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10.2: The Earth’s Water, and the Hydrologic Cycle

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  • The earth would be a boring place without water. Fortunately, there’s a lot of water on Earth. Table 10-1 gives an approximate accounting of where the earth’s water resides. By far the most water is in the oceans. Of the Earth’s fresh water, most is tied up in glacier ice, with groundwater a distant second; the fresh water we are most familiar with—streams, rivers, lakes, and what’s in the atmosphere in the form of water vapor, clouds, and precipitation—is a tiny part of the Earth’s stock of near-surface water.

    The trouble with an accounting technique like that of Table 10-1 is that it doesn’t tell anything about the movements and transformation of the earth’s water. You all know that water on, below, and above the earth’s surface is in a continuous state of movements and transformations. The picture of these movements and transformations, viewed in their entirety, is called the water cycle, or the hydrologic cycle. Figure 10-1 gives one version of the hydrologic cycle. Most of the pathways and processes shown in Figure 12-1 should be easily understandable to you. They vary a lot in their magnitude (how much water, per unit time on average, is involved in each of them), but that’s not shown in the figure. For the purposes of this course, what’s most important is evaporation, condensation, and precipitation.


    Table 10-1. An accounting of where the Earth’s water resides. (From Strahler, 1975.)

    Here’s one interesting fact that underlines the “activeness” of the hydrologic cycle: the mass of water vapor plus condensed water in clouds plus precipitation that’s in the process of falling to the ground, at any given time, amounts to only about one week’s worth of global precipitation.Thismeansthat there must be very rapid recycling of water between the liquid reservoirs at the earth’s surface, on the one hand, and the stock of water in the atmosphere, on the other hand.

    Two incidental notes:

    • The major player in evaporation is the ocean surface: the ocean surface accounts for about 85% of total average evaporation, and the continents account for only about 15%.
    • Evaporation of the ocean exceeds precipitation over the ocean, and precipitation over the continents exceeds evaporation. The balance is maintained mainly by runoff of water from the continents, in the form of rivers and streams, although in certain places large glaciers deliver water to the oceans, as they calve directly into the ocean.


    Figure 10-1. The hydrologic cycle. (From Water, The Yearbook of Agriculture 1955.)