The circulation of water from one part of the general earth system to another is known as the hydrologic cycle. Water is cycled endlessly between the atmosphere, the soil, surface storage, lakes and streams, plants and animals, glacial ice, and the principal reservoir-the oceans.
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Even though the water-vapour content of atmosphere is, on the average, constant, there are local changes because of seasonal and weather variations.
Whatever moisture is lost by precipitation, it is made good by continuous evaporation from large and small water bodies like oceans, lakes, rivers, moist soil and vegetation etc. The entire process of maintaining constant water vapour content of the atmosphere is known as the hydrologic cycle.
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Moisture loss from vegetation is known as transpiration. The combined effects of evaporation and transpiration from vegetation-covered land areas are called evapotranspiration. Thus, the hydrologic cycle involves evaporation, condensation, and precipitation.
Hydrologic cycle denotes unending movement of water from the oceans to atmosphere, from the atmosphere to land, and from the land back to the oceans. “The hydrologic cycle may be characterized as an ongoing cycle, without any known beginning or end”.
In fact, this cyclic movement of water is mainly responsible for the distribution of moisture over the earth’s surface. It is also closely related to all weather phenomena.