Hydrology
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Hydrology (from Greek: Yδρoλoγια, Yδωρ+Λoγos, Hydrologia, the "study of water") is the study of the movement and distribution 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, or civil and environmental engineering.
Domains of hydrology include hydrometeorology, surface hydrology, and hydrogeology, 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 not only in that it allows us to better understand the world in which we live, but also by providing insight for environmental engineering, policy and planning.
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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. Aquaducts were built by the Greeks and Romans, while the Chinese built irrigation and flood control works.
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 independantly 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
Main article: Hydrologic cycle
The central theme of hydrology is that water moves throughout the Earth in different ways 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, which flows down the rivers back to the ocean, completing a cycle.
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.
- Rain gauge - rain and snowfall
- Stream gauge - stream flow (see: discharge (hydrology))
- Radar - cloud properties
- Piezometer - groundwater pressure and, by inferrence, groundwater depth (see: aquifer test)
- Tensiometer - soil moisture
- Satellite
- Disdrometer - precipitation characteristics
- Time domain reflectometer - soil moisture
- Sling psychrometer - humidity
- Infiltrometer - infiltration
Hydrologic Prediction
Observations of hydrologic processes are used to make predictions of future water movement and quantity.
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.
See: return period.
Hydrologic Modeling
With an understanding of how changes in the environment affect the movement of water, hydrologists can also construct models to predict how these changes will happen in the future.
Hydrologic Transport
Water movement is a significant means by which other material, such as soil or pollutants, are transported from place to place.
Applications of Hydrology
- 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;
- 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.
See also
- chemical hydrology
- current (water)
- drainage system
- fresh water
- groundwater
- hydraulic engineering
- hydrograph
- hydroinformatics
- hyetograph
- limnology
- river
- soil moisture
- virtual water
- water abstraction
- water industry
- water table
- watershed
- well water
References
- Introduction to Hydrology, 4e. Viessman and Lewis, 1996. ISBN 0-673-99337-X
- Handbook of Hydrology. ISBN 0070397325
External links
- International Glossary of Hydrology (http://www.cig.ensmp.fr/~hubert/glu/aglo.htm).
- U.S. Geological Survey (http://www.usgs.gov).ca:Hidrologia
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