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: Houston, Texas, spread over prairie and wetland that once soaked up rain. When Hurricane Harvey stalled over the city in 2017, over 1,000 mm of rain fell in a few days; with so much concrete, it ran off and flooded well over 100,000 homes.
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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: before people arrived, about 80% of New Zealand was forest; today only about a quarter is native forest. On cleared hill country, heavy rain runs off quickly, washing soil into rivers and raising flood peaks downstream.
Say up or down: Every point needs a direction: 'less transpiration', 'more run-off'. 'It affects run-off' is not enough.
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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: from the 1960s the Soviet Union diverted the Amu Darya and Syr Darya rivers to irrigate cotton. So little water reached the Aral Sea that it shrank to about a tenth of its former volume, and its fishing industry collapsed.
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'.
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: Egypt's Aswan High Dam, finished in 1970, created Lake Nasser and ended the Nile's yearly floods. The lake loses over 10 km³ of water a year to evaporation, and with the river's silt trapped behind the dam, the Nile Delta coast is now eroding.
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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: in October 2024 more than 400 mm of rain fell in a few hours in parts of Valencia, Spain. Water rushed down the paved ravine suburbs south of the city as a flash flood, and over 200 people died.
How this comes up: Paper 2, Section B (a): outline four ways urbanisation changes the water cycle [4]. Each point = a change in the city linked to a flow.
Dhaka, the capital of Bangladesh, has grown from about 3 million people in 1980 to over 20 million, building over its wetlands and ponds.
Outline four ways in which urbanisation may influence processes in the hydrological cycle.
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