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Global Hydrological Cycles and
World Water Resources

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 onthe 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 ofpeople 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.
All organisms, including humans, require water for their survival. Therefore, ensuring that adequate supplies of water are available is essential for humanwell-being. Although our planet is often called the BBlue Planet,[ warnings of increasing water scarcity in the world are common. However, unlike oil, water circulates, forming closed hydrologic cycles. The amount of water will not diminish on shorter than geological time scales (1). Given this background, how could water scarcity become a widespread reality within a few decades (2)?

A commonexplanation is that even though there is a lot of water on Earth, only about 2.5% is fresh water, and because most of that water is stored as glaciers or deep groundwater, only a small amount of water is easily accessible. This answer is only partly correct: Rather than looking only at the stocks of water resources, assessments should concentrate mainly on the flows (Fig. 1) (1, 3–5). The amount of waterstored in all the rivers in the world is only 2000 km3, much less than the annual water withdrawal of 3800 km3/year (Fig. 1). Clearly, a more adequate measure of water availability is the 45,500 km3/year of annual discharge, which flows mainly through the rivers from continents to the sea.

What Is the Meaning of a Circulating Resource?

Unlike most other natural resources, water circulatesnaturally. When it evaporates, it changes from liquid to gas and eventually recondenses as a liquid. Water assimilated during photosynthesis becomes part of carbohydrates stored in plants, but ultimately reverts to water again by decomposition.

When used, water loses properties such as purity, heat content, and potential gravitational energy, but eventually, most degraded water resources arerefreshed by natural processes in the hydrological cycle, which is mostly driven by solar energy. When considering water flux as the most relevant measure of water resources, the speed of water circulation ecomes crucial. Mean residence times of water molecules—i.e., how long they stay in a given reservoir—can be estimated by dividing the volume of the reservoir by the mean flux into and out of it.For rivers unaffected by human interventions, the mean residential time of water is about two and a half weeks (1). In contrast, the recharge rate of some groundwater aquifers is very slow, and the mean residential 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 tothe original volume stored; in practice, that water is exhausted once it has been used. Because it took so long to accumulate, the groundwater in such aquifers is sometimes called fossil water.

How Much Renewable Fresh Water Is Available?

Can human demand for water be fully met by using only circulating renewable freshwater resources (RFWR)? The answer is both yes and no. Even though RFWR...
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