II.

Storage

Enormous volumes of water are involved in the water cycle. There are about 1.4 billion cu km (about 340 million cu mi) of water on the earth, enough to cover the United States with water 147 km (92 mi) deep. Slightly more than 97 percent of this amount is ocean water and is therefore salty. However, because the water that evaporates from the ocean is almost free of salt, the rain and snow that fall on the earth are relatively fresh. Fresh water is stored in glaciers, lakes, and rivers. It is also stored as groundwater in the soil and rocks. There are about 36 million cu km (about 8.6 million cu mi) of fresh water on the earth.

The atmosphere holds about 12,000 cu km (about 2,900 cu mi) of water at any time, while all the world’s rivers and freshwater lakes hold about 120,000 cu km (about 29,000 cu mi). The world’s two main reservoirs of fresh water are the great polar ice caps, which contain about 28 million cu km (about 6.7 million cu mi), and the ground, which contains about 8 million cu km (about 2 million cu mi).

Almost all of the world’s fresh ice is found in the ice caps of Antarctica and Greenland. These ice caps cover more than 17 million sq km (more than 6.6 million sq mi) of land to an average depth of more than 1.5 km (more than 0.93 mi). Most other glaciers, formed in mountain valleys at high latitudes, are tiny compared to the ice caps. If all of the ice in the ice caps and other glaciers melted, it would raise the sea level by about 80 m (about 260 ft).

The amount of water stored as ice on the land varies with climate. At the peak of the last ice age, about 22,000 years ago, an additional 20 million sq km (8 million sq mi) of land—including almost all of Canada, the northern fringe of the United States, northern Europe, and large tracts in Siberia—were covered with ice about 1.5 km (about 0.93 mi) thick. Because this water came from the oceans, sea level was about 120 m (about 390 ft) lower than it is today. Most water in the ice caps remains frozen for centuries and is not readily accessible.

Most groundwater is more accessible and supplies much of people’s water needs in many regions of the earth. Permafrost, ground that is always frozen, forms an impermeable barrier to the flow of groundwater. Permafrost occurs in places such as northern Canada and Siberia where the annual average temperature is below 0° C (below 32° F).

Almost all groundwater fills the tiny pores and cracks in the soil and rocks. Very little is stored in subterranean caverns. Near the earth’s surface, most soils and sedimentary rocks are so porous that water can occupy from 20 to 40 percent of their volume. As depth increases, the pores and open spaces in the rocks are squeezed shut. As a result, almost all groundwater is found in the top 8 to 16 km (5 to 10 mi) of the earth. Water below this depth is chemically bound in the rocks and minerals and is not readily available, but it can be released as a result of geologic processes such as volcanic eruptions (see volcano).