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Hydrology

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Water covers 70% of the Earth's surface.
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   Water covers 70% of the Earth's surface.

   Hydrology (from Greek: Yδρoλoγια, Yδωρ+Λoγos, Hydrologia, the "study of
   water") 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.

History

   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 Romans, while the Chinese
   built irrigation and flood control works. The ancient Sinhalese used
   hydrology to build complex Irrigation Works of Ancient Sri Lanka, known
   for inventiion 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, and the
   Chezy formula. 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 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.

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 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.

Related fields

     * Aquatic chemistry
     * Civil engineering
     * Climatology
     * Environmental engineering
     * Geomorphology
     * Hydraulic engineering
     * Limnology
     * Oceanography
     * Physical Geography

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.
   Following is a list of devices used by hydrologists and what they are
   used to measure.
     * Disdrometer - precipitation characteristics
     * Evaporation - Symon's evaporation pan
     * Infiltrometer - infiltration
     * Piezometer - groundwater pressure and, by inference, groundwater
       depth (see: aquifer test)
     * Radar - cloud properties
     * Rain gauge - rain and snowfall
     * Satellite
     * Sling psychrometer - humidity
     * Stream gauge - stream flow (see: discharge (hydrology))
     * Tensiometer - soil moisture
     * Time domain reflectometer - soil moisture

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. 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.

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, neural
       networks and system identification. 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, 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

   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

     * Designing riparian restoration projects.
     * Mitigating and predicting flood, landslide and drought risk;
     * 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.

   Retrieved from " http://en.wikipedia.org/wiki/Hydrology"
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   with only minor checks and changes (see www.wikipedia.org for details
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