Hydrology Water covers 70% of the Earths surface. Hydrologyis the study of the movement, distribution, and quality of wateron Earth and other planets, including the hydrologic cycle, water resourcesand environmental watershed sustainability. A practitioner of hydrology is a hydrologist, working within the fields of earthor environmental science, physical geography, geologyor civiland environmental engineering. Hydrology is subdivided into surface hydrology and marine hydrology. Domains of hydrology include hydrometeorology, surface hydrology, hydrogeology, drainage basinmanagement and water quality, where water plays the central role. Oceanographyand meteorologyare not included because water is only one of many important aspects within those fields. Hydrological research can inform environmental engineering, policyand planning. The term hydrologyis from Greek: ὕδωρ,hydōr, water; and λόγος,logos, study. History 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 townswere protected from flooding with high earthen walls. aqueductswere built by the Greeksand Ancient Romans, while the history of Chinashows they built irrigation and flood control works. The ancient Sinhaleseused hydrology to build complex irrigation works in Sri Lanka, also known for invention of the Valve Pit which allowed construction of large reservoirs, anicutsand 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 Earths surface and led to streams and springs in the lowlands. With adoption of a more scientific approach, Leonardo da Vinciand Bernard Palissyindependently 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 Mariotteand 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 Seawas sufficient to account for the outflow of rivers flowing into the sea. Advances in the 18th century included the Bernoulli piezometerand Bernoullis equation, by Daniel Bernoulli, the Pitot tube. The 19th century saw development in groundwater hydrology, including Darcys law, the Dupuit-Thiem well formula, and Hagen- Poiseuilles 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 Shermans unit hydrograph, the infiltration theory of Robert E. Horton, and C.V. Theiss 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). Main article: Outline of hydrology Branches Chemical hydrologyis the study of the chemical characteristics of water. Ecohydrologyis the study of interactions between organisms and the hydrologic cycle. Hydrogeologyis the study of the presence and movement of ground water. Hydroinformaticsis the adaptation of information technology to hydrology and water resources applications. Hydrometeorologyis the study of the transfer of water and energy between land and water body surfaces and the lower atmosphere. Isotope hydrologyis the study of the isotopic signatures of water. Surface hydrologyis the study of hydrologic processes that operate at or near Earths surface. Drainage basinmanagement covers water-storage, in the form of reservoirs, and flood-protection. Water qualityincludes the chemistry of water in rivers and lakes, both of pollutants and natural solutes. Related topics Oceanographyis the more general study of water in the oceans and estuaries. Meteorologyis the more general study of the atmosphere and of weather, including precipitation as snow and rainfall. Limnologyis the study of lakes. 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). [ 1 ] Water resourcesare sources of water that are useful or potentially useful. Hydrology studies the availability of those resources, but usually not their uses. Applications Determining the water balanceof a region. Determining the agricultural water balance. Designing riparian restoration projects. Mitigating and predicting flood, landslideand droughtrisk. Real-time flood forecastingand flood warning. Designing irrigationschemes and managing agriculturalproductivity. Part of the hazard module in catastrophe modeling. Providing drinking water. Designing damsfor water supplyor hydroelectric powergeneration. Designing bridges. Designing sewersand urban drainage system. Analyzing the impacts of antecedent moistureon sanitary sewer systems. Predicting geomorphologicchanges, such as erosionor sedimentation. Assessing the impacts of natural and anthropogenic environmental change on water resources. Assessing contaminanttransport risk and establishing environmental policy guidelines. Chathu Themes Main article: Water cycle The central theme of hydrology is that water circulates 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. 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 modelling these processes either for scientific knowledge or for making prediction in practical applications. Groundwater Hydrology (hydrogeology) considers quantifying groundwater flow and solute transport.[ citation needed]Problems in describing the satuatated zone include the characterization of aquifers in terms of flow direction, groundwater pressure and, by inference, groundwater depth (see: aquifer test). Measurements here can be made using a piezometer. Aquifers are also described in terms of conductivity, storativity and transmisivity. There are a number of geophysical methods [ 2 ]for characterising aquifers. There are also problems in characterising the vadose zone (unsaturated zone). [ 3 ] Infiltration The infiltration of water from precipitation into the soil is an important topic. In some circumstances a dry soil may not absorb rainfall as readily as a soil that is already wet. Infiltration can sometimes be measured by an infiltrometer. Soil moisture Soil moisturecan be measured in various ways; by capacitance probe, time domain reflectometeror Tensiometer. 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 water has reached a river include the stream gauge(see: discharge (hydrology)), 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 (stream-aquifer exchange). While in many geographical regions it is natural to think only of water moving out of aquifers into rivers, the reverse can also happen. Precipitation and evaporation In some considerations, hydrology is thought of as starting at the land-atmosphere boundary[ citation needed]and so it is important to have adequate knowledge of both precipitation and evaporation. Precipitation can be measured in various ways: disdrometerfor precipitation characteristics at a fine time scale; radarfor cloud properties, rain rate estimation, hail and snow detection; Rain gaugefor routine accurate measurements of rain and snowfall; satellite- rainy area identification, rain rate estimation, land-cover/land-use, soil moisture. Evaporationis an important part of the water cycle. It is partly affected by humidity, which can be measured by a 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. A direct measurement of evaporation can be obtained using Symons evaporation pan. Detailed studies of evaporation involve boundary layer considerations as well as momentum, heat flux and energy budgets. Uncertainty analyses Remote sensing Remote sensing of hydrologic processes can provide information of various types.[ citation needed]. Sources include land based sensors, airborne sensors and satellite sensors. Information can include clouds, surface moisture, vegetation cover. Water quality Main article: 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 Scaling in time and space Data assimilation Quality control of data — see for example Double mass analysis Prediction Observations of hydrologic processes are used to make predictionsof the future behaviour 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 analysing the statisticalproperties 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 periodof such events. Other quantities of interest include the average flow in a river, in a year or by season. These estimates are important for engineersand economistsso that proper risk analysiscan 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, industrialand residentialdemands. 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. Two major types of hydrological models can be distinguished:[ citation needed] Models based on data. These models are black boxsystems, 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 models may be linear models, but it is common to deploy non-linear components to represent some general aspects of a catchments 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, 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 hydrological modeling tries to have a more global approach to the understanding of the behavior of hydrologic systemsto make better predictions and to face the major challenges in water resources management.
Posted on: Wed, 16 Oct 2013 17:10:56 +0000
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