1.4.1 The Water Cycle (Cambridge International AS Environmental Management)
Water Cycle Processes
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1.4.1
Water on Earth
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Fresh water only makes up a very small fraction (approximately 2.5% by volume) of the Earth’s water storages
- The rest is saltwater
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Of this fresh water, approximately 68.7% is stored in glaciers and ice sheets and 30% is stored as groundwater
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The remaining 1.3% of freshwater is in rivers, lakes and the atmosphere
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All water (both saltwater and freshwater) is part of the water cycle
Comparison of the world's freshwater stores
Water Cycle
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Globally, the water cycle is a closed system
- This means that there is a constant amount of water on the planet
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Within the water cycle, there are stores and transfers
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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
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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
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These processes transfer the water on Earth from one store to another (e.g. river to ocean or ocean to atmosphere)
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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
The transfers and stores of the water cycle
Storages & Processes
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Storages in the water cycle include:
- Rivers, lakes and oceans
- Groundwater (aquifers)
- Soils
- The atmosphere
- Glaciers and ice caps
- Organisms (e.g. trees)
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The main processes in the water cycle include:
- Condensation
- Precipitation
- Interception
- Infiltration
- Surface run-off
- Through-flow
- Groundwater flow
- Transpiration
- Evaporation
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 |
The loss of water vapour from the leaves of the plant (transpiration) causes water to move upwards through the plant, allowing more water to be drawn in through the roots
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