The natural water cycle

WATER IS THE MOST WIDELY OCCURRING SUBSTANCE on this planet. Globally distributed by the hydrological cycle, driven by the energy cycle, the circulation of water powers most of the other natural cycles and conditions the weather and climate. Water has shaped the Earth’s evolution (Dooge, 1983) and continues to fashion its progress, in marked contrast to those bodies in the solar system without water. While the greater part of the water within the Earth’s hydrological cycle is saline, it is the lesser volume of freshwater within the land-based phase which provides a catalyst for civilization. This is the water precipitated from the atmosphere onto land, where it may be stored in liquid or solid form, and can move laterally and vertically and between one phase and another, by evaporation, condensation, freezing and thawing. On the land surface, this water can travel at widely differing velocities usually by predictable pathways (Young et al., 1994) which can slowly change with time. These pathways combine to form stream networks and rivers within river basins, the water flowing by gravity from the headwaters to the sea. Some basins, such as the Amazon, are massive, others minute. Depending on the nature of the geology, soils and land cover within the basin, a varying proportion of this water may infiltrate to recharge the underlying aquifers, some recharge re-emerging later to sustain river flows.

Sustainable Development Goals:

Clean Water and Sanitation