Water on earth is made of 97.5% of salt water, contained in the oceans and 2.5% of clean water, contained in lakes, rivers, glaciers, groundwater, etc. The maximum amount of clean water by humans is up to 0.7% of the total water supply.
Clean water is constantly renewed though the water cycle. It goes from the sea to the atmosphere, then from the land to the sea, following a cycle whioch is repeated indefinitely in differents stages :
Evaporation and evapotranspiration :
Part of the sea water is transformed into water vapor under the action of the sun as well and water of plantsand animals by evapostranspiration.
Infiltration :
Part of the rainwater going into groundwater.
Runoff :
Another part of the water join surface water: rivers, lakes, etc.
Condensation :
Clouds form in the sky.
Rainfall :
Clouds transformed int rainwater, snow or hail.
Back to the sea:
the water in natural reservoirs evaporates under the action of the sun and returns to the seas and oceans.
Stagnation :
Water is stored in natural reservoirs for short or long periods (ex: 8 days of stagnation in the atmosphere, 17 years in lakes, 2500 years in oceans…)