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FIGURE 2.2 A simplified depiction of the global water cycle. Flows are approximate, in cubic kilometers per year. Downward arrows signify precipitation, upward arrows evapotranspiration (ET). The upper horizontal arrow represents the transfer of moisture from sea to land; the lower arrow represents runoff from land to sea. (Reproduced from Postel et al. 1996, after Gleick 1993.)

FIGURE 2.2 A simplified depiction of the global water cycle. Flows are approximate, in cubic kilometers per year. Downward arrows signify precipitation, upward arrows evapotranspiration (ET). The upper horizontal arrow represents the transfer of moisture from sea to land; the lower arrow represents runoff from land to sea. (Reproduced from Postel et al. 1996, after Gleick 1993.)

at any one time is tiny in comparison to other stores. Only 2.8% of the world's total water occurs on land. Ice caps and glaciers make up the majority (2.24%), and groundwater (0.61%) also is a sizable percentage. Only 0.009% of the total water is stored in lakes, about 0.001% is stored in the atmosphere, and rivers contain ten times less, 0.0001% of the world's water. Because the volume in the atmosphere and rivers at any instant in time is small, the average water molecule cycles through them rapidly, residing only days to weeks, compared with much longer residence times of water in other compartments.

Estimates of the amount of water discharged annually by rivers to the world's oceans vary, but the value of 40,000 km3 is widely used. The world's 16 largest rivers in terms of runoff volume account for nearly one third of the total, and the Amazon alone contributes nearly 15% (Dingman 2002). Within the United States, the Mississippi contributes some 40% of total US discharge, while the Columbia, Mobile, and Sus-quehanna together contribute an additional 20%. Globally, the greatest runoff occurs in tropical and subtropical areas, because these latitudes also receive the greatest rainfall (Milliman 1990). By continent, South America is the wettest, Antarctica the driest, and Australia has the lowest runoff per unit area (Table 2.1).

TABLE 2.1 Water balances of the continents. (From Dingman 2002.)

Continent Precipitation Evapotranspiration Runoff

TABLE 2.1 Water balances of the continents. (From Dingman 2002.)

Continent Precipitation Evapotranspiration Runoff

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