hydrology

The scientific study of the properties, distribution, and effects of water on the earth's surface, in the soil and underlying rocks, and in the atmosphere.

Concept

Hydrology is among the principal disciplines within the larger framework of hydrologic sciences, itself a subcategory of earth sciences study. Of particular importance to hydrology is the hydrologic cycle by which water is circulated through various earth systems above and below ground. But the hydrologic cycle is only one example of the role water plays in the operation of earth systems. Outside its effect on living things, a central aspect of the hydrologic cycle, water is important for its physical and chemical properties. Its physical influence, exerted through such phenomena as currents and floods, can be astounding but no less amazing than its chemical properties, demonstrated in the fascinating realm of karst topography.

How It Works

The Systems Approach

The modern study of earth sciences looks at the planet as a large, complex network of physical, chemical, and biological interactions. This is known as a systems approach to the study of Earth. The systems approach treats Earth as a combination of several subsystems, each of which can be viewed individually or in concert with the others. These subsystems are the geosphere, atmosphere, hydrosphere, and biosphere.

The geosphere is that part of the solid earth on which people live and from which are extracted the materials that make up our world: minerals and rocks as well as the organic products of the soil. In the latter area, the geosphere overlaps with the biosphere, the province of all living and recently living things; in fact, once a formerly living organism has decomposed and become part of the soil, it is no longer part of the biosphere and has become a component of the geosphere.

Overlap occurs between all spheres in one way or another. Thus, the hydrosphere includes all of the planet's waters, except for water that has entered the atmosphere in the form of evaporation. From the time moisture is introduced to the blanket of gases that surrounds the planet until it returns to the solid earth in the form of precipitation, water is a part of the atmosphere. This aspect of the planet's water is treated in the essay on Evapotranspiration and Precipitation.

The Hydrosphere and the Hydrologic Cycle

All aspects of water on Earth, other than evaporation and precipitation, fall within the hydrosphere. This includes saltwater and freshwater, water on Earth's surface and below it, and all imaginable bodies of water, from mountain streams to underground waterways and from creeks to oceans. One of the fascinating things about water is that because it moves within the closed system of Earth, all the planet's water circulates endlessly. Thus, there is a chance that the water in which you take your next bath or shower also bathed Cleopatra or provided a drink to Charlemagne's horse.

On a less charming note, there is also a good chance that the water with which you brush your teeth once passed through a sewer system. Lest anyone panic, however, this has always been the case and always will be; as we have noted, water circulates endlessly, and one particular molecule may serve a million different functions.

Furthermore, as long as water continues to circulate through the various earth systems—that is, as long as it is not left to stagnate in a pond—it undergoes a natural cleansing process. Modern municipal and private water systems provide further treatment to ensure that the water that people use for washing is at least reasonably clean. In any case, it is clear that the movement of water through the hydrologic cycle is a subject complex enough to warrant study on its own (see Hydrologic Cycle).

The Hydrologic Sciences

As noted in Studying Earth, the earth sciences can be divided into three broad areas: the geologic, hydrologic, and atmospheric sciences. Each of these areas corresponds to one of the "spheres," or subsystems within the larger earth system, that we have discussed briefly: geosphere, hydrosphere, and atmosphere.

The hydrologic sciences are concerned with the hydrosphere and its principal component, water. These disciplines include glaciology—the study of ice in general and glaciers in particular—and oceanography. Glaciology is discussed in a separate essay, and oceanography is examined briefly in the present context. Aside from these two areas of study, the central component of the hydrologic sciences is hydrology—its most basic discipline, as geology is to the geologic sciences.

Oceanography

Oceanography is the study of the world's saltwater bodies—that is, its oceans and seas—from the standpoint of their physical, chemical, biological, and geologic properties. These four aspects of oceanographic study are reflected in the four basic subdisciplines into which oceanography is divided: physical oceanography, chemical oceanography, marine geology, and marine ecology. Each represents the application of a particular science to the study of the oceans.

Physical oceanography, as its name implies, involves the study of physics as applied to the world's saltwater bodies. In general, it concerns the physical properties of the oceans and seas, including currents and tides, waves, and the physical specifics of seawater itself—that is, its temperature, pressure at particular depths, density in specific areas, and so on.

Just as physical oceanography weds physics to the study of seawater, chemical oceanography is concerned with the properties of the ocean as viewed from the standpoint of chemistry. These properties include such specifics as the chemical composition of seawater as well as the role the ocean plays in the biogeochemical cycles whereby certain chemical elements circulate between the organic and inorganic realms (see Biogeochemical Cycles, Carbon Cycle, and Nitrogen Cycle).

The biogeochemical focus of chemical oceanography implies an overlap with geochemistry. Likewise, marine geology exists at the nexus of oceanography and geology, involving, as it does, such subjects as seafloor spreading (see Plate Tectonics), ocean topography, and the formation of ocean basins. Finally, there is the realm where oceanography overlaps with biology, a realm known as marine ecology or biological oceanography. This subdiscipline is concerned with the wide array of life-forms, both plant and animal, that live in the oceans as well as the food webs whereby they interact with one another.

Introduction to Hydrology

As noted earlier, hydrology is the central field of the hydrologic sciences, dealing with the most basic aspects of Earth's waters. Among the areas of focus in hydrology are the distribution of water on the planet, its circulation through the hydrologic cycle, the physical and chemical properties of water, and the interaction between the hydrosphere and other earth systems.

Among the subdisciplines of hydrology are these:

• Groundwater hydrology: The study of water resources below ground.
• Hydrography: The study and mapping of large surface bodies of water, including oceans and lakes.
• Hydrometeorology: The study of water in the lower atmosphere, an area of overlap between the hydrologic and atmospheric sciences
• Hydrometry: The study of surface water—in particular, the measurement of its flow and volume.

The Work of Hydrologists

Bringing together aspects of geology, chemistry, and soil science, hydrology is of enormous practical importance. Local governments, for instance, require hydrologic studies before the commencement of any significant building project, and hydrology is applied to such areas as the designation and management of flood plains. Hydrologists also are employed in the management of water resources, wastewater systems, and irrigation projects. The public use of water for recreation and power generation also calls upon the work of hydrologists, who assist governments and private companies in controlling and managing water supplies.

Hydrologists in the field use a variety of techniques, some of them simple and time-honored and others involving the most cutting-edge modern technology. They may make use of highly sophisticated computer models and satellite remote-sensing technology, or they may apply relatively uncomplicated methods for the measurement of snow depth or the flow of rivers and streams. Local hydrologists searching for water may even avail themselves of the services of quasi-mystics who employ a nonscientific practice called dowsing. The latter method, which involves the sensing of underground water with a "magic" divining rod, sometimes is used, with varying degrees of success, to find water in rural areas.

Real-Life Applications

Currents

Ocean waters are continually moving, not only as waves hitting the shore (a function of the Moon's gravitational pull—see Sun, Moon, and Earth) but also in the form of currents. These are patterns of oceanic flow, many of them regular and unchanging and others susceptible to change as a result of shifts in atmospheric patterns and other parameters. Among the factors that affect the flow of currents are landmasses, wind patterns, and the Coriolis effect, or the deflection of water caused by the turning of Earth.

Landmasses on either side of the Atlantic, Pacific, and Indian Oceans act as barriers to the paths of currents. For instance, if Africa were not placed as it is, between the Atlantic and Indian Oceans, water in the equatorial region would flow uniformly from east to west, or from the Indian Ocean to the Atlantic. Likewise, the movement would be uniformly west to east at the poles, as it would be at the equator.

Such is the case just off the shores of Antarctica, where the Antarctic Circumpolar Current, without the obstruction of land barriers, consistently circles the globe in a west-to-east direction. On the other hand, at the southern extremity of Africa, far from the equator, the movement of water also would be uniform, but in this case from west to east. Because of these landmasses, however, the movement of currents is much more complex.

Wind patterns also drive currents. These patterns work in tandem with the Coriolis effect, a term that generally describes a phenomenon that occurs with all particles on a rotating sphere such as Earth. The result of the Coriolis effect is that water tends to rotate in circles, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. (At the equator and poles, by contrast, the Coriolis effect is nonexistent.) Combined with prevailing winds, the Coriolis effect creates vast elliptical (oval-shaped) circulating currents called gyres.

Surface Currents and Their Effect on Climate

Among the basic types of currents are surface, tidal, and deep-water currents. In addition, a fourth type of current, a turbidity current, is of interest to oceanographers, hydrologists, and underwater geologists. Surface currents such as the Gulf Stream are the most well known variety, being the major form of current by which water circulates on the ocean's surface. Caused by the friction of atmospheric patterns—another type of current—moving over the sea, these currents largely are driven by winds. (Winds, in turn, are caused by differences in temperature between packets of air at differing altitudes—see Convection.)

Running as deep as 656 ft. (200 m), surface currents can be a powerful force. As a result of the Gulf Stream, for instance, a craft that sets sail eastward from the Caribbean is likely to be pulled quickly toward England. Of even greater importance is the impact of the Gulf Stream on climate. By moving warm waters in a northeasterly direction, it causes the European climate to be much warmer than it would be normally.

For instance, Boston, Massachusetts, and Rome, Italy, are on the same latitude, but whereas Boston is known for its icy winters, the mention of Rome rightly conjures images of sunshine and warmth. Likewise, London lies north of the 50th parallel, far above any city in the continental United States—including such places as International Falls, Minnesota, and Buffalo, New York, which are noted for their cruel winters. On the other hand, London, while it is far from balmy in wintertime, is many degrees warmer, thanks to the Gulf Stream.

Other Types of Current

Among the other types of currents are tidal currents, which are horizontal movements of water associated with the changing tides of the ocean. Their effect is felt primarily in the area between the continental shelf and the shore, where tidal-current phenomena such as riptides can pose a serious danger to swimmers. Deep-water currents, while they are less noticeable to people, are responsible for 90% of the water circulation that takes place in the ocean. Caused by variations in water density, which is a function of salt content (salinity) and temperature, these are slow-moving currents that move colder, denser water toward the depths of the ocean.

Then there are turbidity currents, which result from the mixing of relatively light water with water that has been made heavy by its sediment content. Earthquakes may cause these currents, which are local, fast movements of water along the ocean floor. Another cause of turbidity currents is the piling of sediment on underwater slopes. Turbidity currents play a major role in shaping the terrain of the ocean floor.

Flooding

Whereas currents arise in areas of Earth where water is "supposed" to be, floods, by definition, do not. They often occur in valleys or on coastlines and can be caused by various natural and man-made factors. Among natural causes are rains and the melting of snow and ice, while human-related causes can include poor engineering of irrigation or other water-management systems as well as the bursting of dams. In addition, the building of settlements too close to rivers and other bodies of water that are prone to flooding has resulted in the increase of human casualties from flooding over the centuries.

In terms of natural causes, changes in weather patterns typically are involved—but not always. For example, a low-lying coastal area may be susceptible to flooding at times when the ocean reaches high tide. (On the other hand, such weather conditions as low barometric pressure and high winds also can bring about heightened high tides.) Additionally, floods can be caused by earthquakes and other geologic phenomena that have no relation to the weather.

From ancient times people have located settlements near water. This settlement pattern resulted from the obvious benefits that accrued from access to water, and even though flooding was naturally a hazard, in some cases flooding itself was found to be beneficial. For the ancient Egyptians, the yearly cycles of flooding on the part of the Nile caused the deposition of rich soil, which played a major part in the fertility of the farmlands that, in turn, made possible the brilliant civilization of the pharaohs.

Along with these benefits, however, ancient peoples learned to fear the changes in weather and other circumstances that could bring about sudden flooding. This feeling is reflected, for instance, in Jesus' parable about the wise and foolish house builders. In the parable, a favorite Sunday school topic, the foolish man builds his house upon the sand, so that when the floods come, they sweep away his household. On the other hand, the wise man builds his house on rock, so that his household withstands the inevitable flood—an illustration about spiritual values that likewise reflects a reality of daily life in the ancient Near East.

Human Causes and Effects

Humans can cause floods by such disastrous practices as clear-cutting of land and runaway grazing. Such activities remove vegetation, which holds soil in place and, in turn, keeps rivers and other bodies of water from flowing over onto the land. In addition, without vegetation to absorb rain, ground becomes saturated and thus susceptible to flooding. Not surprisingly, these unwise agricultural practices have helped bring about other disasters, such as the massive erosion of soil in the United States plains states that culminated in the dust bowl of the mid-1930s (see Soil Conservation).

Less well known than the dust bowl but still massive in its impact was the 1927 flood of the Mississippi River, which left more than a million people homeless. It, too, was in part the result of unwise practices, in this case, inconsistent flood management and civil engineering design, according to John M. Barry, the author of Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America. As Barry indicated in his subtitle, the flood's impact went far beyond its direct effect on human lives or the landscape.

As Barry discusses in the book, the flood was a major cause behind the rise of the poor-white discontent in Louisiana that led to the governor-ship (and, in the opinion of some people, the dictatorship) of the notorious Huey P. Long (1893-1935). Long, who won election on promises to ease the suffering of the underclass, ultimately became the virtual ruler of his state, with a degree of power that in the opinions of some pundits rivaled that of President Franklin D. Roosevelt—if not that of his other contemporaries, Adolf Hitler and Benito Mussolini. Of even greater long-term significance, Barry maintained, the flood brought about the large-scale flight of African Americans to the north and the shift of black political allegiance from the Republican Party to the Democratic Party.

Few people alive today remember the 1927 flooding, but plenty recall the devastating floods of 1993, which killed 52 people and left over 70,000 homeless. Human mismanagement could not be blamed for the flooding itself, an outgrowth of exceptionally high soil moisture levels remaining from the fall of 1992, as well as heavy precipitation that continued in early 1993. However, once again, human attempts to control the flooding were less than successful: of some 1,300 levees or embankments that had been built (partly as a result of the 1927 flood) to keep flood waters back, all but about 200 failed. The floods, which lasted from late June to mid-August, destroyed nearly 50,000 homes and rendered over 12,000 sq mi. (31,000 sq km) of farmland useless. The overall damage estimate was in the range of $15 to $20 billion.

Power and Prevention

It is no wonder that a flood can have such a far-reaching impact, given the enormous power of water running wild in nature. Water is extremely heavy: just a bathtub full of water can weigh as much 750 lb. (340 kg). And it can travel as fast as 20 MPH (32 km/h), giving it tremendous physical force. Under certain conditions, a flood just 1 in. (2.54 cm) in depth can have as much potential energy as 60,000 tons (54,400 metric tons) of TNT. A U.S. study of persons killed in natural disasters during the 20-year period that ended in 1967 found that of 443,000 victims, nearly 40%, or about 173,000, were killed in floods. The other 60% was made up of people killed in 18 different types of other natural disaster, including hurricanes, earthquakes, and tornadoes.

Given this great destructive potential, communities—often with the help of hydrologists—have devised several means to control floods. Among such methods are the construction of dams and the diversion of floodwaters away from populated areas to flood-control reservoirs. These reservoirs then release the water at a slower rate than it would be released in the situation of a flood, thus giving the soil time to absorb the excess water. About one-third of all reservoirs in the United States are used for this purpose.

Hydrologists are particularly important in helping communities protect against flooding by methods known as hazard zoning and minimizing encroachment. By studying historical records, along with geologic maps and aerial photographs, hydrologists and other planners can make recommendations regarding the zoning laws for a particular area, so that builders will take special precautions. In addition, they can help minimize encroachment—that is, ensure that new buildings are not located in such a way that they restrict the flow of water or cause water to pool up excessively.

Karst Topography

In contrast to the dramatic action of flooding or currents, karst topography is a more subtle but no less intriguing aspect of the ways in which water affects the dynamics of Earth. In this case, however, the effect is chemical rather than physical in origin. Karst topography is a particular variety of landscape created where water comes into contact with extremely soluble (easily dissolved in water) varieties of bedrock.

Karst is the German name for Kras, a region of Slovenia noted for its unusual landscape of strangely shaped white rock. In addition to the odd, funhouse forms of nightmarishly steep hills and twisting caves, much of it like something from a Dr. Seuss book, karst topography is noted for its absence of surface water, topsoil, or vegetation. The reason is that the bedrock comprises extremely soluble calcium carbonate minerals, such as limestone, gypsum, or dolomite.

Karst regions form as a result of chemical reactions between groundwater and bedrock. In the atmosphere and on the surface of the solid earth, water combines with carbon dioxide in the air, and this combination acts as a corrosive on calcium carbonate rocks. This corrosive or acidic material seeps into all crevices of the rock, developing into sinkholes and widening fissures over time. Gradually, it carves out enormous underground drainage systems and caves.

Sometimes the underground drainage structure collapses, leaving behind more odd shapes in the form of natural bridges and sinkholes. This is a variety of karst topography known as doline karst. Another type is cone or tower karst, which produces tall, jagged limestone peaks, such as the sharp hills that characterize the river landscape in many parts of China. The United States is home to the world's largest karst region, which includes the Mammoth cave system in Kentucky.

Where to Learn More

Abbott, Patrick. Natural Disasters. Dubuque, IA: WilliamC. Brown Publishers, 1996.

Arnell, Nigel. Hydrology and Global Environmental Change. Upper Saddle River, NJ: Prentice Hall, 2001.

Barry, John M. Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America. New York: Simon and Schuster, 1997.

Dingman, S. Lawrence. Physical Hydrology. 2d ed. Upper Saddle River, NJ: Prentice Hall, 2002.

Gallant, Roy A. Earth's Water. New York: Benchmark Books, 2002.

Global Hydrology and Climate Center (Web site). <http://www.ghcc.msfc.nasa.gov/>.

Hydrology, Marine Science, Freshwater Science and Aquaculture Journals (Web site). <http://www.sciencekomm.at/journals/hydro.html>.

Karst Link Page (Web site). <http://hum.amu.edu.pl/~sgp/spec/linkk.html>.

Karst Waters Institute (Web site). <http://www.karstwaters.org/>.

Skinner, Brian J., Stephen C. Porter, and Daniel B.

Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

The World-Wide Web Virtual Library: Oceanography (Web site). <http://www.mth.uea.ac.uk/ocean/vl/>.

The study of the waters of the Earth: their occurrence, circulation, and distribution; their chemical and physical properties; and their reaction with the environment, including their relation to living things.

Water in liquid and solid form covers most of the crust of the Earth. By a complex process powered by gravity and the action of solar energy, an endless exchange of water, in vapor, liquid, and solid forms, takes place between the atmosphere, the oceans, and the crust. This is known as the hydrologic cycle. Water circulates in the air and in the oceans, as well as over and below the surface of landmasses. The distribution of water in the planet is uneven. General patterns of circulation are present in the atmosphere, the oceans, and the landmasses, but regional features are very irregular and seemingly random in detail. Therefore, while causal relations underlie the overall process, it is believed that important elements of chance affect local hydrological events. See also Atmospheric general circulation.

Whereas the global linkages of the hydrologic cycle are recognized, the science of hydrology has traditionally confined its direct concern to the detailed study of the portion of the cycle limited by the physical boundaries of the land; thus, it has generally excluded specialized investigations of the ocean (which is the subject of the science of oceanography) and the atmosphere (which is the subject of the science of meteorology). The heightened interest in anthropogenically induced environmental impacts has, however, underlined the critical role of the hydrologic cycle in the global transport and budgeting of mass, heat, and energy. Hydrology has become recognized as a science concerned with processes at the local, regional, and global scales. This enhanced status has strengthened its links to meteorology, climatology, and oceanography. See also Climatology; Meteorology; Oceanography.

A number of field measurements are performed for hydrologic studies. Among them are the amount and intensity of precipitation; the quantities of water stored as snow and ice, and their changes in time; discharge of streams; rates and quantities of infiltration into the soil, and movement of soil moisture; rates of production from wells and changes in their water levels as indicators of ground-water storage; concentration of chemical elements, compounds, and biological constituents in surface and ground waters; amounts of water transferred by evaporation and evapotranspiration to the atmosphere from snow, lakes, streams, soils, and vegetation; and sediment lost from the land and transported by streams.

In addition, hydrology is concerned with research on the phenomena and mechanisms involved in all physical and biological components of the hydrologic cycle, with the purpose of understanding them sufficiently to permit quantitative predictions and forecasting. The field investigations and measurements not only provide the data whereby the behavior of each component may be evaluated in detail, permitting formulation in quantitative terms, but also give a record of the historical performance of the entire system. Thus, two principal vehicles for hydrological forecasting and prediction become available: a set of elemental processes, whose operations are expressible in mathematical terms, linked to form deterministic models that permit the prediction of hydrologic events for given conditions; and a group of records or time series of measured hydrologic variables, such as precipitation or runoff, which can be analyzed by statistical methods to formulate stochastic models that permit inferences to be made on the future likelihood of hydrologic events. See also Ground-water hydrology; Hydrosphere.

The study of the earth's water, particularly of water on and under the ground before it reaches the ocean or before it evaporates into the air. This science has many important applications such as flood control, irrigation, domestic and industrial water supply, and the generation of hydroelectric power.

For more information on hydrology, visit Britannica.com.

Water covers 70% of the Earth's surface.

Hydrology (from Greek: Yδωρ, hydōr, "water"; and λόγος, logos, "study") is the study of the movement, distribution, and quality of water throughout Earth, and thus addresses both the hydrologic cycle and water resources. A practitioner of hydrology is a hydrologist, working within the fields of either earth or environmental science, physical geography, geology or civil and environmental engineering.

Domains of hydrology include hydrometeorology, surface hydrology, hydrogeology, drainage basin management and water quality, where water plays the central role. Oceanography and meteorology are not included because water is only one of many important aspects.

Hydrological research is useful as it allows us to better understand the world in which we live, and also provides insight for environmental engineering, policy and planning.

Contents 1 History of hydrology 2 Hydrologic cycle 3 Branches of hydrology 3.1 Related fields 4 Hydrologic measurements 5 Hydrologic prediction 5.1 Statistical hydrology 5.2 Hydrologic modeling 6 Hydrologic transport 7 Applications of hydrology 8 See also 9 Further reading 10 External links 10.1 Other on-line resources 10.2 National and international research bodies 10.3 National and international societies 10.4 Basin- and catchment-wide overviews

History of hydrology

Hydrology has been a subject of investigation and engineering for millennia. For example, about 4000 B.C. the Nile was dammed to improve agricultural productivity of previously barren lands. Mesopotamian towns were protected from flooding with high earthen walls. Aqueducts were built by the Greeks and Ancient Romans, while the History of China shows they built irrigation and flood control works. The ancient Sinhalese used hydrology to build complex irrigation Works in Sri Lanka, also known for invention of the Valve Pit which allowed construction of large reservoirs, anicuts and canals which still function.

Marcus Vitruvius, in the first century B.C., described a philosophical theory of the hydrologic cycle, in which precipitation falling in the mountains infiltrated the Earth's surface and led to streams and springs in the lowlands. With adoption of a more scientific approach, Leonardo da Vinci and Bernard Palissy independently reached an accurate representation of the hydrologic cycle. It was not until the 17th century that hydrologic variables began to be quantified.

Pioneers of the modern science of hydrology include Pierre Perrault, Edme Mariotte and Edmund Halley. By measuring rainfall, runoff, and drainage area, Perrault showed that rainfall was sufficient to account for flow of the Seine. Marriotte combined velocity and river cross-section measurements to obtain discharge, again in the Seine. Halley showed that the evaporation from the Mediterranean Sea was sufficient to account for the outflow of rivers flowing into the sea.

Advances in the 18th century included the Bernoulli piezometer and Bernoulli's equation, by Daniel Bernoulli, the Pitot tube. The 19th century saw development in groundwater hydrology, including Darcy's law, the Dupuit-Thiem well formula, and Hagen-Poiseuille's capillary flow equation.

Rational analyses began to replace empiricism in the 20th century, while governmental agencies began their own hydrological research programs. Of particular importance were Leroy Sherman's unit hydrograph, the infiltration theory of Robert E. Horton, and C.V. Theis's Aquifer test/equation describing well hydraulics.

Since the 1950s, hydrology has been approached with a more theoretical basis than in the past, facilitated by advances in the physical understanding of hydrological processes and by the advent of computers and especially Geographic Information Systems (GIS).

Hydrologic cycle

Main article: Hydrologic cycle

The central theme of hydrology is that water moves throughout the Earth through different pathways and at different rates. The most vivid image of this is in the evaporation of water from the ocean, which forms clouds. These clouds drift over the land and produce rain. The rainwater flows into lakes, rivers, or aquifers. The water in lakes, rivers, and aquifers then either evaporates back to the atmosphere or eventually flows back to the ocean, completing a cycle.

Branches of hydrology

Chemical hydrology is the study of the chemical characteristics of water.

Ecohydrology is the study of interactions between organisms and the hydrologic cycle.

Hydrogeology is the study of the presence and movement of ground water.

Hydroinformatics is the adaptation of information technology to hydrology and water resources applications.

Hydrometeorology is the study of the transfer of water and energy between land and water body surfaces and the lower atmosphere.

Isotope hydrology is the study of the isotopic signatures of water.

Surface hydrology is the study of hydrologic processes that operate at or near Earth's surface.

Related fields

• Aquatic chemistry
• Civil engineering
• Climatology
• Environmental engineering
• Environmental Engineering Science
• Geomorphology
• Hydrography
• Hydraulic engineering
• Limnology
• Oceanography
• Physical geography

Hydrologic measurements

Measurement is fundamental for assessing water resources and understanding the processes involved in the hydrologic cycle. Because the hydrologic cycle is so diverse, hydrologic measurement methods span many disciplines: including soils, oceanography, atmospheric science, geology, geophysics and limnology, to name a few. Here, hydrologic measurement methods are organized by hydrologic sub-disciplines. Each of these subdisciplines is addressed briefly with a practical discussion of the methods used to date and a bibliography of background information.

Quantifying groundwater flow and transport

• Aquifer characterization
  • Flow direction
    • Piezometer - groundwater pressure and, by inference, groundwater depth (see: aquifer test)
    • Conductivity, storativity, transmisivity
    • Geophysical methods

• Vadose zone characterization
  • Infiltration
    • Infiltrometer - infiltration
  • Soil moisture
    • Capacitance probe-soil moisture
    • Time domain reflectometer - soil moisture
    • Tensiometer - soil moisture
    • Solute sampling
    • Geophysical methods

Quantifying surface water flow and transport

• Direct and indirect discharge measurements
  • Stream gauge - stream flow (see: discharge (hydrology))
  • Tracer techniques
  • Chemical transport
  • Sediment transport and erosion
  • Stream-aquifer exchange

Quantifying hydrologic exchange at the land-atmospheric boundary

• Precipitation
  • Bulk rain events
    • Disdrometer - precipitation characteristics
    • Radar - cloud properties, rain rate estimation, hail and snow detection
    • Rain gauge - rain and snowfall
    • Satellite - rainy area identification, rain rate estimation, land-cover/land-use, soil moisture
    • Sling psychrometer - humidity
  • Snow, hail and ice
  • Dew, mist and fog
• Evaporation
  • from water surfaces
  • Evaporation -Symon's evaporation pan
  • from plant surfaces
  • through the boundary layer
• Transpiration
  • Natural ecosystems
  • Agronomic ecosystems
• Momentum
• Heat flux
  • Energy budgets

Uncertainty analyses

Remote sensing of hydrologic processes

• Land based sensors
• Airborne Sensors
• Satellite sensors

Water quality

• Sample collection
• In-situ methods
• Physical measurements (includes sediment concentration)
• Collection of samples to quantify Organic Compounds
• Collection of samples to quantify Inorganic Compounds
• Analysis of aqueous Organic Compounds
• Analysis of aqueous Inorganic Compounds
• Microbiological sampling and analysis

Integrating measurement and modeling

• Budget analyses
• Parameter estimation
• Scaling in time and space
• Data assimilation
• Quality control of data — see for example Double mass analysis

Hydrologic prediction

Observations of hydrologic processes are used to make predictions of the future behaviour of hydrologic systems (water flow, water quality). One of the major current concerns in hydrologic research is the Prediction in Ungauged Basins (PUB), i.e. in basins where no or only very few data exist.

Statistical hydrology

By analysing the statistical properties of hydrologic records, such as rainfall or river flow, hydrologists can estimate future hydrologic phenomena, assuming the characteristics of the processes remain unchanged.

These estimates are important for engineers and economists so that proper risk analysis can be performed to influence investment decisions in future infrastructure and to determine the yield reliability characteristics of water supply systems. Statistical information is utilised to formulate operating rules for large dams forming part of systems which include agricultural, industrial and residential demands.

See: return period.

Hydrologic modeling

Hydrologic models are simplified, conceptual representations of a part of the hydrologic cycle. They are primarily used for hydrologic prediction and for understanding hydrologic processes. Two major types of hydrologic models can be distinguished:

• Models based on data. These models are black box systems, using mathematical and statistical concepts to link a certain input (for instance rainfall) to the model output (for instance runoff). Commonly used techniques are regression, transfer functions, and system identification. The simplest of these model models are may be linear models, but is common to deploy non-linear components to represent some general aspects of a catchment's response without going deeply into the real physical processes involved. An example of such an aspect is the well-known behaviour that a catchment will respond much more quickly and strongly when it is already wet than when it is dry..

• Models based on process descriptions. These models try to represent the physical processes observed in the real world. Typically, such models contain representations of surface runoff, subsurface flow, evapotranspiration, and channel flow, but they can be far more complicated. These models are known as deterministic hydrology models. Deterministic hydrology models can be subdivided into single-event models and continuous simulation models.

Recent research in hydrologic modeling tries to have a more global approach to the understanding of the behaviour of hydrologic systems to make better predictions and to face the major challenges in water resources management.

Hydrologic transport

See main article: Hydrologic transport model

Water movement is a significant means by which other material, such as soil or pollutants, are transported from place to place. Initial input to receiving waters may arise from a point source discharge or a line source or area source, such as surface runoff. Since the 1960s rather complex mathematical models have been developed, facilitated by the availability of high speed computers. The most common pollutant classes analyzed are nutrients, pesticides, total dissolved solids and sediment.

Applications of hydrology

• Determining the water balance of a region.
• Determining the agricultural water balance
• Designing riparian restoration projects.
• Mitigating and predicting flood, landslide and drought risk.
• Real-time flood forecasting and flood warning.
• Designing irrigation schemes and managing agricultural productivity.
• Part of the hazard module in catastrophe modeling.
• Providing drinking water.
• Designing dams for water supply or hydroelectric power generation.
• Designing bridges.
• Designing sewers and urban drainage system.
• Analyzing the impacts of antecedent moisture on sanitary sewer systems.
• Predicting geomorphological changes, such as erosion or sedimentation.
• Assessing the impacts of natural and anthropogenic environmental change on water resources.
• Assessing contaminant transport risk and establishing environmental policy guidelines.

See also

• Hydropower
• International Hydrological Programme
• Hydrology (agriculture)
• Water cycle
• Water distribution on Earth
• Water management
• Nash–Sutcliffe model efficiency coefficient

Further reading

• Introduction to Hydrology, 4e. Viessman and Lewis, 1996. ISBN 0-673-99337-X
• Handbook of Hydrology. ISBN 0-07-039732-5
• Encyclopedia of Hydrological Sciences. ISBN 0-471-49103-9
• Hydrological Processes, ISSN: 1099-1085 (electronic) 0885-6087 (paper), John Wiley & Sons
• Journal of Hydroinformatics, ISSN: 1464-7141, IWA Publishing
• Hydrology Research (formerly Nordic Hydrology), ISSN: 0029-1277, IWA Publishing
• Journal of Hydrologic Engineering, ISSN: 0733-9496, ASCE Publication
• Hydrologic Analysis and Design. McCuen, Third Edition, 2005. ISBN 0-13-142424-6

External links

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Look up hydrology in Wiktionary, the free dictionary.

Other on-line resources

• The Modular Curriculum for Hydrologic Advancement - MOCHA
• International glossary of hydrology.
• Virtual campus in hydrology and water resources
• Decision tree to choose an uncertainty method for hydrological and hydraulic modelling
• Catchment Modelling Toolkit
• CITY DRAIN - Open source software for integrated modelling in urban drainage (Platform: MAtlab/Simulink, citydrain.bplaced.net)
• Experimental Hydrology Wiki
• Hydrognomon - Open source software for hydrological time series processing and analysis

National and international research bodies

• U.S. Geological Survey - Water Resources of the United States
• Centre for Ecology and Hydrology - UK
• eawag - aquatic research, ETH Zürich, Switzerland
• Institute of Hydrology, Albert-Ludwigs-University of Freiburg, Germany
• NOAA's National Weather Service - Office of Hydrologic Development, USA
• National Hydrology Research Center - Canada
• eWater - Australian Government’s Cooperative Research Centres programme
• Hydrologic Research Center - A Non-Profit Research Corporation, USA
• Economics of Water Resource Management from Improved Forecasting, NOAA Economics, USA
• University of Oklahoma- Natural Hazards and Disaster Center/USA
• UNESCO-IHE Institute for Water Education

National and international societies

• International Association of Hydrological Sciences (IAHS)
• British Hydrological Society
• Russian Geographical Society (Moscow Centre) - Hydrology Commission
• International Assoc for Environmental Hydrology
• American Water Resources Association

Basin- and catchment-wide overviews

• Connected Waters Initiative, University of New South Wales — Investigating and raising awareness of groundwater and water resource issues in Australia

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Dansk (Danish)
n. - hydrologi

Nederlands (Dutch)
hydrologie (studie van water)

Français (French)
n. - hydrologie

Deutsch (German)
n. - Hydrologie (Lehre vom Wasser)

Ελληνική (Greek)
n. - υδρολογία

Italiano (Italian)
idrologia

Português (Portuguese)
n. - hidrologia (f)

Русский (Russian)
гидрология

Español (Spanish)
n. - hidrología

Svenska (Swedish)
n. - hydrologi

中文（简体）(Chinese (Simplified))
水文学, 水文地理学

中文（繁體）(Chinese (Traditional))
n. - 水文學, 水文地理學

한국어 (Korean)
n. - 수문학

日本語 (Japanese)
n. - 水文学

العربيه (Arabic)
‏(الاسم) الهيدرولوجيا أو المائيات أو علم المياه, علم يبحث في خصائص المياه وظواهرها وتوزيعها فوق سطح الارض وفي التربه وتحت الصخور, وفي الجو‏

עברית (Hebrew)
n. - ‮הידרולוגיה, תורת המים‬

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