The tiny fraction of freshwater not bound up in ice sheets and glaciers comprises only a very small fraction of total global water volume (about 0.79 %). Global use of that freshwater, however, has been growing at roughly twice the rate of global population for the past century. Even so, this volume of unfrozen freshwater is still more than adequate to meet all human needs. However, this essential resource, which is mostly stored as groundwater, is distributed quite unevenly around the globe. Furthermore, physical and economic constraints make it impractical in most cases to move great volumes of water from areas of surplus to areas of need