The Water Cycle

This lesson covers the water cycle as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe the main stages of the water cycle, explain the roles of evaporation, condensation, precipitation and transpiration, and understand how water is recycled through ecosystems.

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Why the Water Cycle Matters

Water is essential for all living organisms:

• It is the solvent for biochemical reactions in cells
• It is needed for photosynthesis (a reactant)
• It transports dissolved substances in the blood and xylem
• It helps maintain body temperature (sweating, transpiration)

There is a finite amount of water on Earth. The water cycle continuously recycles water between the atmosphere, land and oceans so that it is available for living organisms.

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The Water Cycle — Overview

graph TD
    A["Oceans, lakes, rivers"] -->|"Evaporation (heat from the Sun)"| B["Water vapour in atmosphere"]
    B -->|"Condensation (cooling)"| C["Clouds form (tiny water droplets)"]
    C -->|"Precipitation (rain, snow, hail)"| D["Water falls on land"]
    D -->|"Surface run-off"| A
    D -->|"Percolation"| E["Groundwater"]
    E -->|"Groundwater flow"| A
    D -->|"Absorbed by plant roots"| F["Plants"]
    F -->|"Transpiration"| B
    A -->|"Evaporation"| B

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Key Stages of the Water Cycle

1. Evaporation

Evaporation is the process by which liquid water is converted into water vapour (gas) by heat energy from the Sun.

• Occurs from the surface of oceans, lakes, rivers, soil and puddles
• The warmer the temperature, the faster the rate of evaporation
• Water vapour rises into the atmosphere

2. Transpiration

Transpiration is the loss of water vapour from the leaves of plants through stomata (tiny pores on the leaf surface).

• Water is absorbed by plant roots from the soil
• It travels up the plant through the xylem
• It evaporates from the leaf surface into the atmosphere

Transpiration is a major contributor to the water cycle, especially in areas with dense vegetation like forests.

3. Condensation

Condensation occurs when water vapour in the atmosphere cools and changes from a gas back into tiny liquid water droplets.

• This happens at higher altitudes where the air is cooler
• The tiny droplets gather around dust particles in the air
• Billions of these droplets form clouds

4. Precipitation

Precipitation is any form of water that falls from the atmosphere to the Earth's surface:

Form	Description
Rain	Liquid water droplets
Snow	Ice crystals
Hail	Balls of ice
Sleet	Mixture of rain and snow

Precipitation occurs when water droplets in clouds become too heavy to remain suspended in the air.

5. Surface Run-off and Percolation

Once water reaches the ground, it can:

• Flow over the surface as surface run-off into rivers, lakes and oceans
• Soak into the ground (percolation) to become groundwater
• Be absorbed by plant roots

Groundwater flows slowly through porous rock and eventually reaches rivers, lakes or the ocean.

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The Role of Living Organisms in the Water Cycle

Organism	Role in water cycle
Plants	Absorb water through roots; release water vapour through transpiration
Animals	Take in water by drinking or eating; release water in breath, urine and sweat
Decomposers	Release water as a product of aerobic respiration during decomposition

Exam Tip: Plants play a double role — they absorb water from the soil (reducing groundwater) and release water vapour into the atmosphere (adding to the water cycle through transpiration).

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Factors Affecting the Rate of Transpiration

Since transpiration is a key part of the water cycle, you should know what affects it:

Factor	Effect on transpiration rate
Temperature	Higher temperature → faster evaporation → more transpiration
Humidity	Higher humidity → smaller concentration gradient → less transpiration
Wind speed	More wind → water vapour blown away from leaf → faster transpiration
Light intensity	More light → stomata open wider → more transpiration

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Water Cycle and Ecosystems

The water cycle is vital for maintaining ecosystems:

• Freshwater habitats (rivers, lakes) depend on precipitation and groundwater flow
• Soil moisture (from precipitation and groundwater) is essential for plant growth
• Nutrient transport — water dissolves minerals in soil and carries them to plant roots
• Wetlands act as natural water filters and flood defences

Exam Tip: If asked how the water cycle supports life, focus on water availability for organisms, nutrient transport in soil and the role of transpiration in moving water through plants.

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Human Impact on the Water Cycle

Activity	Effect
Deforestation	Fewer trees → less transpiration → less water vapour returned to atmosphere; increased surface run-off and soil erosion
Urbanisation	Concrete and tarmac prevent percolation → more surface run-off → increased flood risk
Irrigation	Removes water from rivers and groundwater for farming → can lower water tables
Climate change	Rising temperatures → increased evaporation → more extreme weather (droughts, floods)

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Comparing the Water Cycle and Carbon Cycle

Feature	Water Cycle	Carbon Cycle
What is recycled	Water (H₂O)	Carbon (C) in various compounds
Key processes	Evaporation, condensation, precipitation, transpiration	Photosynthesis, respiration, combustion, decomposition
Energy source	Heat from the Sun drives evaporation	Light energy for photosynthesis; chemical energy for respiration
Reservoirs	Oceans, ice caps, groundwater, atmosphere	Atmosphere (CO₂), fossil fuels, biomass, oceans, soil

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Summary

• The water cycle recycles water between the atmosphere, land and oceans.
• Key stages: evaporation → condensation → precipitation → run-off / percolation.
• Transpiration from plants is a major part of the water cycle.
• Living organisms (plants, animals, decomposers) all contribute to the water cycle.
• Temperature, humidity, wind speed and light intensity affect transpiration rate.
• Human activities (deforestation, urbanisation, climate change) can disrupt the water cycle.
• Water is essential as a solvent, reactant and transport medium in living organisms.

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