Water for people.pdf - WHO Thailand Digital Repository

T H E N A T U R A L W A T E R C Y C L E / 6 5We made from water every living thing.The Koran (Sura 21:30)WATER IS THE MOST WIDELY OCCURRING SUBSTANCE on thisplanet. Globally distributed by the hydrological cycle, drivenby the energy cycle, the circulation of water powers most ofthe other natural cycles and conditions the weather and climate. Waterhas shaped the Earth’s evolution (Dooge, 1983) and continues to fashionits progress, in marked contrast to those bodies in the solar systemwithout water. While the greater part of the water within the Earth’shydrological cycle is saline, it is the lesser volume of freshwater withinthe land-based phase which provides a catalyst for civilization. This isthe water precipitated from the atmosphere onto land, where it may bestored in liquid or solid form, and can move laterally and vertically andbetween one phase and another, by evaporation, condensation, freezingand thawing. On the land surface, this water can travel at widelydiffering velocities usually by predictable pathways (Young et al., 1994)which can slowly change with time. These pathways combine to formstream networks and rivers within river basins, the water flowing bygravity from the headwaters to the sea. Some basins, such as theAmazon, are massive, others minute. Depending on the nature of thegeology, soils and land cover within the basin, a varying proportion ofthis water may infiltrate to recharge the underlying aquifers, somerecharge re-emerging later to sustain river flows.