7.1.1

Global Hydrological Cycle

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The Global Hydrological Cycle

There is a fixed amount of water on the Earth. This is called a closed system because water cannot enter or leave the Earth and its atmosphere. The three components of the cycle are stores, flows and processes.

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Global stores

  • When we talk about stores we include all water stored as either water vapour, ice, saline or freshwater.
  • Examples of global stores of water include:
    • Oceans.
    • Lakes.
    • Aquifers (underground lakes).
    • The cryosphere (glaciers and ice sheets).
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Local stores

  • Examples of local stores of water include:
    • Vegetation storage.
    • Surface storage.
    • Soil moisture.
    • Groundwater storage.
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Flows

  • Flows are how water moves from one store to another:
    • Infiltration.
    • Throughflow.
    • Percolation.
    • Stem flow.
    • Base flow.
    • Channel flow.
    • Surface runoff.
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Processes

  • The following processes drive the flows between the stores:
    • Precipitation.
    • Evaporation.
    • Transpiration.
    • Cryosphere (glaciers and ice sheets) exchanges.
    • Runoff.

Sizes of Water Stores

Water is found on earth as a gas called water vapour, as liquid water, and as solid ice. Although the amount of water in each store changes over the year, each store has a relative size.

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Total global water supply

  • 96.5% of Earth's water is in the oceans and seas.
  • 2.5% of Earth’s water is in freshwater (not salty).
  • 0.9% of Earth’s water is in other saline (salty) water sources.
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Freshwater

  • Freshwater is water that is not salty.
    • 68.8% of Earth's freshwater is in the cryosphere (glaciers, ice caps).
    • 30% of Earth's freshwater is groundwater.
    • 1.2% of Earth's fresh water is surface water.
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Surface water

  • Water on the Earth's surface is split between the following places:
    • 69% of Earth's surface water is in ground ice and permafrost.
    • 20% of Earth's surface water is in lakes.
    • 3.8% of Earth's surface water is in soil moisture.
    • 3.0% of Earth's surface water is in the atmosphere.
    • 2.6% of Earth's surface water is in swamps and marshes.
    • 0.5% of Earth's surface water is in rivers.
    • 0.26% of Earth's surface water is in the biosphere (living things).

Jump to other topics

1Tectonic Processes & Hazards

2Option 2A: Glaciated Landscapes & Change

3Option 2B: Coastal Landscapes & Change

4Globalisation

5Option 4A: Regenerating Places

6Option 4B: Diverse Places

7The Water Cycle & Water Insecurity (A2 only)

8The Carbon Cycle & Energy Security (A2 only)

9Superpowers (A2 only)

10Option 8A: Health & Human Rights (A2 only)

11Option 8B: Migration & Identity (A2 only)

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