water cycle

About one-third of the solar energy reaching the Earth is used in evaporating water. About 380,000 cubic km/95,000 cubic mi is evaporated each year. The entire contents of the oceans would take about one million years to pass through the water cycle.

Dams, such as this one at Marmolada in the Dolomites, Italy, are a common sight in many upland areas. They trap and store water, making it available for use during periods of water shortage. Highland areas are favoured because of the higher rainfall there (known as orographic or relief rainfall). Hard rocks, which form the high ground, make a good base and building material for the reservoir, and the valleys in such areas provide narrow sites ideal for flooding, to become the reservoir basin.

Natural circulation of water through the upper part of the Earth. It is a complex system involving a number of physical and chemical processes (such as evaporation, precipitation, and infiltration) and stores (such as rivers, oceans, and soil).

Water is lost from the Earth's surface to the atmosphere by evaporation caused by the Sun's heat on the surface of lakes, rivers, and oceans, and through the transpiration of plants. This atmospheric water is carried by the air moving across the Earth, and condenses as the air cools to form clouds, which in turn deposit moisture on the land and sea as precipitation. The water that collects on land flows to the ocean overland – as streams, rivers, and glaciers – or through the soil (infiltration) and rock (groundwater). The boundary that marks the upper limit of groundwater is called the water table.

The oceans, which cover around 70% of the Earth's surface, are the source of most of the moisture in the atmosphere.