One part of the water cycle that is obviously essential to all life on Earth is the freshwater existing on the land surface. Just ask your neighbor, a tomato plant, a trout, or that pesky mosquito. Surface water includes the streams (of all sizes, from large rivers to small creeks), ponds, lakes, reservoirs and canals (man-made lakes and streams), and freshwater wetlands. The definition of freshwater is water containing less than 1,000 milligrams per liter of dissolved solids, most often salt.
As a part of the water cycle, Earth's surface-water bodies are generally thought of as renewable resources, although they are very dependent on other parts of the water cycle. The amount of water in our rivers and lakes is always changing due to inflows and outflows. Inflows to these water bodies will be from precipitation, overland runoff, ground-water seepage, and tributary inflows. Outflows from lakes and rivers include evaporation and discharge to ground water. Humans get into the act also, as people make great use of surface water for their needs. So, the amount and location of surface water changes over time and space, whether naturally or with human help. Certainly during the last ice age when glaciers and snowpacks covered much more land surface than today, life on Earth had to adapt to different hydrologic conditions than those which took place both before and after. And the layout of the landscape certainly was different before and after the last ice age, which influenced the topographical layout of many surface-water bodies today. Glaciers are what made the Great Lakes not only "great, " but also such a huge storehouse of freshwater.
Surface water keeps life going
Satellite picture of lights in southern Europe and North Africa at night, with the lights along the Nile Delta circled for emphasis. As this picture of the Nile Delta in Egypt shows, life can even bloom in the desert if there is a supply of surface water (or ground water) available. Water on the land surface really does sustain life, and this is as true today as it was millions of years ago. I'm sure dinosaurs held their meetings at the local watering hole 100 million years ago, just as antelopes in Africa do today. And, since ground water is supplied by the downward percolation of surface water, even aquifers are happy for water on the Earth's surface. You might think that fish living in the saline oceans aren't affected by freshwater, but, without freshwater to replenish the oceans they would eventually evaporate and become too saline for even the fish to survive.
As we said, everybody and every living thing congregates and lives where they can gain access to water, especially freshwater. Just ask the 6 billion people living on Earth! Here's a satellite picture of the Mediterranean region during night (the full picture of the Earth is available from NASA). The most obvious thing you can see is that people live near the coasts, which, of course, is where water, albeit saline, is located. But the interesting thing in this picture are the lights following the Nile River and Nile Delta in Egypt ( the circled area). In this dry part of the world, surface-water supplies are essential for human communities. And if you check the price of lakefront property in your part of the world, it probably sells for much more than other land.
Usable fresh surface water is relatively scarce
To many people, streams and lakes are the most visible part of the water cycle. Not only do they supply the human population, animals, and plants with the freshwater they need to survive, but they are great places for people to have fun. You might be surprised at how little of Earth's water supply is stored as freshwater on the land surface, as shown in the diagram and table below. Freshwater represents only about three percent of all water on Earth and freshwater lakes and swamps account for a mere 0.29 percent of the Earth's freshwater. Twenty percent of all fresh surface water is in one lake, Lake Baikal in Asia. Another twenty percent (about 5,500 cubic miles (about 23,000 cubic kilometers)) is stored in the Great Lakes. Rivers hold only about 0.006 percent of total freshwater reserves. You can see that life on Earth survives on what is essentially only a "drop in the bucket" of Earth's total water supply! People have built systems, such as large reservoirs and small water towers (like this one in South Carolina, created to blend in with the peach trees surrounding it) to store water for when they need it. These systems allow people to live in places where nature doesn't always supply enough water or where water is not available at the time of year it is needed.
| Water source | Water volume, in cubic miles | Water volume, in cubic kilometers | Percent of freshwater | Percent of total water |
|---|---|---|---|---|
| Lakes, swamps | 24,600 | 102,500 | 0.29% | 0.008% |
| Rivers | 509 | 2,120 | 0.006% | 0.0002% |
| Total global fresh water | 8,404,000 | 35,030,000 | 100% | 2.5% |
| Total global water | 332,500,000 | 1,386,000,000 | -- | 100% |
| Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp.817-823. | ||||
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