Water use in the world:  present situation / future needs

To reliably assess in detail future water resources and present water availability, it is insufficient to rely simply on volume data and natural variations in river runoff. It is also necessary to take into account changes due to human activities. During recent decades natural variations in river runoff and quantitative and qualitative characteristics of renewable water resources have been much affected by a whole complexity of anthropogenic impacts.

The hydrological cycle

Water is the most widespread substance to be found in the natural environment. Water exists in three states: liquid, solid, and invisible vapour. It forms the oceans, seas, lakes, rivers and the underground waters found in the top layers of the Earth's crust and soil cover. In a solid state, it exists as ice and snow cover in polar and alpine regions. A certain amount of water is contained in the air as water vapour, water droplets and ice crystals, as well as in the biosphere. Huge amounts of water are bound up in the composition of the different minerals of the Earth's crust and core.