Global Hydrological Cycle
Geography: Edexcel A Level, topic 7.1.1
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.
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).
Local stores
- Examples of local stores of water include:
- Vegetation storage.
- Surface storage.
- Soil moisture.
- Groundwater storage.
Flows
- Flows are how water moves from one store to another:
- Infiltration.
- Throughflow.
- Percolation.
- Stem flow.
- Base flow.
- Channel flow.
- Surface runoff.
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.
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.
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.
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).
Test yourself on Global Hydrological Cycle
Which of the following is an example of a global water store?
How much of the Earth's entire water supply is found in the oceans and seas?
Where is the Earth's surface water found?
Where is 30% of the Earth's freshwater found?
Relative Sizes of the Earth's Freshwater Stores