Water can change states in four ways:
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evaporating,
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condensing,
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melting,
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freezing.
The hydrological cycle is a closed system - water is not added or lost as it goes around the water cycle.
Water is what makes Earth unique and different from the other planets, without it life couldn't exist.
The water cycle is a CLOSED SYSTEM - no water is added or taken away (there is the same amount of water on Earth now as 50, 500 or 5 million years ago).
The water is just stored in different ways - oceans, ice, lakes, rivers etc. At any point there may be changes in the amount of water in any of these stores.
The stores of water are linked by processes which transfer water into and out of the stores.
The BIOSPHERE and GEOSPHERE (LITHOSPHERE) play a vital role in the water cycle and act as a sub-cycle (a cycle within the water cycle).
For example:
In a river basin system, precipitation falls, trees intercept precipitation, over half is then transpired and evaporated back into the atmosphere without ever reaching the ground.
SOME DEFINITIONS
Infiltration - Movement of water into the soil from the surface.
Percolation - Movement of water into underlying rocks.
Groundwater storage - Water stored in rocks following percolation.
Saturation - When soil is full of moisture.
Water Table - The level at which saturation occurs in the ground or soil.
Inputs - Things which enter the system.
Outputs - Things which leave the system.
Transfers or flows - Movements within the system.
Stores - Held within the system.
