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Hydrology (from
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
ὕδωρ, ''hýdōr'' meaning "water" and
λόγος ''Logos'' (, ; grc, wikt:λόγος, λόγος, lógos; from , , ) is a term in Western philosophy, psychology, rhetoric, and religion derived from a Greek word variously meaning "ground", "plea", "opinion", "expectation", "word", "speech", " ...
, ''lógos'' meaning "study") is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships ...

water cycle
,
water resources Water resources are natural resources of water that are potentially useful as a source of water supply. 97% of the water on the Earth is salt water and only three percent is fresh water; slightly over two thirds of this is frozen in glaciers a ...
, and environmental watershed sustainability. A practitioner of hydrology is called a hydrologist. Hydrologists are scientists studying
earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...
or
environmental science Environmental science is an interdisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of two or more academic disciplines into one activity (e.g., a research project). It draws knowledge from several oth ...
,
civil Civil may refer to: *Civic virtue, or civility *Civil action, or lawsuit *Civil affairs *Civil and political rights *Civil disobedience *Civil engineering *Civil (journalism), a platform for independent journalism *Civilian, someone not a member ...
or
environmental engineering Environmental engineering is a professional engineering discipline Discipline is action ACTION is a bus operator in Canberra Canberra ( ) is the capital city of Australia. Founded following the Federation of Australia, federation o ...
, and
physical geography Physical geography (also known as physiography) is one of the two fields of geography Geography (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hell ...
. Using various analytical methods and scientific techniques, they collect and analyze data to help solve water related problems such as
environmental preservation Environmentalism or environmental rights is a broad philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about Metaphysics, existence, reason, Epistemology, knowledge, Ethics, values, Philos ...
,
natural disaster A natural disaster is a major adverse event An adverse event (AE) is any untoward medical occurrence in a patient or clinical investigation subject administered a pharmaceutical product and which does not necessarily have a causal relations ...
s, and
water management Water resource management is the activity of planning, developing, distributing and managing the optimum use of water resources. It is an aspect of water cycle management. Water is essential for our survival. The field of water resources manageme ...
. Hydrology subdivides into surface water hydrology, groundwater hydrology (hydrogeology), and marine hydrology. Domains of hydrology include
hydrometeorology Hydrometeorology is a branch of meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, ...
, surface hydrology,
hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is lan ...
, management, and
water quality Water quality refers to the chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touche ...

water quality
, where water plays the central role.
Oceanography Oceanography (from the Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period ...
and
meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw modest progress in the f ...
are not included because water is only one of many important aspects within those fields. Hydrological research can inform environmental engineering,
policy Policy is a deliberate system of guideline A guideline is a statement by which to determine a course of action. A guideline aims to streamline particular processes according to a set routine or sound practice. Guidelines may be issued by a ...
, and
planning Planning is the process A process is a series or set of Action (philosophy), activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business pro ...
.


Branches

*
Chemical hydrology Water quality refers to the chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements b ...
is the study of the chemical characteristics of water. *
Ecohydrology Ecohydrology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxim ...
is the study of interactions between organisms and the hydrologic cycle. *
Hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is lan ...
is the study of the presence and movement of groundwater. *
Hydrogeochemistry Geochemistry is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanatio ...
is the study of how terrestrial water dissolves minerals
weathering Weathering is the deterioration of Rock (geology), rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little o ...
and this effect on water chemistry. *
HydroinformaticsHydroinformatics is a branch of informatics which concentrates on the application of information and communications technologies (ICTs) in addressing the increasingly serious problems of the equitable and efficient use of water Water is a ...
is the adaptation of information technology to hydrology and water resources applications. *
Hydrometeorology Hydrometeorology is a branch of meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, ...
is the study of the transfer of water and energy between land and water body surfaces and the lower atmosphere. *
Isotope hydrology Isotope hydrology is a field of geochemistry and hydrology Hydrology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a cou ...
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 A drainage basin is any area of land where precipitation collects and drains off into a common outlet, such as into a river, bay, or other body of water. The drainage basin includes all the surface water from surface runoff, rain runoff, snowm ...

Drainage basin
management covers water storage, in the form of reservoirs, and floods protection. *
Water quality Water quality refers to the chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touche ...

Water quality
includes the chemistry of water in rivers and lakes, both of pollutants and natural solutes.


Applications

* Calculation of
rainfall Rain is liquid water in the form of droplet Rain water flux from a canopy. Among the forces that govern drop formation: cohesion, Van der Waals force">Cohesion_(chemistry).html" ;"title="surface tension, Cohesion (chemistry)">cohesion, ...

rainfall
. * Calculating surface runoff and precipitation. * Determining the
water balance The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. In hydrology, a water balance equation can be used to describe the flow of water in and out of a ...

water balance
of a region. * Determining the agricultural water balance. * Designing riparian restoration projects. * Mitigating and predicting
flood A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide Tides are the rise and fall of sea level Mean sea level (MSL) (often ...

flood
,
landslide Landslides, also known as landslips, are several forms of mass wasting Mass wasting, also known as mass movement, is a general term for the movement of rock Rock most often refers to: * Rock (geology) A rock is any naturally occurri ...

landslide
and drought risk. * Real-time flood forecasting and
flood warning A flood warning is closely linked to the task of flood forecasting. The distinction between the two is that the outcome of flood forecasting is a set of forecast time-profiles of channel flows or river levels at various locations, while "flood war ...
. * Designing
irrigation Irrigation is the agricultural Agriculture is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in seden ...

irrigation
schemes and managing agricultural productivity. * Part of the hazard module in
catastrophe modeling:''This article refers to the use of computers to estimate losses caused by disasters. For other meanings of the word catastrophe, including catastrophe theory In mathematics, catastrophe theory is a branch of bifurcation theory in the study of d ...
. * Providing
drinking water Drinking water, also known as potable water, is water that is safe to drinking, drink or use for food preparation. The amount of drinking water required to maintain good health varies, and depends on physical activity level, age, health-related ...

drinking water
. * Designing
dams A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, tap water, human consumption, Indus ...
for
water supply Water supply is the provision of water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for ...
or
hydroelectric power Hydroelectricity, or hydroelectric power, is electricity produced from hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to produce electricity or to power machin ...
generation. * Designing bridges. * Designing sewers and urban drainage system. * Analyzing the impacts of
antecedent moisture{{Refimprove, date=November 2008 In hydrology Hydrology (from Greek: wikt:ὕδωρ, ὕδωρ, "hýdōr" meaning "water" and wikt:λόγος, λόγος, "lógos" meaning "study") is the scientific study of the movement, distribution, and manage ...
on sanitary sewer systems. * Predicting changes, such as
erosion In earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific ...

erosion
or
sedimentation Sedimentation is the deposition of sediments Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks In geology Geology (from the Ancient Greek γ ...
. * Assessing the impacts of natural and anthropogenic environmental change on
water resources Water resources are natural resources of water that are potentially useful as a source of water supply. 97% of the water on the Earth is salt water and only three percent is fresh water; slightly over two thirds of this is frozen in glaciers a ...
. * Assessing
contaminant Contamination is the presence of a constituent, impurity Impurities are chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by ...

contaminant
transport risk and establishing environmental policy guidelines. * Estimating the water resource potential of river basins.


History

Hydrology has been a subject of investigation and engineering for millennia. For example, about 4000 BC the Nile was dammed to improve agricultural productivity of previously barren lands.
Mesopotamia Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in th ...

Mesopotamia
n towns were protected from flooding with high earthen walls.
Aqueducts Aqueduct may refer to: Bridges *Aqueduct (bridge) Aqueduct of Vanvitelli, Italy, built by Luigi Vanvitelli. It is a World Heritage Site and one of the finest examples of an aqueduct in Europe. Aqueducts or water bridges are bridges constructe ...
were built by the
Greeks The Greeks or Hellenes (; el, Έλληνες, ''Éllines'' ) are an ethnic group An ethnic group or ethnicity is a grouping of people A people is any plurality of person A person (plural people or persons) is a being that has cer ...
and
Romans Roman or Romans usually refers to: *Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, ...
, while
history History (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxima ...

history
shows that the Chinese built irrigation and flood control works. The ancient
Sinhalese Sinhala may refer to: * Something of or related to the Sinhalese people of Sri Lanka * Sinhalese people * Sinhala language, one of the three official languages used in Sri Lanka * Sinhala script, a writing system for the Sinhala language ** Sinhala ...
used hydrology to build complex irrigation works in
Sri Lanka Sri Lanka (, ; si, ශ්‍රී ලංකාව, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon, and officially the Democratic Socialist Republic of Sri Lanka, is ...

Sri Lanka
, also known for the invention of the Valve Pit which allowed construction of large reservoirs,
anicut An anicut (Originated from Tamil Tamil may refer to: * Tamils, an ethnic group native to India, Sri Lanka and some other parts of Asia **Sri Lankan Tamils, Tamil people native to Sri Lanka **Tamil Malaysians, Tamil people native to Malaysia * Tam ...
s and canals which still function. , in the first century BC, 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 the adoption of a more scientific approach,
Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian of the who was active as a painter, , engineer, scientist, theorist, sculptor and architect. While his fame initially rested on his achievements as a painter, he als ...

Leonardo da Vinci
and
Bernard Palissy Bernard Palissy (c. 1510c. 1589) was a Huguenot, French Huguenot pottery, potter, Hydraulics, hydraulics engineer and craftsman, famous for having struggled for sixteen years to imitate Chinese porcelain. He is best known for his so-called "rustic ...

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 Pierre Perrault (29 June 1927 – 24 June 1999) was a Québécois documentary film director A film director controls a film's artistic and dramatic aspects and visualizes the screenplay (or script) while guiding the technical crew and acto ...
,
Edme Mariotte Edme Mariotte (; ; c. 162012 May 1684) was a French physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of interest. In classi ...

Edme Mariotte
and
Edmund Halley Edmond (or Edmund) Halley (; – ) was an English astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such ...

Edmund Halley
. By measuring rainfall, runoff, and drainage area, Perrault showed that rainfall was sufficient to account for the flow of the Seine. combined velocity and river cross-section measurements to obtain a discharge value, again in the Seine. Halley showed that the evaporation from the
Mediterranean Sea The Mediterranean Sea is a connected to the , surrounded by the and almost completely enclosed by land: on the north by and and , on the south by , and on the east by the . The Sea has played a central role in the . Although the Mediterrane ...
was sufficient to account for the outflow of rivers flowing into the sea. Advances in the 18th century included the
BernoulliBernoulli can refer to: People *Bernoulli family of 17th and 18th century Swiss mathematicians: ** Daniel Bernoulli (1700–1782), developer of Bernoulli's principle ** Jacob Bernoulli (1654–1705), also known as Jacques, after whom Bernoulli numbe ...
piezometer A piezometer is either a device used to measure liquid pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ...
and
Bernoulli's equation Video of a venturi meter used in a lab experiment In fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, inc ...
, by
Daniel Bernoulli Daniel Bernoulli Fellows of the Royal Society, FRS (; – 27 March 1782) was a Swiss people, Swiss mathematician and physicist and was one of the many prominent mathematicians in the Bernoulli family from Basel. He is particularly remembered for ...
, and the
Pitot tube , combines a pitot tube (right) with a static port and an angle-of-attack vane (left). Air-flow is right to left. helicopter A pitot ( ) tube, also known as pitot probe, is a flow measurement Flow measurement is the quantification of bulk ...

Pitot tube
, by
Henri Pitot Henri Pitot (; May 3, 1695 – December 27, 1771) was a France, French hydraulic engineer and the inventor of the pitot tube. In a pitot tube, the height of the fluid column is proportional to the square of the velocity of the fluid at the depth ...
. The 19th century saw development in groundwater hydrology, including
Darcy's law Darcy's law is an equation that describes the flow of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), phase of m ...
, the Dupuit-Thiem well formula, and Hagen-
Poiseuille The poiseuille (symbol Pl) has been proposed as a derived SI unit of dynamic viscosity, named after the French physicist Jean Léonard Marie Poiseuille (1797–1869). In practice the unit has never been widely accepted and most international stan ...

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 , the infiltration theory of Robert E. Horton, and C.V. Theis' 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 A geographic information system (GIS) is a type of database containing Geographic data and information, geographic data (that is, descriptions of phenomena for which location is relevant), combined with Geographic information system software, sof ...
(GIS). (See also GIS and hydrology)


Themes

The central theme of hydrology is that water circulates throughout the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

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 modeling these processes either for scientific knowledge or for making a prediction in practical applications.


Groundwater

Ground water is water beneath Earth's surface, often pumped for drinking water. Groundwater hydrology (
hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is lan ...
) considers quantifying groundwater flow and solute transport. Problems in describing the saturated zone include the characterization of aquifers in terms of flow direction, groundwater pressure and, by inference, groundwater depth (see:
aquifer testAn aquifer test (or a pumping test) is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" (drawdown (hydrology), drawdown) in observation Water well, wells. Aquifer testing ...
). Measurements here can be made using a
piezometer A piezometer is either a device used to measure liquid pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ...
. Aquifers are also described in terms of hydraulic conductivity, storativity and transmissivity. There are a number of geophysical methods for characterizing aquifers. There are also problems in characterizing the vadose zone (unsaturated zone).


Infiltration

Infiltration is the process by which water enters the soil. Some of the water is absorbed, and the rest down to the
water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. Th ...

water table
. The infiltration capacity, the maximum rate at which the soil can absorb water, depends on several factors. The layer that is already saturated provides a resistance that is proportional to its thickness, while that plus the depth of water above the soil provides the driving force (
hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum A vertical datum, altimetric datum, or height datum is a reference surface for vertical position Vertical position or vertical location ...

hydraulic head
). Dry soil can allow rapid infiltration by
capillary action Capillary action (sometimes capillarity, capillary motion, capillary effect, or wicking) is the ability of a liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the ...
; this force diminishes as the soil becomes wet.
Compaction Compaction may refer to: * Soil compaction In geotechnical engineering#REDIRECT geotechnical engineering {{Redirect category shell, 1= {{R from other capitalisation ..., soil compaction is the process in which stress applied to a soil causes ...
reduces the porosity and the pore sizes. Surface cover increases capacity by retarding runoff, reducing compaction and other processes. Higher temperatures reduce
viscosity The viscosity of a fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...

viscosity
, increasing infiltration.


Soil moisture

Soil moisture can be measured in various ways; by
capacitance probeCapacitance sensors (or Dielectric sensors) use capacitance Capacitance is the ratio of the amount of electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic fie ...
,
time domain reflectometerA time-domain reflectometer (TDR) is an electronic instrument used to determine the characteristics of Transmission line, electrical lines by observing Reflection (electrical), reflected waveforms. It can be used to characterize and locate faults in ...
or
Tensiometer Tensiometer may refer to one of a number of devices. The two most common are: *Tensiometer (surface tension) an instrument used to measure the surface tension of liquids *Tensiometer (soil science) an instrument to determine matric water potentia ...
. Other methods include solute sampling and geophysical methods.


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 the water has reached a river include the
stream gauge A stream gauge, streamgage or stream gauging station is a location used by hydrologists Hydrology (from Greek: ὕδωρ, "hýdōr" meaning "water" and wikt:λόγος, λόγος, "lógos" meaning "study") is the scientific study of the move ...
(see: discharge), 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. Groundwater/surface water interactions in streams and aquifers can be complex and the direction of net water flux (into surface water or into the aquifer) may vary spatially along a stream channel and over time at any particular location, depending on the relationship between stream stage and groundwater levels.


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 both precipitation and evaporation. Precipitation can be measured in various ways:
disdrometer A disdrometer is an instrument used to measure the Raindrop size distribution, drop size distribution and velocity of falling hydrometeors. Some disdrometers can distinguish between rain, graupel, and hail. The uses for disdrometers are numerou ...

disdrometer
for precipitation characteristics at a fine time scale;
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

radar
for cloud properties, rain rate estimation, hail and snow detection;
rain gauge A rain gauge (also known as an udometer,pluvia metior, pluviometer, ombrometer, and hyetometer) is an instrument used by meteorologist A meteorologist is a scientist who studies and works in the field of meteorology Meteorology is a branch of ...
for routine accurate measurements of rain and snowfall;
satellite In the context of spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either human spaceflight, with or uncrewed spaceflight, without humans on board. Most spaceflight ...

satellite
for rainy area identification, rain rate estimation, land-cover/land-use, and soil moisture, for example.
Evaporation Evaporation is a type of vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phe ...

Evaporation
is an important part of the water cycle. It is partly affected by humidity, which can be measured by a
sling psychrometer image:Haar-Hygrometer.jpg, A hair tension dial hygrometer with a nonlinear scale. A hygrometer is an instrument used to measure the amount of water vapor in air, in soil, or in confined spaces. Humidity measurement instruments usually rely on mea ...

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. Direct measurement of evaporation can be obtained using Simon's . Detailed studies of evaporation involve boundary layer considerations as well as momentum, heat flux, and energy budgets.


Remote sensing

Remote sensing of hydrologic processes can provide information on locations where ''in situ'' sensors may be unavailable or sparse. It also enables observations over large spatial extents. Many of the variables constituting the terrestrial water balance, for example
surface water Surface water is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of ...
storage,
soil moisture Soil moisture is the water content 300px, Soil composition by Volume and Mass, by phase: air, water, void (pores filled with water or air), soil, and total. Water content or moisture content is the quantity of water Water is an Inorganic ...
,
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...

precipitation
,
evapotranspiration Evapotranspiration (ET) is the sum of water evaporation and transpiration from a surface area to the Atmosphere of Earth, atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and ...

evapotranspiration
, and
snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). ...

snow
and
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

ice
, are measurable using remote sensing at various spatial-temporal resolutions and accuracies. Sources of remote sensing include land-based sensors, airborne sensors and satellite sensors which can capture
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...
, thermal and near-infrared data or use
lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object with a laser A laser is a device that emits light Light or visible light is electromagnetic radiati ...
, for example.


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 Estimation theory is a branch of statistics that deals with estimating the values of Statistical parameter, parameters based on measured empirical data that has a random component. The parameters describe an underlying physical setting in such ...
* Scaling in time and space *
Data assimilation Data assimilation is a mathematical discipline that seeks to optimally combine theory (usually in the form of a numerical model) with observations. There may be a number of different goals sought, for example—to determine the optimal state estim ...
* Quality control of data – see for example Double mass analysis


Prediction

Observations of hydrologic processes are used to make predictions of the future behavior 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 analyzing 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 (business), 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 utilized to formulate operating rules for large dams forming part of systems which include agricultural, industrial and residential demands.


Modeling

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, within the general field of scientific modeling. Two major types of hydrological models can be distinguished: * Models based on data. These models are Black box (systems), black box systems, using mathematical and statistical concepts to link a certain input (for instance rainfall) to the model output (for instance Surface runoff, runoff). Commonly used techniques are Regression analysis, 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 behavior 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 Evapotranspiration (ET) is the sum of water evaporation and transpiration from a surface area to the Atmosphere of Earth, atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and ...

evapotranspiration
, and open channel flow, channel flow, but they can be far more complicated. Within this category, models can be divided into conceptual and deterministic. Conceptual models link simplified representations of the hydrological processes in an area, whereas deterministic models seek to resolve as much of the physics of a system as possible. These 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 behavioral modeling in hydrology, behavior of hydrologic systems to make better predictions and to face the major challenges in water resources management.


Transport

Water movement is a significant means by which other materials, such as soil, gravel, boulders or pollutants, are transported from place to place. Initial input to receiving waters may arise from a point source (pollution), point source discharge or a line source or Area source (pollution), 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.


Organizations


Intergovernmental organizations

* International Hydrological Programme (IHP)


International research bodies

* International Water Management Institute (IWMI) * UN-IHE Delft Institute for Water Education


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 * National Oceanic and Atmospheric Administration, 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, United States * University of Oklahoma Center for Natural Hazards and Disasters Research, USA * National Hydrology Research Centre, Canada * National Institute of Hydrology, India


National and international societies

* American Institute of Hydrology (AIH) *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) * Statistics in Hydrology Working Group (subgroup of IAHS) * German Hydrological Society (DHG: Deutsche Hydrologische Gesellschaft) * Italian Hydrological Society (SII-IHS) – http://www.sii-ihs.it * Nordic Association for Hydrology * British Hydrological Society * Russian Geographical Society (Moscow Center) – Hydrology Commission * International Association for Environmental Hydrology * International Association of Hydrogeologists
Society of Hydrologists and Meteorologists – Nepal


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 * Murray Darling Basin Initiative, Department of Environment and Heritage, Australia


Research journals


International Journal of Hydrology Science and Technology
* ''Hydrological Processes'', (electronic) 0885-6087 (paper), John Wiley & Sons * ''Hydrology Research'', , IWA Publishing (formerly ''Nordic Hydrology'') * ''Journal of Hydroinformatics'', , IWA Publishing * ''Journal of Hydrologic Engineering'', , American Society of Civil Engineers, ASCE Publication * ''Journal of Hydrology'' * ''Water Research'' * ''Water Resources Research'' * ''Hydrological Sciences Journal ''- ''Journal of the International Association of Hydrological Sciences'' ''(IAHS)'' (Print), (Online)


See also

* Aqueous solution * Climatology * Environmental engineering science * Green Kenue a software tool for hydrologic modellers * Hydraulics * Hydrography * Hydrology (agriculture) * International Hydrological Programme * Nash–Sutcliffe model efficiency coefficient * Outline of hydrology * Socio-hydrology * Soil science * Water distribution on Earth * WEAP (Water Evaluation And Planning) software to model catchment hydrology from climate and land use data * Catchment hydrology ; Other water-related fields *
Oceanography Oceanography (from the Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period ...
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, rivers and wetlands ecosystems. 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.


References


Further reading

* Eslamian, S., 2014, (ed.) Handbook of Engineering Hydrology, Vol. 1: Fundamentals and Applications, Francis and Taylor, CRC Group, 636 Pages, USA. * Eslamian, S., 2014, (ed.) Handbook of Engineering Hydrology, Vol. 2: Modeling, Climate Change and Variability, Francis and Taylor, CRC Group, 646 Pages, USA. * Eslamian, S, 2014, (ed.) Handbook of Engineering Hydrology, Vol. 3: Environmental Hydrology and Water Management, Francis and Taylor, CRC Group, 606 Pages, USA. * * * * * *


External links


Hydrology.nl – Portal to international hydrology and water resources

Decision tree to choose an uncertainty method for hydrological and hydraulic modelling

Experimental Hydrology Wiki
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