Water cycle is the continuous recycling of the earth's water into the hydrosphere, the atmosphere and the lithosphere (soil - subsoil).

The continuous circular process of the water cycle is achieved due to solar radiation. The water of the planet is constantly changing physical state from solid ice to liquid form of rivers, lakes and seas and the gaseous water vapor. More specifically, because of the heat and winds on the Earth's surface, the water evaporate and gather as creating water vapor clouds. The vapors are condensed, liquefied and then fall as rain or other forms of precipitation, thus enriching the land water storage, whether these superficial as seas and lakes, or is underground.

The water cycle is the subject of Hydrology discipline for what is happening or occurring in the territory and Meteorology for what goes on in this atmosphere. Especially in Meteorology the hydrological cycle is the greatest weather phenomenon as a whole individual phenomena. This regulates the soil moisture, the splendor of the day, and finally the frequency and intensity of hydrometeors, except that giant project of energy transfer from small to high latitudes. From the sea, lakes and rivers evaporate every minute an amount of 1,000,000,000 (one billion) cubic meters of water that returns to the atmosphere. The transpiration of plants is another feature that delivers water vapor in the atmosphere. Evaporation and transpiration from land often not visible and thus speak of evapotranspiration.

A small amount of water vapor in the atmosphere comes from sublimation, through which molecules of ice and snow are transformed directly to vapor without passing through the liquid. However, the water of precipitation is not only flowing into the rivers. Certain quantities penetrate the ground by the operation of the filter and form the underground water. Part of this water can rediscover its way to the surface water bodies (and the ocean) as groundwater-charging. When finding passages to the surface of the earth appears as sources.

Another part of the underground water goes deeper and enrich the underground aquifers, which can store huge amounts of water for long periods. Even though this water continues to move and the time of the re-enters the oceans where the water cycle "ends" and "starts" again.