Water cycle

Where it happens

The water cycle happens across oceans, rivers, lakes, soil, the atmosphere, and living things, moving the same water endlessly between these places.

Inputs and outputs

• Input: heat energy from the sun, which drives evaporation from water bodies and soil.
• Input: liquid water in oceans, rivers, lakes, soil and the bodies of living things.
• Input from living things: water vapour released by transpiration from plants and, in smaller amounts, by respiration and sweating in animals.
• Output to the atmosphere: water vapour that later condenses into clouds.
• Output to the land and sea: precipitation as rain, which feeds surface run-off, groundwater and the water absorbed by roots.
• Net result: the total amount of water on Earth stays the same; only its location and state change.

The steps

1. Evaporation: heat energy from the sun turns water on the surface of oceans, rivers and lakes into water vapour.
2. Transpiration: plants lose water vapour from their leaves into the atmosphere through open stomata, adding further water vapour to the air.
3. Condensation: water vapour rises, cools, and condenses into tiny droplets, forming clouds.
4. Precipitation: water falls back to Earth as rain (or other forms), replenishing rivers, lakes, soil and groundwater.
5. The water then flows over land as surface run-off or seeps through soil, eventually returning to rivers and oceans, and the cycle repeats.
6. Infiltration: some rainwater soaks into the soil, where it is held as soil water for plant roots or moves deeper to become groundwater that slowly feeds rivers and springs.
7. Uptake by living things: plants absorb soil water through their root hairs, and animals drink water or take it in with food; this water is later returned by transpiration, respiration, excretion and decomposition.

Why it matters and how it is controlled

The water cycle continually renews the fresh water available on land, supplying rivers, lakes and groundwater that living things depend on. Because transpiration from plants adds a significant amount of water vapour to the atmosphere, large-scale loss of forests can reduce local rainfall over time.

The cycle has no central control; its rate is set by physical factors. Higher temperatures increase evaporation and transpiration, while high humidity slows both.

Vegetation cover matters because a forest canopy returns a large share of rainfall to the air as transpiration, and its roots and leaf litter slow run-off so that more water infiltrates the soil.

Human activity changes the balance. Deforestation reduces transpiration and increases surface run-off, which leads to flash floods, soil erosion and lower groundwater levels.

Paved urban surfaces have a similar effect because rain cannot infiltrate. Pollution of rivers and groundwater does not stop the cycle but reduces the amount of clean water it delivers.

For the ecosystem chapter, the water cycle links to the carbon and nitrogen cycles: water is a reactant in photosynthesis, the solvent for mineral ions taken up by roots, and the medium in which decomposers release nutrients back into the soil.

How it is examined

You may be asked to name and order the stages of the water cycle, to explain the role of transpiration in adding water vapour to the atmosphere, or to explain why forests influence local rainfall patterns.

Diagram-based questions give an unlabelled cycle and ask you to name the processes at each arrow. Marks depend on using the correct terms, evaporation, transpiration, condensation, precipitation, run-off and infiltration, and on placing transpiration at the plant, not the soil.

Application questions describe a change such as clearing a forest for a plantation or building a housing estate, then ask you to predict and explain the effect on local rainfall, flooding or groundwater. Full-mark answers state the change in transpiration or infiltration first, then trace the consequence through the cycle.

Common misconceptions

Worked exam-style question

Question. A hillside forest was cleared to build a housing estate. Residents later reported that the nearby river flooded more often after heavy rain and that the local well dried up during the dry season.

(a) Name the process by which trees return water to the atmosphere. (b) Explain why the river floods more often after the forest was cleared.

(c) Explain why the well dried up. (d) Suggest one measure that could reduce both problems.

Model answer. (a) Transpiration. (b) With the trees gone, there is no canopy to intercept rain and no root system or leaf litter to hold water, so less rain infiltrates the soil and more flows as surface run-off.

The run-off reaches the river quickly and in larger volume, so the river overflows. (c) Less infiltration means less water reaches the groundwater that feeds the well.

In the dry season the groundwater is not replenished, so the water table falls below the level of the well. (d) Replant trees or keep vegetated buffer zones on the slope, so that roots and leaf litter slow run-off and increase infiltration.

Source:SRC-DSKP-EN

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