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A SURVEY AND REVIEW OF
MODELING FOR TMDL APPLICATION
IN TEXAS WATERCOURSES
By George H. Ward, Jr. & Jennifer Benaman
ABSTRACT
A model in its broadest sense is a simplified
depiction of a natural entity that in some way
exhibits its important features while eliminating or suppressing
matters of irrelevant detail. In
science and engineering, an essential attribute of a model is that
it be quantitative, that is, that it
yield a numerical value for a feature of the natural entity, as
a surrogate for a measurement. A
quantitative model can be used to explore cause-and-effect relations
and to determine values of physical variables that are too costly
or difficult to measure directly.
The above paragraph is general and could
apply to any discipline or field of study. In the
specific area of water resources, examples of models include an
arrangement of laboratory tanks and retorts in which microorganisms
behave as they do in a lake, and a scale-model of a river or estuary
in which the movement of contaminants can be visualized by dye plumes.
Another, and very important, example of a model is a mathematical
relation, which might be embodied in a graph or equation, referred
to as a "mathematical model." Equations representing flow
in a stream as a function of water level, and the longitudinal profile
of dissolved oxygen downstream from a sewage outfall are mathematical
models. As the equations are extended to include various, often
interacting variables of the watercourse, and to accommodate the
effect of more external factors, the resulting mathematical model
can become extremely complex, until its solution must be carried
out on a computer. For this reason, it is now common to refer to
specific computer programs, which solve such equations, as "models."
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