It represents the exchange of water between air, land and sea and between the living organisms and the environment.

Two overlapping water cycles are operating in nature:

(i) Global water cycle: Water evaporates from the hydrosphere (oceans, lakes, rivers seas, ponds) by the heat of the sun and forms clouds. Clouds are carried over to the land by winds. After getting cooled, water in the clouds falls on the earth in the form of rain, sleet and hail. This water again evaporates in the atmosphere and completes the global water cycle.

(ii) Biological water cycle: Here, the living organisms, i.e., plants and animals, consume water from the global water cycle and return it back. Plants release water in the form of water vapours by the process of transpiration. Animals return water to the air as vapours by the process of respiration and to the soil in the form of fluid by the process of excretion.