For any understanding at all of the world water situation it is necessary to understand what is called the hydrologic cycle. This is the succession of processes by which water is evaporated (mostly from the sea), carried by winds over the oceans and continents, and precipitated back down to earth.

It then runs off in streams and rivers, or accumulates in lakes and ponds, where it is known as surface water, or penetrates below the surface to become groundwater. A lot accumulates as mountain glaciers or polar icecaps in the form of frozen water, while a lot remains in the atmosphere as water vapour which, under the right conditions, returns to earth as dew.

Other sources of water which will become more important in the future sea water, from which the salt can be removed (at high cost) by de-salination, and wastewater, or sewage, which is already recycled in quantities which may surprise a lot of people.