Chapter 11 Summary
Most of the water in the hydrosphere at any given time is in the oceans; most of the remaining fresh water is stored in ice sheets. Relatively little water is found in lakes and streams. A portion of the fresher waters on the continents—both surface water and ground water—is diverted for human use. The availability of ground water is influenced by such factors as the presence of suitable aquifers, water quality, and rate of recharge relative to rate of water use. Where ground water is plentiful and subsurface rocks are soluble, dissolution may create caves below ground and sinkholes at the surface, producing a distinctive landscape (karst). Karst aquifers may be productive, with rapid water flow, but the supply may be erratic and rapidly polluted.
In the United States, more than half of offstream water use (withdrawal) is for industrial purposes and thermoelectric power generation. However, these industrial applications generally consume very little water. Of the water actually consumed in the United States, approximately 80% is as water loss associated with irrigation. While three-fourths of the water withdrawn for use is surface water, close to half of the water consumed is ground water, which, in many cases, is being consumed faster than recharge can replace it. Adverse consequences of such rapid consumption of ground water include lowering water tables, surface subsidence, and, in coastal areas, saltwater intrusion. Conservation, interbasin transfers of surface water, and desalination are possible ways to extend the water supplies of high-demand regions. Desalinated water is presently too expensive to use for irrigation on a large scale, which means that it is unlikely to have a significant impact on the largest consumptive water use for some time to come.
