Water is constantly moving among Earth’s different spheres—the hydrosphere, the atmosphere, the geosphere, and the biosphere. This unending circulation of water is called the hydrologic cycle. Earth is the only planet in our solar system that has a global ocean and a hydrologic cycle.

Earth’s Water

Water is almost everywhere on Earth—in the oceans, glaciers, rivers, lakes, air, soil, and living tissue. All these reservoirs constitute Earth’s hydrosphere. Figure 1 reminds us that the vast bulk of Earth’s water, about 96.5 percent, is stored in the global ocean. Ice sheets and glaciers account for most of our planet’s freshwater, leaving just a fraction of  percent to be divided among lakes, streams, groundwater, and the atmosphere. Although the percentage of Earth’s total water found in each of the latter sources is just a small fraction of the total inventory, the absolute quantities are great.

Water’s Paths

The hydrologic cycle is a gigantic, worldwide system powered by energy from the Sun, in which the atmosphere provides a vital link between the oceans and continents (Figure 2). Evaporation, the process by which liquid water changes into water vapor (gas), is how water enters the atmosphere from the ocean and, to a much lesser extent, from the land. Winds often transport moisture-laden air great distances. Complex processes of cloud formation eventually result in precipitation. The precipitation that falls into the ocean has completed its cycle and is ready to begin again. The water that falls on the continents, however, must make its way back to the ocean.

What happens to precipitation once it has fallen on land? A portion of the water soaks into the ground (called infiltration), slowly moving downward, then moving laterally, and finally seeping into lakes, streams, or directly into the ocean. When the rate of rainfall exceeds ground’s ability to absorb it, the surplus water flows over the surface into lakes and streams, a process called runoff. Much of the water that infiltrates or runs off eventually returns to the atmosphere because of evaporation from the soil, lakes, and streams. Also, some of the water that soaks into the ground is absorbed by plants, which then release it into the atmosphere. This process is called transpiration. Because both evaporation and transpiration involve the transfer of water from the surface directly to the atmosphere, they are often considered together as the combined process of evapotranspiration. When precipitation falls in very cold places—at high elevations or high latitudes—the water may not immediately soak in, run off, or evaporate. Instead, it may become part of a snowfield or glacier. Snow that accumulates and compresses in polar or mountainous regions can form glaciers, in which large quantities of water are stored.

Water Balance

Figure 2 also shows Earth’s overall water balance, or the volume that passes through each part of the cycle annually. The amount of water vapor in the air at any one time is just a tiny fraction of Earth’s total water supply. But the absolute quantity of water in the hydrologic cycle is immense—more than 500,000 cubic kilometers (about 120,000 cubic miles) of water moves through the cycle every year.

It is important to know that the hydrologic cycle is balanced. Because the total amount of water vapor in the atmosphere remains about the same, the average annual precipitation worldwide must be equal to the quantity of water evaporated. For all the continents taken together, precipitation exceeds evaporation. Conversely, over the oceans, evaporation exceeds precipitation. To balance the cycle, about 46,000 cubic kilometers (~110,000 cubic miles) of water annually makes its way from the land back to the ocean through runoff, stream flow, and groundwater flow.

About one-quarter of global precipitation falls on land and flows on and below the surface. This water is the most important force sculpting Earth’s land surface. In the rest of this chapter, we will observe the work of water running over the surface, including floods, erosion, and the formation of valleys. Then we will look underground, at the slow labors of groundwater as it forms springs and caverns on its long migration to the sea. As we explore the work of water on Earth, we will also discuss environmental issues associated with its movement and its use by humans.

Water as a Resource

Humans play a significant role in Earth’s hydrological cycle, and global human population growth puts ever-increasing pressure on the 1 percent of Earth’s water readily available for human use. Yearly, humans obtain, use, and discard 24,000 cubic kilometers (~5800 cubic miles) of water from Earth, almost entirely from groundwater and some surface water sources. This human water cycle involves the use of water for everyday activities, such as energy production, drinking water, agriculture, manufacturing goods, and recreation. Worldwide, the largest human consumption of water is for agriculture, accounting for 70 percent of global water use. Globally, industrial uses of water account for about 19 percent, and drinking water and household water use account for 12 percent. These percentages vary by region; in the Americas, 51 percent of water use is for agriculture while 34 percent is for industrial use. Recent studies suggest that human demands for water will increase 20 to 50 percent by the year 2050.

• Water moves through the hydrosphere’s many reservoirs by evaporating, condensing into clouds, and falling as precipitation.

• Once it reaches the ground, rain can either soak in, evaporate, be returned to the atmosphere by plant transpiration, or run off.

• Running water is the most important agent sculpting Earth’s varied landscapes.

• evaporation: The process by which a liquid converts to a gas.

• evapotranspiration: The combined effects of evaporation and transpiration.

• hydrologic cycle: The circulation of Earth’s water supply, powered by energy from the Sun and characterized by continuous exchanges of water among the oceans, the atmosphere, and the continents.

• infiltration: The movement of surface water into rock or soil through cracks and pore spaces.

• runoff: Water that flows over land rather than infiltrating into the ground.

• transpiration: The release of water vapor to the atmosphere by plants.