Encyclopedia of Ecology

Abstract

The principles of the perpetual water cycle on Earth are explained. It is the largest movement of matter in the Earth’s system, with essential phase changes. The largest water transfers are related to changes from the liquid to the gaseous phase – by evapotranspiration and from the gaseous to the liquid phase – by condensation. Water cycle can be interpreted as movement of water between different, abstract, reservoirs (stores), which can be defined in several ways. Quantitative estimates of water fluxes between stores are given. Hydrological processes partaking in the water cycle are reviewed. Continuity (conservation of mass) principle is discussed and aggregate sojourn times of a water particle in different stores are presented. Water is an excellent solvent and carrier of substances, and hence plays a significant role in fundamental processes in the geosphere and the biosphere. Water cycle is the basic component of the life support system of the planet. Human interaction with the natural water cycle has reached an unprecedented dimension, under active presence of 6.5 billion people, leading to high water withdrawals and deterioration of water quality. Indirect human impacts on the water cycle, due to the anthropogenic climate change driven by increasing carbon emissions and decreasing carbon sinks, are discussed.