The Water Cycle (Cambridge (CIE) AS Environmental Management)

Revision Note

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Expertise

Biology & Environmental Systems and Societies

Fresh water only makes up a very small fraction (approximately 2.5% by volume) of the Earth’s water storages
- The rest is saltwater
Of this fresh water, approximately 68.7% is stored in glaciers and ice sheets and 30% is stored as groundwater
The remaining 1.3% of freshwater is in rivers, lakes and the atmosphere
All water (both saltwater and freshwater) is part of the water cycle

Globally, the water cycle is a closed system
- This means that there is a constant amount of water on the planet
Within the water cycle, there are stores and transfers
The water cycle is a series of processes in which water is constantly recycled through the system
- The cycle also shapes landscapes, transports minerals and is essential to all life on Earth
The main processes (transfers) occurring within the water cycle are:
- Evaporation - the sun evaporates surface water into vapour
- Condensation - water vapour condenses and precipitates
- Flows - water runs off the surface into streams and reservoirs or beneath the surface as groundwater flow
These processes transfer the water on Earth from one store to another (e.g. river to ocean or ocean to atmosphere)
The water cycle involves energy exchange, leading to local temperature fluctuations
- As water evaporates, it absorbs energy from its surroundings
- This effectively cools the environment
- The reverse happens when water condenses (heat is released)
- This heat exchange influences the local climate

Water Cycle Processes

Process	Description
Condensation	The process through which water vapour in the atmosphere transforms into liquid water, forming clouds or dew, as a result of cooling
Precipitation	The process of water falling from the atmosphere to the Earth's surface in the form of rain, snow, sleet, or hail
Interception	The process through which precipitation is prevented from reaching the ground, usually by being caught on leaves or branches
Infiltration	The process of water seeping into the soil from the surface, entering the soil layers and becoming groundwater
Surface run-off	The movement of water over the Earth's surface, typically occurring when the ground is saturated or impermeable, leading to excess water
Through-flow	The movement of water through the soil between the groundwater store (water table) and the surface
Groundwater flow	The movement of water through underground rock layers
Transpiration	The process by which plants absorb water from the soil through their roots and release it as water vapour through tiny openings called stomata in their leaves
Evaporation	The process by which liquid water changes into a gaseous state (water vapour) and enters the atmosphere from water bodies such as oceans, lakes, and rivers

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