Water cycles from oceans, to land, and back to oceans in a flow known as the hydrological cycle. The sun drives the hydrological cycle by providing the energy to evaporate surface waters. Most of the evaporation occurs from the oceans, and a portion of the evaporated ocean water then falls on land. Gravity completes the hydrological cycle by driving water from the land back to the oceans via rivers, coastal runoff, and subterranean flows.


The hydrological cycle operates because more water is evaporated from the oceans than is returned to them as precipitation. The excess water is transported over land, where it falls as precipitation. Gravity then delivers water back to the ocean through rivers, coastal runoff, and groundwater flows. Water that falls on land renews water reservoirs, with water renewal being fastest (about 4 years) in lakes and rivers, important sources of our fresh water. In a sense, the hydrological cycle is a continually operating distillation machine, evaporating water from the salty oceans and dropping fresh water back on land.

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Textbook Reference: Concept 45.3 Certain Biogeochemical Cycles Are Especially Critical for Ecosystems