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United States Department of Agriculture

Agricultural Research Service

Title: Modeling Water Quality

Author
item Lowrance, Robert

Submitted to: Water Science
Publication Type: Book / Chapter
Publication Acceptance Date: October 10, 2002
Publication Date: January 31, 2003
Citation: Lowrance, R.R. 2003. Modeling water quality. Encyclopedia of Water Science. In Press.

Interpretive Summary: Although water quality models can be physical representations of the real world such as channels and ditches built to scale, mathematical or formal models are more common. Water quality models are based on some representation of hydrology and may include movement of surface water, ground water, and mixing of water in lakes and water bodies. Water quality models simulate a combination of sediment, nutrients, heavy metals, other chemical compounds, and aquatic biology. Mathematical water quality models are quantitative expressions of how water and things moving with water behave in the real-world. Most water quality models are set up so that for any given set of inputs there is only one unique set of outputs. If random occurrences are accounted for in the model, it is said to be stochastic. Continuous simulation models represent ground water flow and the interaction with surface water. Event based models usually represent only processes that occur during rainfall events or snow melt. A lumped parameter model represents land uses, soils, vegetation, and the inputs for the model as the same over large areas. A distributed parameter model represents attempts to represent certain aspects of the land more like what occurs in the real world. Water quality models are used to estimate the risk posed by chemicals that might end up in the drinking water supply and to evaluate different ways of doing things on the land in order to determine what are the best management practices (BMPs) Water quality models are also used to determine what are the sources and/or impacts of pollutants and to help explain how large-scale complex systems behave.

Technical Abstract: Water quality models are based on some representation of hydrology and may include movement of surface water, ground water, and mixing of water in lakes and water bodies. Water quality models simulate some combination of sediment, nutrients, heavy metals, xenobiotics, and aquatic biology. Although water quality models can be physical representations of the real world such as channels and ditches built to scale, mathematical or formal models are more common. Mathematical water quality models are quantitative expressions of processes or phenomena that are known to occur in the real-world. The expressions are simplifications of real-world systems through a series of equations governed by conservation of mass. Mathematical models can be classified as deterministic or stochastic; continuous simulation or event based; lumped or distributed parameter. Most water quality models are deterministic models in the sense that one set of inputs will provide only one set of outputs. Continuous simulation models provide at least some representation of ground water /surface water interactions, while event-based models represent only processes that take place during precipitation events. A lumped parameter model represents landscape units as homogeneous with respect to the parameters and inputs that drive the model. A distributed parameter model represents certain aspects of the landscape structure, typically by representing areas that are homogeneous with respect to soils, vegetation, and/or land use. Water quality models are used for risk assessment of pesticides; evaluation of best management practices (BMPs); evaluation of sources and/or impacts of pollutants; and explanation of large-scale systems behavior.

Last Modified: 12/22/2014
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