Digital Signal Processing Reference
In-Depth Information
more generalized mass balance approach and by the inclusion of additional processes such as benthic
oxygen demand, benthic scour and deposition, photosynthesis and respiration of aquatic plants, and
nitrification. The more comprehensive water quality models have been developed to include the
nitrogen and phosphorus cycle and the lower trophic levels of phytoplankton and zooplankton. A
number of investigations have modeled the algal nutrient, Silica.
A number of chemical constituents have been selected and modeled by assuming thermo-dynamic
equilibrium. The fate of toxicants such as pesticides, metals, and polyCHL-aorinated biphenyls
(PCBs) is very complicated involving adsorption-desorption reactions, flocculation, precipitation,
sedimentation, and biological uptake. Examination of toxicants and their impact on biological
populations requires ecological models. The water quality modeling methodology requires considering
the following:
The water quality parameters to be modeled
The dimensional and temporal resolution of the model
Data requirements for model building
Water Quality parameters
The water quality parameters most frequently simulated include salinity, light, temperature, DO, BOD,
coliform bacteria, algae, nitrogen, and phosphorus . Each of these parameters interacts with each
others, but the significance of their dependencies varies among constituents, and their inclusion in a
numerical water quality model depends upon the study objectives and the water body under
consideration.
Dimensional and Temporal Resolution of Model
In a numerical water quality model the choice is between a 1-dimensional model and one that
incorporates two or three spatial dimensions. A long, narrow, and vertically well-mixed water body
may be represented by a one-dimensional model consisting of a series of segments averaged over the
cross section. Where there is pronounced vertical stratification, it is likely that a laterally averaged
two-dimensional model will be needed. In other situations where there are marked lateral
heterogeneities in water quality but the water body is well mixed, a vertically averaged two-
dimensional model is indicated. If significant lateral heterogeneities are accompanied by pronounced
stratification, a three-dimensional model may be required. Most existing water quality models are one-
dimensional. Practical applications of two-dimensional depth and breadth integrated models have been
made and are feasible. Three-dimensional water quality models are presently research tools; data
requirements for calibration and verification make them prohibitively expensive at present for
practical application.
The basis of all water quality models is a velocity field either specified by empirical measurements or
computed by numerical hydrodynamic models. The current trend in hydrodynamic modeling is toward
development of 3-dimensional models with increased spatial and temporal resolution in order to
resolve important scales and to minimize the need for parameterization. As a result, modern time-
dependent hydrodynamic models normally have time steps on the order of minutes to one hour. The
chemical and biological equations of water quality models have characteristic time scales determined
by the kinetic rate coefficients. These time scales are usually on the order of one to ten days. The
phenomena of interest, such as depletion of DO and excessive plant growth, occur on time scales of
days to several months.
Direct coupling of hydrodynamic and water quality models provides potential spatial and temporal
resolution that cannot be effectively interpreted. The reasons are that present field sampling programs
resolve constituent concentrations on the order of a kilometer to tens of kilometers in the horizontal,
meters in the vertical, and days to weeks in time. In addition, the kinetic rate coefficients presently
used in water quality models resolve dynamics on the order of days to weeks. Direct coupling
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