Hydrologic Cycle

views updated

Hydrologic Cycle

Introduction

The continuous circulation of water through regions of the global environment—including the ocean, glaciers, underground reservoirs of water (aquifers), and surface waters such as lakes, rivers, and streams—and through the atmosphere, is vital for life on Earth and is responsible for the planet's climate. This cycling of water is called the hydrologic cycle.

The total amount of water present on Earth as a solid, liquid, and gas is constant, but the water shifts between the various states in various environmental locations. For example, at different times a given molecule of water can be part of a frozen lake, water vapor in the atmosphere, or a raindrop falling back to the surface.

Although the total amount of water remains the same, it is not distributed evenly. Temperate regions north and south of the equator receive much precipitation, while other arid areas of Earth receive so little water that they are desert-like.

This disparity will increase due to global warming, according to the Intergovernmental Panel on Climate Change (IPCC). In its 2007 Assessment Report, the IPCC notes that increased runoff and less severe winters

may be affecting hydrological systems, particularly in the polar regions. Although the consensus among scientists is that the changing hydrologic cycle will influence climate, it is not clear whether the change will be slight or great.

Historical Background and Scientific Foundations

The cycling of water between Earth's surface and the surrounding environment has been occurring since the atmosphere developed nearly five million years ago. The amount of water that formed as Earth cooled is about the same as the volume present in 2007. The water has been endlessly cycling since the atmosphere formed.

Some of Earth's freshwater has not been part of the hydrological cycle, having become bound as frozen water in glacial or polar ice. Yet, this previously inaccessible water is now increasingly joining the global water cycle, as polar ice melts under the influence of the warming atmosphere.

The hydrologic cycle consists of five parts: condensation, precipitation, infiltration, runoff, and evaporation. Condensation is the process where water changes from a gas (water vapor) to a liquid due to cooling temperature. Clouds form due to condensation. Moisture released from clouds as precipitation falls to the surface and can percolate down into the soil (infiltration) or run along the surface of impermeable surfaces (runoff) to collect in stream, rivers, and larger bodies of water. Some of the water that has infiltrated into Earth will move underground and emerge at surface water sources as well. Water also remains underground where it becomes a source of moisture for vegetation and drinking water. Finally, the heat from the sun will cause evaporation of the surface water, changing it from a liquid to water vapor that rises in the atmosphere, where it cools and condenses, beginning another round of the hydrologic cycle.

The seemingly simple nature of the water cycle that can be depicted on paper is more complex in real life. Influences include the amount of vegetation present, nature of the surface (soil absorbs water better than rockier terrain), and atmospheric temperature. All these are influenced by human activities.

Impacts and Issues

The hydrologic cycle is vital to Earth's climate, and is being affected by the increasing temperature of the planet's atmosphere. Increased evaporation in equatorial areas increases the amount of water vapor in the atmosphere, which can help retain heat. The result in the twenty-first century may, according to some climate scientists, be a vicious circle, whereby global warming accelerates the hydrologic cycle, which then speeds up global warming.

Other scientists argue that this scenario fails to account for the complexity of the interactions that occur between Earth's surface and the atmosphere. Indeed, different computer models produce different predictions of the extent of global warming. However, climate models all agree that warming of the atmosphere will continue, and that the hydrologic cycle is involved.

Analysis of data from the past 150 years has established that human activities influence the hydrologic cycle. For example, the removal of large numbers of trees by deforestation to produce agricultural land or urban development reduces the amount of water that is transferred to the atmosphere. In its 2007 assessment report, the IPCC concluded that it is very likely that natural systems, including the hydrologic cycle, are being affected by human-mediated climate change.

Humans have used parts of the hydrologic cycle to create power for centuries. Two thousand years ago, waterfalls were already being used to move stone wheels that ground wheat into flour. Damming of rivers allows the use of water energy to generate electricity. These types of hydroelectric developments cause local changes in hydrology, especially by reducing variations of flow and sometimes by unpredictable spillage of water at times when the storage capacity of the reservoir is full.

Human activities may be having a much greater influence on the hydrologic cycle than local activities such as deforestation and urbanization. In polar areas, the freshwater content of surface and deeper water of the North Atlantic has been increasing since the mid-twentieth century with the increased warming of the atmosphere, according to the IPCC. At the same time, water loss from tropical surface waters has been increasing due to the increased evaporation caused by global warming. These are indications that the hydrologic cycle is changing.

WORDS TO KNOW

CLIMATE MODEL: A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

HYDROLOGY: The science that deals with global water (both liquid and solid), its properties, circulation, and distribution.

PRECIPITATION: Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, their water droplets slowly being pulled down by gravity. Because the water droplets are so small and light, it can take 21 days to fall 1,000 ft (305 m), and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles, then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

IN CONTEXT: IMPACTS ON WATER RESOURCES

According to the National Academy of Sciences: “Climate-driven changes in precipitation in certain regions could have significant impacts on water availability for agriculture, residential and industrial use, and recreation. Such regional impacts will be much more noticeable than projected changes in global average temperature.”

SOURCE: Staudt, Amanda, Nancy Huddleston, and Sandi Rudenstein. Understanding and Responding to Climate Change. National Academy of Sciences, 2006.

The effect of a changed global water cycle is not clear. Climate models have shown that the increased source of freshwater in more northern latitudes could affect ocean currents, in particular the Gulf Stream. In 2005, a paper published in the journal Nature reported that the northern flow of the warm Gulf Stream had decreased by 30% as compared to earlier surveys. The scientific consensus is that an altered Gulf Stream would affect the climates of eastern North America and the United Kingdom (the U.K. would become colder). But, the extent of the temperature change and whether the changed Gulf Stream flow was only a single incident or the beginning of a more persistent change that is related to a global climate change was unclear as of 2007.

BIBLIOGRAPHY

Books

DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.

Web Sites

“Climate and the Water Cycle.” National Center for Atmospheric Research. <http://www.ncar.ucar.edu/research/earth_system/watercycle.php> (accessed November 30, 2007).

“Common Misconceptions About Abrupt Climate Change.” Woods Hole Oceanographic Institution, August 30, 2007. <http://www.whoi.edu/page.do?pid=12455&tid=282&cid=10149#ocean_9> (accessed November 30, 2007).

Bill Freeman

Climate Change: In Context