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Role Blood Plasma Transport
Although rocks contain large quantities of water, this "locked-in" water plays a very small role in the hydrological cycle.

because no individual, even of the longest-lived species, lives very long in geological terms.

Before we describe the cycles of these elements, however, we must discuss water and fire. As we have just seen, the movement of water transfers many elements between the atmosphere, land, fresh waters, and oceans. Fire is a powerful agent that speeds the cycling of chemical elements.

Water transfers materials from one compartment to another

The cycling of water through the oceans, atmosphere, fresh waters, and land is known as the hydrological cycle. The hy-drological cycle operates because more water is evaporated from the surface of the oceans than is returned to them as precipitation (rain or snow). The excess evaporated water is carried by winds over the land, where it falls as precipitation.

Water also evaporates from soils, from lakes and rivers, and from the leaves of plants (transpiration), but the total amount evaporated from those surfaces is less than the amount that falls on them as precipitation. The excess terrestrial precipitation eventually returns to the oceans via rivers, coastal runoff, and groundwater flows (Figure 58.4).

Earth's 16 largest rivers account for more than one-third of total water discharge; more than half of the discharge comes from the three largest rivers (Amazon, Congo, and Yangtze). Despite their relatively small volume, rivers play a disproportionate role in the hydrological cycle because the average residence time of water in lakes and rivers is only 4.3 years, compared with 2,640 years in the oceans. The average turnover time of water in living things is much shorter—about 5.6 days.

By building dams, canals, and reservoirs and by diverting huge quantities of water to irrigated fields, humans have had major effects on the temporal and spatial distribution of fresh water on Earth. The most important consequence of human activities is that more water now evaporates from land and less flows to the oceans than before the Industrial Revolution. In addition, freshwater flow patterns are being seriously altered. For example, dams on the Columbia River in Washington State are

^y 58.4 The Global Hydrological n7 Cycle The numbers show the rela-I i tive amounts of water (expressed as ^ / units of 1018 g) held in or exchanged annually by ecosystem compartments. The widths of the arrows are proportional to the sizes of the fluxes.

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Essentials of Human Physiology

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