1. Water cycles

1.1 Natural water cycle

Water is singular in its properties and changes of state (liquid, solid and vapor), and is present everywhere, visible or hidden (seas, oceans, water tables, glaciers, ice floes, clouds...).

The closed-circuit water cycle is a complex phenomenon. Although water covers 70% of the Earth's surface (which is a lot on the surface), 97% of this water is saline, and only 3% is freshwater, which is sometimes difficult to access, even though it is essential for life. Freshwater comes mainly from precipitation, and is present at various levels:

• 76% in glaciers and ice caps ;

• 22.5% underground: groundwater and deep, captive groundwater ;

• ...