Chapter Summary

The hydrologic cycle constantly purifies and redistributes fresh water, providing an endlessly renewable resource. The physical processes that make this possible—evaporation, condensation, and precipitation—depend upon the unusual properties of water, especially its ability to absorb and store solar energy. More than 97 percent of all water in the world is salty ocean water. Of the 33,400 km³ (8.8 quintillion gal) that is fresh, 99 percent is locked up in ice or snow or buried in groundwater aquifers. Lakes, rivers, and other surface freshwater bodies make up only about 0.01 percent of all the water in the world, but they provide more than half of all water for human use and habitat and nourishment for aquatic ecosystems that play a vital role in the chain of life.

Water is essential for nearly every human endeavor. In the United States, direct personal use constitutes about one-tenth of the water we withdraw from our resources. Our two largest water uses are agricultural irrigation and industrial cooling. Altogether, the amount of water we withdraw from underground aquifers and surface reservoirs represents about 19 percent of the surplus runoff in the United States. Only about half the water we withdraw is consumed or degraded so that it is unsuitable for other purposes; much could be reused or recycled. Water conservation and recycling would have both economic and environmental benefits.

Water shortages in many parts of the world result from rising demand, unequal distribution, and increased contamination. Arid zones are especially vulnerable to the effects of natural droughts and land abuse by humans and domestic animals. Lakes, rivers, and groundwater reservoirs are being depleted at an alarming rate, leading not only to water shortages, but also to subsidence, sinkhole formation, saltwater intrusion, and permanent loss of aquifers.

Water storage and transfer projects are a response to flooding and water shortages. Giant dams and diversion projects can have environmental and social costs that are not justified by the benefits they provide. Among the problems they pose are evaporation and infiltration losses, siltation of reservoirs, and loss of recreation and wildlife habitat. Watershed management and small dams are preferred by many conservationists as means of flood control and water storage.

There is much we can do to save water. Charging users the true cost of water is a good start toward conservation. We can each use less water in our personal lives, and society can encourage development of water-saving appliances, natural yards, recycling, and efficient water use. Perhaps the most important change we can make is to treat wastes at their sources rather than use precious water resources for waste disposal. Not everyone can live upstream.