A key characteristic of the world's freshwater resources is their uneven distribution in time and space. Until recently water resource management focused almost exclusively on redistributing water to when and where people want it for their use. Green water the rainfall that is stored in the soil and then evaporates or is incorporated in plants and organisms is the main source of water for natural ecosystems and for rainfed agriculture, which produces 60" of the world's food. Renewable water resources represent the water entering a countrys river and ground water systems. Usable water resources represent the water that could be used if all economically and technically feasible storage and diversion structures were built. Freshwater ecosystems have been declining in some parts of the world, for hundreds of years threatening the economic, social, and environmental security of human society and terrestrial ecosystems. Freshwater biodiversity is high relative to the limited portion of the earths surface covered by freshwater.