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Water covers 70% of the Earth's
surface.
Hydrology (from Greek: |Ὓδωρ,
hydōr,
"water"; and λόγος,
logos, "study") is the study of the
movement, distribution, and quality of
water
throughout 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,
geology or
civil
and
environmental
engineering.
Domains of hydrology include
hydrometeorology,
surface hydrology,
hydrogeology,
drainage basin management and
water chemistry, 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 as it allows us to better
understand the world in which we live, and also provides insight
for
environmental
engineering,
policy and
planning.
History of hydrology
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).
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 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.
Related fields
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
- Direct and indirect discharge measurements
- Stream gauge - stream flow (see:
discharge )
- Tracer techniques
- Chemical transport
- Sediment transport and erosion
- Stream-aquifer exchange
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
- Transpiration
- Natural ecosystems
- Agronomic ecosystems
- Momentum
- Heat flux
Uncertainty analyses
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
- Quality control of data — see for example Double mass analysis
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, assuming 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, and system identification. The simplest of
these model models are may be linear models, but 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 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
- 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.
Applications of hydrology
See also
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
- Hydrology
Research , 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
External links
Other on-line resources
National and international research bodies
National and international societies
Basin- and catchment-wide overviews