Water covers 70 percent of the Earth's surface.

Hydrology (from the Greek word Yδρoλoγια, hydrologia, the "study of water") is the study of the movement, distribution, and quality of water throughout the Earth. It addresses both the hydrologic cycle and water resources. A practitioner of hydrology, or hydrologist, may work in any of several fields: earth science, environmental science, physical geography, civil engineering, and environmental engineering.


Hydrological research is useful in that it allows engineers to (a) design irrigation schemes, water-supply systems, dams, bridges, and sewers; (b) predict and mitigate the risk of floods, droughts, landslides, erosion, and sedimentation; and (c) assess the risk of contaminant transport. In this manner, it provides insights for environmental engineering, policy, and planning.


Hydrology has been a subject of investigation and engineering for millennia. For example, around 4000 B.C.E., 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 Romans, while the Chinese built irrigation and flood control works.

In the first century B.C.E., Marcus Vitruvius described a philosophical theory of the hydrologic cycle, according to which precipitation falling in the mountains infiltrated the Earth's surface and led to streams and springs in the lowlands. With the 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 seventeenth 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 evaporation from the Mediterranean Sea was sufficient to account for the outflow of rivers flowing into the sea.

Advances in the eighteenth century included the Bernoulli equation and piezometer by Daniel Bernoulli, the Pitot tube, and the Chezy formula. The nineteenth 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 twentieth 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 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 the advent of computers.

Hydrologic cycle and transport

The central theme of hydrology is that water moves throughout the Earth by different pathways and at different rates. The most striking image of this is in the evaporation of water from the ocean, to form 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 into the atmosphere or eventually flows back to the ocean, completing a cycle.

Moreover, water movement is a significant means by which other material, such as soil or pollutants, is transported from place to place. The initial input in 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.

Branches of hydrology

  • Chemical hydrology is the study of the chemical characteristics of water. It examines how water is affected as it comes into contact with different materials on and below the Earth's surface. This field includes studies on the mechanisms by which salts are transported by such processes as erosion, runoff, evaporation, and precipitation.
  • Ecohydrology is the study of ecological processes in the hydrologic cycle. As these processes occur in the soil and plant foliage, ecohydrologists study how the hydrologic system affects plant physiology, soil moisture, and plant diversity and spatial orientation in various regions over a period of time. Ecohydrology has four main components: infiltration of precipitation into the soil, evapotranspiration, leakage of water into deeper portions of the soil not accessible to the plant, and runoff from the ground surface.
  • Hydrogeology (or geohydrology) is the study of the distribution and movement of water in aquifers and shallow porous media—that is, the porous layers of rock, sand, silt, and gravel below the Earth's surface. Hydrogeology examines the rate of diffusion of water through these media as the water moves down its energy gradient. The flow of water in the shallow subsurface is also pertinent to the fields of soil science, agriculture, and civil engineering. The flow of water and other fluids (hydrocarbons and geothermal fluids) in deeper formations is relevant to the fields of geology, geophysics, and petroleum geology.[1]
  • Hydroinformatics is the adaptation of information technology to hydrology and water resources applications. Its purpose is to facilitate decision-making for many critical applications. Hydrological data are collected, stored, processed, and analyzed using modeling techniques and simulations, based on the knowledge of particular systems. Three common types of hydrological data collected are: the rate of flow of major rivers and streams, precipitation, and water height in wells.[2]
  • Hydrometeorology is the study of the transfer of water and energy between land and water body surfaces and the lower atmosphere. Hydrometeorology incorporates meteorology to solve hydrological problems. These problems include forcasting flood or drought, or determining water resources and the safety of dams. Hydrometeorologists try to determine, through empirical data or theory, how the dynamics of water in the atmosphere affect the greatest levels of precipitation reaching the ground. The domain of hydrometeorology in the physical sciences is not very clearly defined, as it involves cloud physics, climatology, weather forecasting, and hydrology, to name a few.[3][4]
  • Hydromorphology is the study of the physical characteristics of bodies of water on the Earth's surface, including river basins, channels, streams, and lakes. Water quality, levels of pollution, and biological components needed for ecological system maintenance are a few areas assessed when classifying water systems. Hydromorphology studies the dynamics of groundwater flow into channels, lakes, and streams. It measures flow patterns and geometry as well as routing flows to avoid flooding or drought.[5]
  • Isotope hydrology is the study of the isotopic signatures of water. This subfield of hydrology utilizes isotopic dating to determine the origin and age of water throughout its movement within the hydrologic cycle. Isotopic dating involves measuring the levels of deviation in the isotopes of oxygen and hydrogen in water. Researchers are able to determine groundwater dated as far back as the Ice Age by using these techniques. Isotope hydrology deals with water usage policy, mapping aquifers, conservation of water resources, and maintaining pollution levels. One way isotopic hydrology is applied today is in the mitigation of arsenic levels in the drinking water of Bangladesh.
  • Surface-water hydrology is the study of bodies of water on or near the Earth's surface. Rivers, dams, lakes, and reservoirs are all part of this area of study, which further includes the systems used in recreational activity and transportation. Surface hydrology addresses issues pertaining to eroding soils and streams due to surface flow. Flooding, nutrient runoff, and pollutants are a few of the effects addressed, as well as the destruction of civil constructions such as dams. Methods of hydraulic and hydrologic design regulation are also undertaken in this field of study, as researchers simulate the long and short-term effects of anthropogenically manipulated surface water forms.[6]

Related fields

  • Aquatic chemistry: Aquatic chemistry studies chemical reactions in aqueous solutions, including acid-base reactions, oxidation-reduction reactions, precipitation reactions, and dissolution reactions. It can be applied to addressing issues on water pollution and treatment and creating sustainable methods of production with little environmental impact.
  • Civil engineering: Related to the study of hydrology, civil engineers contribute to the planning, design, construction, and maintenance of structures associated with hydraulics. For instance, the engineers are involved in controlling water flow by way of draining swamps, municipal sewage disposal, flood control, and irrigation. They also work in creating structures that help route water flow in dams and bridges.[7]
  • Climatology: Climatology is the study of climate, which is scientifically defined as weather conditions averaged over a period of time. It is a branch of the atmospheric sciences. Average precipitation and temperature trends are measured over specific regions.
  • Environmental engineering: Environmental engineering combines science and engineering principles to address ways in which to improve the quality of air, land, and water for living organisms. Chemical, biological, and geological sciences are incorporated into the techniques of mechanical, civil, and chemical engineering to address issues of public health and policy. Remediation of polluted sites, sanitary engineering, and waste reduction and prevention are keys areas of concern.[8]
  • Physical Geography: Physical geography deals with topics concerning the Earth's surface, including glaciers, landforms, rivers and oceans, climate, and hydrological processes driven by the Sun. It involves the systematic study of patterns in the biosphere, lithosphere, hydrosphere, and atmosphere.
  • Geomorphology: Geomorphology is the study of landforms, including their origin and evolution, and the processes that shape them. A combination of field observation, physical experiment, and numerical modeling help geomorphologists to understand landform history and dynamics, and predict future changes. Applications of geomorphology include landslide prediction and mitigation, river control and restoration, coastal protection, and assessing the presence of water on Mars.
  • Hydraulic engineering: Hydraulic engineering is a subdiscipline of civil engineering that focuses on the flow and conveyance of fluids, particularly water. It involves the design and construction of hydraulic structures such as bridges, dams, canals, channels, and levees, and also aligns itself with the goals of sanitary and environmental engineering.
  • Limnology: Limnology involves the study of inland waters, both saline and fresh. Specifically, it is the study of lakes, ponds, and rivers (natural and manmade), including their biological, physical, chemical, and hydrological aspects.
  • Oceanography: Oceanography is the study of the Earth's seas and oceans. It includes the geological movement of tectonic plates under the Earth's surface, physical oceanographic characteristics, chemical processes, and marine biological processes taking place in these bodies of water.

Hydrologic measurements

The movement of water through the Earth can be measured in a number of ways. This information is important for both assessing water resources and understanding the processes involved in the hydrologic cycle. The following is a list of devices used by hydrologists and what they are used to measure.

  • Disdrometer - precipitation characteristics
  • Symon's evaporation pan - evaporation
  • Infiltrometer - infiltration
  • Piezometer - groundwater pressure and, by inference, groundwater depth
  • Radar - cloud properties
  • Rain gauge - rain and snowfall
  • Satellite - topographic patterns of surface water
  • Sling psychrometer - humidity
  • Stream gauge - stream flow
  • Tensiometer - soil moisture
  • Time domain reflectometer - soil moisture

Hydrologic prediction

Observations of hydrologic processes are used to make predictions of future water movement and quantity.

Statistical hydrology

By analyzing the statistical properties of hydrologic records, such as rainfall or river flow, hydrologists can estimate future hydrologic phenomena. This approach, however, assumes that the characteristics of the processes remain unchanged.

These estimates are important for engineers and economists so that they can perform proper risk analysis for future decisions in infrastructure and to determine the yield reliability characteristics of water supply systems. Statistical information is utilized to formulate operating rules for large dams that are part of systems set up to meet agricultural, industrial, and residential demands.

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 use mathematical and statistical concepts to link a certain input (such as rainfall) to the model output (such as runoff). These models are known as stochastic hydrology models.
  • Models based on process descriptions: These models try to represent the physical processes observed in the real world. Typically, they 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.

Applications of hydrology

  • Mitigating and predicting the risk of flood, landslide, and drought.
  • Designing irrigation schemes and managing agricultural productivity.
  • 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.


  • Anderson, Malcom G., and Jeffrey J. McDonnell Encyclopedia of Hydrological Sciences. Wiley, 2005. ISBN 0-471-49103-9
  • Maidment, David R. Handbook of Hydrology. McGraw Hill, 1993. ISBN 0-07-039732-5
  • Viessman, Warren, and Gary L. Lewis. Introduction to Hydrology, 5 ed. Prentice Hall, 2002. ISBN 0-673-99337-X


  1. Todd, David Keith. Groundwater Hydrology, 2nd ed. New York: John Wiley & Sons, 1980.
  2. Soh, Leen-kiat et al. "A Task-Based Approach to User Interface Design for a Web-Based Hydrologic Information Systems." Transactions in GIS, vol. 10 (3). Blackwell Synergy, May 2006.
  3. Bruce, J.P. et al. Introduction to Hydrometeorology. London: Pergamon Press, 1966.
  4. Hydrometeorology.com Retrieved September 25, 2007.
  5. Ward, Andy D., and William J. Elliot. Environmental Hydrology. Boca Raton: Lewis Publishers, 1995.
  6. Ward, Andy D., and William J. Elliot. Environmental Hydrology, Boca Raton: Lewis Publishers, 1995.
  7. Engineering Terms Retrieved September 25, 2007.
  8. Environmental Engineering Retrieved September 25, 2007.

External links

All links retrieved March 29, 2014.

General subfields within the earth sciences
Atmospheric sciences | Geodesy | Geology | Geophysics | Glaciology
Hydrology | Oceanography | Soil science
Environmental science
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Related fields: Biology | Chemistry | Environmental design | Environmental economics | Environmental ethics | Environmental law | Physics |

Sustainability | Waste management

Environmental technology

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