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Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability. A practitioner of hydrology is a hydrologist, working within the fields of 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 within those fields.
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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).
- 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.
- Drainage basin management covers water-storage, in the form of reservoirs, and flood-protection.
- Water quality includes the chemistry of water in rivers and lakes, both of pollutants and natural solutes.
Related topics 
- Oceanography is the more general study of water in the oceans and estuaries.
- Meteorology is the more general study of the atmosphere and of weather, including precipitation as snow and rainfall.
- Limnology is the study of lakes. It covers the biological, chemical, physical, geological, and other attributes of all inland waters (running and standing waters, both fresh and saline, natural or man-made).
- Water resources are sources of water that are useful or potentially useful. Hydrology studies the availability of those resources, but usually not their uses.
- 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 geomorphologic 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.
The central theme of hydrology is that water circulates 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. Water changes its state of being several times throughout this cycle.
The areas of research within hydrology concern the movement of water between its various states, or within a given state, or simply quantifying the amounts in these states in a given region. Parts of hydrology concern developing methods for directly measuring these flows or amounts of water, while others concern modelling these processes either for scientific knowledge or for making prediction in practical applications.
Hydrology (hydrogeology) considers quantifying groundwater flow and solute transport. Problems in describing the satuatated zone include the characterization of aquifers in terms of flow direction, groundwater pressure and, by inference, groundwater depth (see: aquifer test). Measurements here can be made using a piezometer. Aquifers are also described in terms of conductivity, storativity and transmisivity. There are a number of geophysical methods for characterising aquifers. There are also problems in characterising the vadose zone (unsaturated zone).
The infiltration of water from precipitation into the soil is an important topic. In some circumstances a dry soil may not absorb rainfall as readily as a soil that is already wet. Infiltration can sometimes be measured by an infiltrometer.
Soil moisture 
Surface water flow 
Hydrology considers quantifying surface water flow and solute transport, although the treatment of flows in large rivers is sometimes considered as a distinct topic of hydraulics or hydrodynamics. Surface water flow can include flow both in recognizable river channels and otherwise. Methods for measuring flow once water has reached a river include the stream gauge (see: discharge (hydrology)), and tracer techniques. Other topics include chemical transport as part of surface water, sediment transport and erosion.
One of the important areas of hydrology is the interchange between rivers and aquifers (stream-aquifer exchange). While in many geographical regions it is natural to think only of water moving out of aquifers into rivers, the reverse can also happen.
Precipitation and evaporation 
In some considerations, hydrology is thought of as starting at the land-atmosphere boundary and so it is important to have adequate knowledge of the both precipitation and evaporation. Precipitation can be measured in various ways: disdrometer for precipitation characteristics at a fine time scale; radar for cloud properties, rain rate estimation, hail and snow detection; Rain gauge for routine accurate measurments of rain and snowfall; satellite - rainy area identification, rain rate estimation, land-cover/land-use, soil moisture.
Evaporation is an important part of the water cycle. It is partly affected by humidity, which can be measured by a sling psychrometer. It is also affected by the presence of snow, hail and ice and can relate to dew, mist and fog. Hydrology considers evaporation of various forms: from water surfaces; as transpiration from plant surfaces in natural and agronomic ecosystems. A direct measurement of evaporation can be obtained using Symon's evaporation pan.
Detailed studies of evaporation involve boundary layer considerations as well as momentum, heat flux and energy budgets.
Uncertainty analyses 
Remote sensing 
Remote sensing of hydrologic processes can provide information of various types.. Sources include land based sensors, airborne sensors and satellite sensors. Information can include clouds, surface moisture, vegetation cover.
Water quality 
In hydrology, studies of water quality concern organic and inorganic compounds, and both dissolved and sediment material. In addition, water quality is affected by the interaction of dissolved oxygen with organic material and various chemical transformations that may take place. Measurements of water quality may involve either in-situ methods, in which analyses take place on-site, often automatically, and laboratory-based analyses and may include microbiological analysis.
Integrating measurement and modelling 
- Budget analyses
- Parameter estimation
- Scaling in time and space
- Data assimilation
- Quality control of data — see for example Double mass analysis
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 "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. When making assessments of how often relatively rare events will occur, analyses are made in terms of the return period of such events. Other quantities of interest include the average flow in a river, in a year or by season.
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.
Hydrological models are simplified, conceptual representations of a part of the hydrologic cycle. They are primarily used for hydrological prediction and for understanding hydrological processes. Two major types of hydrological 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 models may be linear models, but it 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 hydrological 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.
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.
International research bodies 
National research bodies 
- Centre for Ecology and Hydrology – UK 
- Centre for Water Science, Cranfield University, UK 
- eawag – aquatic research, ETH Zürich, Switzerland 
- Institute of Hydrology, Albert-Ludwigs-University of Freiburg, Germany
- United States Geological Survey – Water Resources of the United States
- NOAA's National Weather Service – Office of Hydrologic Development, USA
- US Army Corps of Engineers Hydrologic Engineering Center, USA
- Hydrologic Research Center, USA
- NOAA Economics and Social Sciences, USA
- University of Oklahoma Center for Natural Hazards and Disasters Research, USA 
- National Hydrology Research Centre, Canada
National and international societies 
- Geological Society of America (GSA) – Hydrogeology Division
- American Geophysical Union (AGU) – Hydrology Section
- National Ground Water Association (NGWA)
- American Water Resources Association
- Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHSI)
- International Association of Hydrological Sciences (IAHS)
- Italian Hydrological Society (SII-IHS) - http://www.sii-ihs.it
- Statistics in Hydrology Working Group (subgroup of IAHS)
- British Hydrological Society 
- Russian Geographical Society (Moscow Centre) – Hydrology Commission
- International Association for Environmental Hydrology
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
Research journals 
- 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
See also 
- Aquatic chemistry
- Civil engineering
- Environmental engineering
- Environmental engineering science
- Green Kenue a software tool for hydrologic modellers
- International Hydrological Programme
- Hydrology (agriculture)
- Water distribution on Earth
- Nash–Sutcliffe model efficiency coefficient
- WEAP (Water Evaluation And Planning) software to model catchment hydrology from climate and land use data
- Wetzel, R.G. (2001) Limnology: Lake and River Ecosystems, 3rd ed. Academic Press. ISBN 0-12-744760-1
- Vereecken, H.; Kemna, A.; Münch, H. M.; Tillmann, A.; Verweerd, A. (2006). "Aquifer Characterization by Geophysical Methods". Encyclopedia of Hydrological Sciences. John Wiley & Sons. doi:10.1002/0470848944.hsa154b. ISBN 0471491039.
- Wilson, L. Gray; Everett, Lorne G.; Cullen, Stephen J. (1994). Handbook of Vadose Zone Characterization & Monitoring. CRC Press. ISBN 978-0873716109.
- "International Water Management Institute (IWMI)". IWMI. Retrieved 8 March 2013.
- "UNESCO-IHE Institute for Water Education". UNIESCO-IHE. Retrieved 8 March 2013.
- "CEH Website". Centre for Ecology & Hydrology. Retrieved 8 March 2013.
- "Cranfield Water Science Institute". Cranfield University. Retrieved 8 March 2013.
- "Eawag aquatic research". Swiss Federal Institute of Aquatic Science and Technology. 2012-01-25. Retrieved 8 March 2013.
- "Professur für Hydrologie". University of Freiburg. 2010-02-23. Retrieved 8 March 2013.
- "Water Resources of the United States". USGS. 2011-10-04. Retrieved 8 March 2013.
- "Office of Hydrologic Development". National Weather Service. NOAA. 2011-10-28. Retrieved 8 March 2013.
- "Hydrologic Engineering Center". US Army Corps of Engineers. Retrieved 8 March 2013.
- "Hydrologic Research Center". Hydrologic Research Center. Retrieved 8 March 2013.
- "NOAA Economics and Social Sciences". NOAA Office of Program Planning and Integration. Retrieved 8 March 2013.
- "Center for Natural Hazard and Disasters Research". University of Oklahoma. 2008-06-17. Retrieved 8 March 2013.
- "National Hydrology Research Centre (Saskatoon, SK)". Environmental Science Centres. Environment Canada. Retrieved 8 March 2013.
- "Hydrogeology Division". The Geological Society of America. 2011-09-10. Retrieved 8 March 2013.
- "Welcome to AGU's Hydrology (H) Section". American Geophysical Union. Retrieved 8 March 2013.
- "National Ground Water Association". Retrieved 8 March 2013.
- "American Water Resources Association". 2012-01-02. Retrieved 8 March 2013.
- "CUAHSI". Retrieved 8 March 2013.
- "International Association of Hydrological Sciences (IAHS)". Associations. International Union of Geodesy and Geophysics. 2008-12-01. Retrieved 8 March 2013.
- "International Association of Hydrological Sciences". Retrieved 8 March 2013.
- "International Commission on Statistical Hydrology". STAHY. Retrieved 8 March 2013.
- "The British Hydrological Society". Retrieved 8 March 2013.
- "Гидрологическая комиссия" [Hydrological Commission] (in Russian). Russian Geographical Society. Retrieved 8 March 2013.
- "Hydroweb". The International Association for Environmental Hydrology. Retrieved 8 March 2013.
- "Connected Waters Initiative (CWI)". University of New South Wales. Retrieved 8 March 2013.
Further reading 
- Anderson, Malcom G.; McDonnell, Jeffrey J., eds. (2005). Encyclopedia of hydrological sciences. Hoboken, NJ: Wiley. ISBN 0-471-49103-9.
- Hendriks, Martin R. (2010). Introduction to physical hydrology. Oxford: Oxford University Press. ISBN 9780199296842.
- Maidment, David R., ed. (1993). Handbook of hydrology. New York: McGraw-Hill. ISBN 0-07-039732-5.
- McCuen, Richard H. (2005). Hydrologic analysis and design (3rd ed.). Upper Saddle River, N.J.: Pearson-Prentice Hall. ISBN 0-13-142424-6.
- Viessman, Jr., Warren; Gary L. Lewis (2003). Introduction to hydrology (5th ed.). Upper Saddle River, N.J.: Pearson Education. ISBN 0-673-99337-X.
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