Water is a naturally circulating resource that is constantly recharged, unlike oil. The amount of water on our planet will not diminish on shorter than geological time scales (Oki, 2005). Therefore, even though stocks of water in natural and artificial reservoirs are helpful to meet our water resource needs, it is the flow of water that should be the main focus in water resource assessments (Shiklomanov, 1997; Korzun, 1978; Oki and Kanae, 2006).
For example, the amount of water stored in all the rivers in the world is only 2000 km3 , which is much less than the annual water withdrawal of 3,800 km3/year. A more accurate measure of water availability is the 45,500 km3/year of annual discharge flowing through the rivers to the sea (Oki and Kanae, 2006).
Water flux is the most relevant measure of water resources. Therefore, the speed of water circulation is crucial. How long water molecules stay in a given reservoir, that is, their mean residence time, can be estimated by dividing the volume of the reservoir by the mean flux into and out of it. For rivers unaffected by humans, the mean residence time of the water is about two and a half weeks (Oki, 2005). In contrast, the recharge rate of some groundwater aquifers is very slow, and the mean residence time is considered to be hundreds or even thousands of years. When water is extracted from such an aquifer, it will take a very long time, measured on a human time scale, to return to the original volume stored; in practice, the water is exhausted once it has been used. For this reason, the groundwater in such aquifers is called fossil water.
The global population will continue to grow for at least a few decades, and water demand will thus increase. Reliance upon slowly recharging groundwater aquifers is not sustainable as a global strategy. Circulating renewable freshwater resources are essential to meeting the growing human demand for water.
Korzun, V. I. 1978. World Water Balance and Water Resources of the Earth, Vol. 25 of Studies and Reports in Hydrology. UNESCO, Paris.
Oki, T. 2005. In Encyclopedia of Hydrological Sciences, M.G. Anderson, J. McDonnell, Eds., Wiley, New York,Vol. 1, pp. 13-22.
Oki, Taikan and Kanae, Shinjiro. 2006. Global hydrological cycles and world water resources. Science 313:1068-1072.
Shiklomanov, I. A., Editor. 1997. Assessment of Water Resources and Water Availability in the World. World Meteorological Organization/Stockholm Environment Institute, Geneva.
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