Water (H2O) is a chemical compound containing one oxygen and two hydrogen atoms that are connected by covalent bonds. It is the only compound that exists in nature in solid, fluid and gaseous state. Water covers 71% of the Earth's surface. It is vital for all known forms of life. On Earth, 96.5% of the planet's water is found in seas and oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation. Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in rivers and lakes and only the tiny fraction of 0.04% of the freshwater is dispersed in gaseous form in the atmosphere.

Fresh water in fluid form is the most precious resource for the terrestrial flora and fauna and hence mankind.

In all kind of exploitation of earth resources, protection of the fresh water has highest priority. Most conflicts of use are concerned with fresh water. A sound understanding of hydrological systems and cycles in the subsurface is fundamental for any impact assessment.