18. Water requirements

Water and energy are intimately linked. Most of the water on earth (97%) is salty. It is found in oceans and seas. A small proportion (2.5%) is freshwater in the form of ice (pack ice, icebergs...) and only 0.5% is liquid freshwater, found mainly in aquifers, in the form of rain, in lakes, reservoirs and rivers .

A distinction is usually made between :

• blue" water, i.e. water from rivers, lakes, groundwater, etc. ;

• green" water, which is free rainwater;

• grey" water, which is polluted...