The freshwater supply is an continual motion through the hydrologic cycle. An equal amount fall back to earth as rain, sleet, hail or snow. The process, however, returns more water to the land than is evaporated therefrom. The World Resources Institute estimates that this cycle distills and transfers 41,000 cubic kilometres of water from the oceans to the continents annually. To complete the natural cycle, the water then makes its way back to the ocean as runoff.
Water quality is the most important aspect of freshwater management.
Presently, more than one-fifth of the world's population do not have access to safe drinking water and more than one half of humanity lack adequate sanitation. Water quality has been an issue in the developing world where three out of five persons lack access to clean water supplies. The problem of ensuring adequate and quality water assumed such a proportion that the United Nations declared the 1980s to be the International Drinking Water Supply and Sanitation Decade and instituted a programme to provide safe drinking water and appropriate sanitation for everyone by 1990. The commitments of this decade, however, have not been fully met. Population growth, increasing and unplanned urbanisation offered challenges in the efforts made under the programmes of this decade.