Only a relatively small fraction of the total amount of water in the biosphere is found as free water on continental landmasses. The oceans contain about 97.6 percent of the biosphere’s water, and polar ice, groundwater, and water vapour take up another 2.4 percent. Thus, less than 1 percent exists as continental free water, which is generally referred to as inland water. In spite of this small percentage, inland water is an essential element of the biosphere. It occurs in a wide variety of forms and is inhabited by a diverse set of biological communities, quite distinct from the communities of marine and terrestrial ecosystems.
All inland waters originate from the ocean, principally through evaporation, and ultimately return to this source. This process is part of the global hydrologic cycle. A major feature of this cycle is that more water evaporates from the ocean than is directly precipitated back into it. The balance of water vapour is precipitated as rain, snow, or hail over continental landmasses whence it either evaporates into the atmosphere (about 70 percent) or drains into the sea. (For more information and a schematic representation of the hydrologic cycle, see hydrosphere.)
On the surface of the land, free water habitats can be classified as either lotic (running-water) or lentic (standing-water). Lotic habitats include rivers, streams, and brooks, and lentic habitats include lakes, ponds, and marshes. Both habitats are linked into drainage systems of three major sorts: exorheic, endorheic, and arheic. Exorheic regions are open systems in which surface waters ultimately drain to the ocean in well-defined patterns that involve streams and rivers temporarily impounded by permanent freshwater lakes. Endorheic regions are considered closed systems because, rather than draining to the sea, surface waters drain to inland termini whence they evaporate or seep away. Typically, the termini are permanent or temporary lakes that become saline as evaporation concentrates dissolved salts that either have been introduced by rainwater or have been leached out of substrata within the drainage basin.