timed mock exams and get detailed feedback on every answer. See exactly where you're losing marks.
A dam does not change how much rain falls, but it changes where water is stored and when it flows.
The points to remember
- A dam creates a new store: the reservoir.
- Flow downstream becomes smaller and steadier: fewer floods, but less water for wetlands.
- Sediment is trapped behind the dam, so deltas and coasts downstream erode.
- A big reservoir loses a lot of water by evaporation.
- Water seeping from the reservoir can raise groundwater nearby.
Above the dam
- A new reservoir store
- Evaporation from its surface
- Sediment settles out
Below the dam
- Smaller, steadier flow
- Fewer floods
- Less sediment: deltas erode
Real example: China's Three Gorges Dam on the Yangtze holds a reservoir about 600 km long. Since it filled, the sediment the Yangtze carries to the sea has fallen by over two thirds, and parts of the river's delta near Shanghai are eroding.
Questions often give a figure of where rain goes in a forest and in a town, and ask you to calculate or explain a change.
Reading the figure
- Read each flow as a percentage of the rain.
- Infiltration = shallow + deep: add them before comparing.
- Give the units (%) in every calculated answer.
- Flash-flood factors: paving, poor drains, steep slopes, thin soils, heavy rain or melt, felled slopes, dam failure.
- A storage question wants a store (groundwater, biomass), not a flow.
Worked example: the difference in infiltration: Formula: difference = forest infiltration - city infiltration.
Forest: 25% + 25% = 50%. City: 10% + 5% = 15%. Difference = 50% - 15% = 35%.
Real example: Thimphu, Bhutan's capital, lies in a steep valley fed by glaciers. Concrete is spreading over the valley floor and its storm drains are poor, so heavy summer rain brings flash floods.
Get feedback like a real examiner
Submit your answers and get instant feedback — what you did well, what's missing, and exactly what to write to score full marks.
How this comes up: Paper 1: outline two factors that raise the risk of flash flooding in a city [2], from a fact file.
Islamabad, Pakistan, lies at the foot of the Margalla Hills. In July 2001 about 620 mm of rain fell in 10 hours, and the Nullah Lai stream flooded the city.
Outline two factors that may increase the risk of flash flooding in Islamabad.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.