Water: A Brief Global Perspective
The global water cycle, or hydrologic cycle, involves the movement or transfer of water from one of Earth’s storage compartments, such as the ocean, lakes, and the atmosphere, to another. In its simplest form, the water cycle can be viewed as water moving from the oceans to the tmosphere, falling from rain and snow, and then returning to the oceans as surface runoff and subsurface flow or to the atmosphere by evaporation. The cyclic nature of this global movement of water is illustrated in Figure 12.2. The major processes are evaporation, precipitation, transpiration (loss of water by plants to the atmosphere), surface runoff, and subsurface groundwater flow. These are quantitatively. The annual volume of water transferred from the ocean to the land is balanced by the same volume returning by river and groundwater flow to the ocean, and there is a balance between total evaporation and precipitation. The water that returns to the ocean is changed because it carries with it gravel, sand, silt, and clay that has eroded from the land. The return flow also carries many chemicals. Most of the chemicals are natural; however, many human-made and human-induced compounds are trapped in the return flow, such as organic waste and nutrients, as well as thousands of chemicals used in our agricultural, industrial, and urban processes.
Water is a heterogeneous resource that can be found in liquid, solid, or gaseous form on or near Earth’s surface. Water’s residence time may vary from a few days to many thousands of years, depending on its specific location. Furthermore, more than 99 percent of Earth’s water is unavailable or unsuitable for beneficial human use either because of its salinity, as with seawater, or its form and location, as is the case with water stored in ice caps and glaciers. Thus, all people compete for less than 1 percent of Earth’s water supply.