Water cycle

Each year, the global water cycle delivers 110,000 cubic kilometers of water over land in the form of rain, sleet, and snow. (These values are approximate; various models have produced different estimates.) About 64 percent of that precipitation returns to the atmosphere through evaporation or transpiration (the use of water by plants, crops, grass

by an equal excess of precipitation over evaporation on land; consequently the volume of water carried to the sea by glaciers, rivers and coastal springs is close to 27 billion acre-feet per year. About 13 billion acre-feet is carried by 68 major river systems from a drainage area

Generally about three-fourths of the applied water is lost to evapotranspiration. The rest, which contains all the originally dissolved salts (except for the tiny amounts incorporated in the crop itself), percolates downward and laterally through the soil. It may enter an underground aquifer or it may reappear somewhere downstream as seepage into a

globe. A planetary boundary for freshwater resources needs to secure water flows to regenerate precipitation, support terrestrial ecosystem functioning and services (such as carbon sequestration, biomass growth, food production, and biological diversity), and also ensure the availability of water for aquatic ecosystems.