Earth is often called the water planet. Water makes up most of the volume of every living cell, maintains the temperature of the climate, and reshapes mountains and land. Although it covers more than 70 percent of the surface of the blue planet (Stanley 2004, 19), only a tiny percentage is available for human use (Figure 7.1). The freshwater that is not frozen and is available for use makes up only about 0.76 percent of the total. About 97.5 percent of Earth’s water is salt water. The remaining 2.5 percent is freshwater, most of which is frozen in ice

and snow. About 30.1 percent of the freshwater is available as groundwater. The water we can see-rivers, lakes, wetlands, clouds, soil moisture, plants and animals-makes up the remaining 0.4 percent of the freshwater (Black and King 2009, 21). Some water, including water in rivers, soil, and living things, recirculates within a time scale of days. Other water, including saltwater in the oceans and freshwater held in glaciers and underground aquifers, has a residence time measured in thousands of years (Cunningham and Cunningham 2010, 374).