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Science 25 August 2006:
Vol. 313. no. 5790, pp. 1068 - 1072
DOI: 10.1126/science.1128845
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Review

Global Hydrological Cycles and World Water Resources

Taikan Oki1,2,3*{dagger} and Shinjiro Kanae1,4*

Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should be the main focus in water resources assessments. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect. Reducing current vulnerability will be the first step to prepare for such anticipated changes.

1 Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.
2 Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchishi, Saitama 332-0012, Japan.
3 Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa, Kanazawa-ku, Yokohama 236-0001, Japan.
4 Research Institute for Humanity and Nature, 457-7 Motoyama, Kamigamo, Kita-ku, Kyoto 603-8047, Japan.

* These authors contributed equally to this work.

{dagger} To whom correspondence should be addressed. E-mail: taikan{at}iis.u-tokyo.ac.jp

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Science. ISSN 0036-8075 (print), 1095-9203 (online)