Biological importance: Water is essential to all organisms.

Forms available to life: Liquid water is the primary phase in which water is used, though some organisms can harvest water vapor.

Reservoirs: The oceans contain 97% of the water in the biosphere. About 2% is bound in glaciers and polar ice caps, and the remaining 1% is in lakes, rivers, and ground water.

Key processes: The main processes are evaporation of liquid water, condensation into clouds, and precipitation.

Transpiration by plants also moves significant volumes of water.