Hydrological cycle

The amounts of water that are moved through the hydrological cycle can also be estimated and evaluated in numerical terms. The tool we use is the global hydrogeological balance of the Earth. The total amount of water that evaporates from the surface of the oceans is more than the water that gets straight into them from precipitations. The difference is part of the amount of water that falls down on the continents. The total amount of water that falls on the continents actually consists of that which has evaporated, not only from the seas or oceans, but straight from the soil as well. The global hydrological balance differs according to climatic conditions – especially the extent of precipitations – and is different, therefore, in each region of our planet. If we take, for instance, the equatorial climate, we can see that there are no water shortages during the year: there is always plenty of water available since the precipitations can easily cover the losses. Conversely, in the hot desert climate, the high temperatures promote the evapo-transpiration that the few precipitations cannot make up for: in this case, there are serious shortages all through the year and little water available. The differences found between the hydrological balances of these two types of climates have immediate repercussions on the vegetal and animal species that live there, whose survival is closely linked to the amount of water that is available and usable.

Special reports

From the Multimedia section

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    Availability of water resources

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    The water cycle

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    Map of water scarcity

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    Annual availability of freshwater

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    Hydrosphere

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    Surface tension

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    Availability of water resources

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    The water cycle

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    Map of water scarcity

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Facts