All water on Earth is in continual motion. It is constantly being exchanged between the ground and the air. This exchange is caused by the heat of the sun and the force of gravity. Water evaporates from moist ground, from the leaves of vegetation, and from such bodies of water as lakes, streams, and reservoirs. It is then carried into the atmosphere as water vapor, a gas. When the water vapor condenses, it is transformed from a gas to a liquid and falls as precipitation, in the form of rain, mist, sleet, hail, or snow.
FIGURE 1.3
The water cycle
The precipitation, in turn, replenishes the Earth's surface and underground waters, which eventually join the oceans and seas. Evaporation from both the land and the ocean puts the water (as vapor) back into the air, in a constant exchange, leaving impurities behind. In this way, water travels from the ground to the atmosphere and back to the ground continuously. The exchange of water between the ground and the air is called the hydrologic cycle or water cycle. (See Figure 1.3.) The term "hydrologic" is derived from two Greek words: hydro, which refers to things involving water, and loge, an ancient Greek word meaning "knowledge of."
The hydrologic cycle is a natural, constantly running distillation and pumping system. As a cycle, this flow has no beginning and no end. Within the hydrologic cycle water is neither lost nor gained; it simply changes form as it progresses through the cycle. The molecules of water in the world's oceans, lakes, rivers, ponds, streams, and atmosphere today are the same molecules that were formed four billion years ago.
Although constantly in motion, water is transferred between phases of the hydrologic cycle at different rates depending on where it is located. For instance, a molecule of water exists as water vapor in the atmosphere an average of eight days, but when it enters the ocean it may remain there for the next 2,500 years.
The hydrologic cycle has shaped and sustained life on Earth in its present form. It is largely responsible for determining climate and types of vegetation. Because it is a "closed system," actions that affect any phase of the cycle may have consequences not only today and tomorrow but for centuries to come.
Soil Moisture
Although it represents only a very small percentage of Earth's water supply, soil moisture is extremely significant. It supplies water to plants, a vital link in the food chain. Some plants grow directly in water or in marshy ground, but most live on "dry" land. This is possible because the land is truly dry in just a few places, and often only temporarily.
Dust is generally considered dry, but the dust kicked up by a car on a "dry" dirt road may contain up to 15% water by weight. Vegetation, however, could not grow and flourish on that road because soils hold small percentages of moisture so strongly that plant roots cannot get it out. Other than desert plants, which store water in their own tissues during infrequent wet periods, most plants can grow only where there is extractable water in the soil. Since the Earth's vegetation continually withdraws moisture from the ground in large amounts, frequent renewals of soil moisture, either by precipitation or irrigation, are needed.
Atmospheric Moisture
Rain, snow, sleet, and hail are all forms of precipitation. This moisture in the air comes from the evaporation of water from the ground and from bodies of water such as lakes, rivers, and, especially, the oceans. Plants also release moisture into the air through their leaves. This process is called transpiration. For example, an acre of corn gives off 3,000 to 4,000 gallons of water to the atmosphere every day. A large tree may release fifty gallons per day. The plant moisture is first taken up by the roots from the soil, moves up the plant in the sap, and then emerges from the plant through thousands of tiny holes on the underside of each leaf.
Transpiration from plants is one of the important sources of water vapor in the air, and usually produces more moisture than evaporation from the ground, lakes, and streams. The most important source of water vapor in the air, however, is evaporation from the oceans, especially those parts of the ocean that are located in the warmest parts of the planet. Heat is required to change water from a liquid to a vapor. Thus, the higher the temperature, the faster the water evaporates from oceans. The winds in the upper air carry this moisture far from the oceans. Someone who lives in the central part of the United States may receive rain that is composed of water particles evaporated from the ocean near the equator or the Gulf of Mexico.
Ice Caps and Glaciers
About three-fourths of all freshwater in the world is stored as ice. After the oceans, the single greatest body of water is the Antarctic ice sheet. It covers about six million square miles, with a total volume of between six and seven million cubic miles—90% of all existing ice and about 70% of all freshwater. To get an idea of how much water covers Antarctica, if the ice sheet were to melt, it could supply the Mississippi River for more than 50,000 years, or all the rivers in the United States for about 17,000 years. It could supply the Amazon River for approximately 5,000 years, or all the rivers in the world for about 750 years.
The Greenland ice cap is about 667,000 square miles in area and is, on average, 5,000 feet thick; its total volume is 630,000 cubic miles. If melted, it would produce enough water to keep the Mississippi River flowing for more than 4,700 years.
A glacier is any large mass of snow or ice that persists on land for many years. Glaciers are formed in locations where, over a number of years, more snow falls than melts. As this snow accumulates, it is compressed and changed into dense, solid ice. Glacial masses tend to flow due to their own weight—downhill if on a slope or in all directions from the center if on a flat surface.
Although most people associate glaciers with remote, frozen regions such as Antarctica, there are more than 1,650 glaciers in the lower forty-eight U.S. states, most of them quite small. They cover about 227 square miles in parts of California, Colorado, Idaho, Montana, Nevada, Oregon, Washington, and Wyoming. Alaska has uncounted numbers of glaciers that cover many thousands of square miles.
Because the study of glaciers is a relatively new science, there is considerable debate over the significance of the expansion or shrinkage of glaciers or ice caps over five- to twenty-year time frames. Some studies suggest that many glaciers are melting and shrinking in size. Other scientists argue that these are short-term changes that occur routinely in geologic time, and should not be given too much significance. Extensive studies are underway in Antarctica, Greenland, and other areas of the world to investigate the formation, duration, and melting of glaciers and ice caps, but it will be many years before these phenomena are well understood.
Permafrost
Permafrost is permanently frozen ground, which underlies approximately one-fifth of Earth's entire land
FIGURE 1.4
Typical cross section of permafrost terrain
In North America, the discovery of gold in Alaska and the Yukon in the early 1900s sparked an increased interest in the nature of the vast areas of permafrost. After World War II (1939–45), more and more nonnative people migrated to areas of frozen ground, and the construction of roads, railroads, and buildings, and the clearing of land, led to the disruption and thawing of previously undisturbed permafrost. This caused unstable ground, landslides, mudflows, and, consequently, dangerous living conditions. Some scientists believe that the total area of permafrost is declining. Like glaciers and ice caps, permafrost regions are areas of scientific scrutiny and intense debate.
Snowmelt
Except for the disruption of day-to-day life caused by winter snowstorms in certain areas of the United States, most Americans are largely unaware of the importance of snow. Unlike many other countries, the United States is economically dependent on snow. Almost all the water in the arid West that can be tapped on a large-volume basis comes directly from spring snowmelt. The amount of water in a given year's snowpack varies greatly from one year to another. The snowpack volume is of crucial importance to regional economics. Too much snow can cause flooding and extensive damage to crops, livestock, businesses, and homes. Too little can mean shortages in water for drinking, irrigation, and hydroelectric power, affecting their availability and cost.
The importance of snow was highlighted in a May 2003 article in the Denver Post ("San Luis Valley Still in the Grip of Record Drought. Snowpack Goes into Ground, Not Streams"). Mark H. Hunter described drought conditions in northern Colorado, caused in part by a significant decrease in snowpack in the upper Rio Grande basin. According to the article, in 2003 snowpack in the upper Rio Grande basin was only 71% of average, with a snow-water content of thirteen inches. Drought conditions had affected the region for three consecutive years, diminishing the San Luis Valley's underground aquifer. The aquifer sustains thousands of acres of natural wetlands and half a million acres of farm land and ranch land. The area's snowpack improved somewhat in 2004 and 2005, but the effects of the drought were still being felt.
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