2.4 Summary of Section 2
The water cycle involves the movement of water, in all its forms, over, on and through the rocks near the surface of the Earth in a cycle. This cycle is driven by the Sun's energy and the Earth's gravity. The total volume of water in the cycle is virtually constant. Magmatic water adds small amounts of water to the cycle, and formation water removes small amounts of water from the cycle. Water is stored in the natural reservoirs of the hydrosphere: in the oceans, underground, in the ice caps, and in lakes, rivers, the soil and the atmosphere. There is a transfer of water to the oceans from the land surface by rivers and by outflow of underground water.
The residence time for water in a reservoir is the average length of time that water remains in that reservoir. It is calculated by dividing the mass in a particular reservoir by the rate of transfer to or from the reservoir. Residence time is a measure of the rate at which water in different parts of the cycle is renewed: it is fastest in the atmosphere (about 11 days) and rivers (a few weeks). Only about 4% of the water in the water cycle is not seawater. The proportion of fresh water which can be used for water supplies is less than this, about 1% of the total.
Precipitation has a very uneven global distribution, but is greatest near the Equator. On a smaller scale, precipitation is greatest over mountainous areas on land. Interception is the process by which precipitation is prevented from reaching the ground by vegetation.
Water is returned to the atmosphere by evaporation and transpiration. Transpiration is the process by which plants draw water from the soil, transfer it to their leaves and it then evaporates. Evaporation and transpiration can be combined into one parameter, evapotranspiration. A maximum theoretical value for evapotranspiration, called potential evapotranspiration, can be calculated from meteorological parameters for any area.