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Water covers 70% of the Earth's surface.
Hydrology (from Greek: Yδωρ, hudōr, "water"; and λόγος, logos, "study") is the study of the movement, distribution, and quality of water throughout the 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 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 in that it allows us to better understand the world in which we live, and also provides insight for environmental engineering, policy and planning.
[edit] History of hydrology
Hydrology has been a subject of investigation and engineering for millennia. For example, in 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 1950's, 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).
[edit] Hydrologic cycle
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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.
[edit] 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 water in aquifers.
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 the Earth's surface.
[edit] Related fields
[edit] 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
- Soil moisture
- Quantifying surface water flow and transport
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- Direct and indirect discharge measurements
- 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
- from plant surfaces
- through the boundary layer
- Transpiration
- Natural ecosystems
- Agronomic ecosystems
- Momentum
- Heat flux
- 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
[edit] 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.
[edit] Statistical hydrology
By analysing the statistical properties of hydrologic records, such as rainfall or river flow, hydrologists can estimate future hydrologic phenomena. This, however, assumes 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.
[edit] 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 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.
[edit] 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.
[edit] Applications of hydrology
[edit] See also
[edit] 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
- 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
[edit] External links
[edit] Other on-line resources
[edit] National and international research bodies
[edit] National and international societies
[edit] Basin- and catchment-wide overviews
