ABSTRACT
The circulation of water from the ocean and land surfaces into the atmosphere and back again creates a resource which is unlimited in global terms. The supply of freshwater on Earth is entirely due to this hydrological cycle. This is reinforced by observations of slight traces of water vapour in the present Martian atmosphere. The global hydrological system consists of four major reservoirs or stores, the ocean, terrestrial waters, terrestrial ice and the atmosphere, and the flows or 'fluxes' between them. The gaseous and liquid fluxes dominate exchanges; it is worth noting that the formation of ice within the atmosphere is an important precursor to rainfall and currently produces some 74 per cent of world rainfall. The water surplus in the Arctic is critical for the water balances of Europe. It accrues because of low evaporation losses and large volumes of freshwater runoff from major rivers like the Mackenzie, Lena, Ob and Yenisei.
