hydrology

hydrology

[hahy-drol-uh-jee]
hydrology, study of water and its properties, including its distribution and movement in and through the land areas of the earth. The hydrologic cycle consists of the passage of water from the oceans into the atmosphere by evaporation and transpiration (or evapotranspiration), onto the lands, over and under the lands as runoff and infiltration, and back to the oceans. Hydrology is principally concerned with the part of the cycle after the precipitation of water onto the land and before its return to the oceans; thus meteorology and oceanography are closely related to hydrology. Hydrologists study the cycle by measuring such variables as the amount and intensity of precipitation, the amount of water stored as snow or in glaciers, the advance and retreat of glaciers, the rate of flow in streams, and the soil-water balance. Hydrology also includes the study of the amount and flow of groundwater. Though the flow of water cannot be seen under the surface, hydrologists can deduce the flow by understanding the characteristics, including permeability, of the soil and bedrock; how water behaves near other sources of water, such as rivers and oceans; and fluid flow models based on water movements on the earth's surface. Hydrology is also important to the study of water pollution, especially of groundwater and other potable water supplies. Knowledge of hydrology is extensively used to determine the movement and extent of contamination from landfills, mine runoff, and other potentially contaminated sites to surface and subsurface water. See water supply.

Scientific discipline concerned with the waters of the Earth, including their occurrence, distribution, circulation via the hydrologic cycle, and interactions with living things. It also deals with the chemical and physical properties of water in all its phases.

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or hydrochemistry

Subdivision of hydrology that deals with the chemical characteristics of the water on and beneath the surface of the Earth. Water in all forms is affected chemically by the materials with which it comes into contact, and it can dissolve many elements in significant quantities. Chemical hydrology is concerned with the processes involved and thus includes study of phenomena such as the transport of salts from land to sea (by erosion of rocks and surface runoff) and from sea to land (by evaporation, cloud formation, and precipitation) and the age and origin of groundwater in desert regions and of ice sheets and glaciers.

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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.

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).

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

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
  • Quantifying surface water flow and transport
    • 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
    • Transpiration
      • Natural ecosystems
      • Agronomic ecosystems
  • Momentum
  • Heat flux
    • Energy budgets
  • 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

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, 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

''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

External links

Other on-line resources

National and international research bodies

National and international societies

Basin- and catchment-wide overviews

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