The water cycle is a continuous purification process of water on Earth due to the movement of water species among various reservoirs. This cycle is vital for Earth’s life, ecosystems, and living organisms. The water cycle includes mainly four processes. Water is evaporated from ocean and land surfaces driven by solar heating. The resulting water vapor rises upwards into the atmosphere, transported by the winds, cools, and due to low air temperature condensates into liquid droplets and ice crystals to form clouds. The ice or/and liquid droplets collide, increase their size, and precipitate as snow or rain to Earth’s surface and oceans. The subtraction of energy (latent heat of evaporation) at low latitudes related to the evaporation processes as well as its release (latent heat of condensation) at higher latitudes related to the condensation processes is a formidable way to guarantees the heat transport from the warmer part of the Earth to the colder ones mantaining local air temperature more compatible with the human life. The starting point of the water cycle is not unique, but the oceans can be selected as the initial reservoir. Other important reservoirs are considered ice sheets, lakes, and rivers.
The hydrosphere is defined by the various water reservoirs which are characterized by different residence times – the time spends the water molecules in a reservoir. The water residence time – the rate at which the water comes out the reservoirs – varies for each reservoir extending from hundreds (Greenland Ice Sheet) or thousands of years (Antarctic Ice Sheet) to years and days for rivers and lakes, respectively. It also defines the energy transferred from the Earth to the Atmosphere which increases for short-term residence times. In long-term temporal scales, this energy is defined as the evaporation rate (E) and balances with the precipitation rate (P). This global energy balance breaks for shorter time scales depending also on the local and regional climate. For example, in regions located in the Inter-Tropical Convergence Zone (ITCZ), the energy balance in the water cycle does not exist since the precipitation rate is much higher than the evaporation rate (P>>E) due to the horizontal movement of converging trade winds.