The amount of water on earth has been constant for a very long time. Water travels the earth in a constant cycle through atmosphere, land mass and oceans. Precipitation is the main source of water for all human uses and for ecosystems; its main driver is the sun. Precipitation is taken up by plants and soils, evaporates into the atmosphere via evapotranspiration, and runs off to the sea via rivers, and to lakes and wetlands. This water supports forests, rain-fed cultivated and grazing lands, and ecosystems. Some of the precipitation can be stored for centuries in snow and ice, and some of it can also be stored temporarily in aquifers before it again comes out (springs, pumps etc.). However, most water falls back into the oceans or lands as rain, where it flows over the ground as surface runoff and again flows into rivers and eventually the sea