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Hydrology is the science of water. It is concerned with the occurrence and circulation of water on and within Earth, the physical and chemical properties of bodies of water, the relationship between water and other parts of the environment, and societal or economic aspects of water resources. Hydrology is an interdisciplinary field of study, and hydrologists have academic backgrounds that include geology, engineering, biology, chemistry, geography, soil science, economics, and mathematics.

The two major sub-disciplines of hydrology are surface water hydrology, which is concerned with water on or at Earth's surface, and groundwater hydrology (sometimes referred to as hydrogeology or geohydrology), which his concerned with the water beneath Earth's surface. Surface water hydrology includes the analysis and prediction of floods (as well as the meteorological events that produce them); the transfer of water from Earth's surface to its atmosphere by evaporation, transpiration, and sublimation; the study of sediment erosion, transportation, and deposition by flowing water; and investigations of water quality in lakes and streams. Groundwater hydrology involves the study of soils and rocks that comprise aquifer systems, the exploration for new groundwater resources using geological and geophysical methods, monitoring groundwater flow directions and velocities, and the remediation of contaminated groundwater.

Atmospheric water, surface water, and groundwater are linked together by the hydrologic cycle, which describes the continuous movement of water in its various forms and phases. The hydrologic cycle has no beginning or end. Surface water is transformed from a liquid to atmospheric water vapor as it is evaporated from open bodies of water and transpired by plants, or from solid to vapor by sublimation of snow at high elevations. Atmospheric pressure and temperature changes then transform the water vapor into liquid or solid water that falls to Earth's surface as rain or snow. A portion of the rain and snow is returned to Earth's surface by rivers and streams. Another portion seeps into Earth's crust to become groundwater via a process known as infiltration. Groundwater can be pumped from aquifers by humans or flow naturally into surface water bodies by way of seeps and springs. In some cases, particularly in arid and semi-arid regions, rain can be returned to the atmosphere by evaporation before it reaches the ground. This cycle, with many variations, occurs continuously as water is recycled through the environment. Therefore, one of the principal activities of hydrologists is the development of water balances that quantify the different components of the water cycle in a particular region.

Hydrologists rely on many techniques to collect the data they need; some are simple and straightforward, such as the measurement of snow depth and the discharge of rivers and streams. Others are more elaborate, such as the use of remote-sensing techniques to assess the quantity and quality of water resources. The development and application of computer models that simulate hydrologic systems is also an important aspect of hydrology.

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Science EncyclopediaScience & Philosophy: Hydrazones to Incompatibility