ABSTRACT
A THE GLOBAL HYDROLOGICAL CYCLE
The global hydrosphere can be regarded as a series of reservoirs interconnected by water cycling in various phases. These reservoirs are the oceans; ice sheets and glaciers; terrestrial water (i.e. rivers, soil moisture, lakes and ground water); the biosphere (i.e. water in plants and animals); and the atmosphere. The oceans, with a mean depth of 3.8 km and covering 71 per cent of the earth's surface, hold 97 per cent of all the earth's water (1.31 × 1018 m3). Approximately 75 per cent of the total fresh water is locked up in ice sheets and glaciers, while almost all of the remainder is ground water. It is an astonishing fact that at any instant rivers and lakes hold only 0.33 per cent of all fresh water and the atmosphere a mere 0.035 per cent (about 12 × 1012m3 (Figure 3.1). Water cycling is accomplished by evaporation, the transport of water vapour in the atmosphere, condensation, precipitation and terrestrial runoff. The average residence time of water within these reservoirs varies from hundreds or thousands of years for the oceans and polar ice to only about 10 days for the atmosphere.
