Water cycles from oceans, to land, and back to oceans in a flow known as the hydrologic cycle. The sun drives the hydrologic 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 hydrologic cycle by driving water from the land back to the oceans via rivers, coastal runoff, and subterranean flows.
The oceans contain most of Earth's water. The evaporation and precipitation of water, on land and in the ocean, drive the cycling of water throughout the globe.
Water leaves the ocean through evaporation. Most of this evaporated water returns again to the ocean in the form of precipitation. However, a small portion does not immediately return, and is transported by winds over land.
Water also evaporates from the land, primarily from lakes and rivers, but also from the leaves of plants (a process called transpiration). This evaporated water joins the evaporated ocean water and precipitates back on land, replenishing the terrestrial water supplies.
Excess water that precipitates on land returns to the oceans via rivers, coastal runoff, and subterranean flows. As this water returns, it completes the hydrological cycle.
The hydrologic 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 hydrologic cycle is a continually operating distillation machine, evaporating water from the salty oceans and dropping fresh water back on land.