Water cycle on planet Earth
The Earth is called the blue planet because it looks blue when seen from space. The blue color is due to water (oceans) that covers 71% of the surface of the Earth (Fig.1). Our watery planet holds about 1.5 x 1018 t (1 tonne = 1000 kg) of water, which in turn contains 0.05 x 1018t of dissolved salts. This part of the hydrosphere interacts chemically with the Earth’s rocks (geosphere), with the air (atmosphere), and with living things (biosphere).
Water evaporates from the oceans as water vapour, which condenses later in the atmosphere to fall as rain or snow, collectively called precipitation (Fig. 29). It is estimated that 0.42 x 1015 t of water circulates annually in this water cycle. Precipitated water finally returns to the oceans via streams and rivers and carries weathered and eroded material from the Earth’s crust into the oceans. Each year, 8 x 1012 t of material is transported from the land into the oceans. About two-thirds of this consists of insoluble material (rocks, pebbles, sand, silt, etc.) and the rest remains in dissolved state.