INTEGRATING TOOLS FOR SURFACE WATER QUALITY MANAGEMENT - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 51

purposes mostly the well-known, cross-sectionally integrated (1D) Saint Venant equations or
approximations to these equations are used (Mahmood and Yevjevich, 1975; Abbott, 1979).
Many different forms and approximations to the Saint Venant equations are known, depending upon
whether the flow is steady or unsteady and which simplifications are made. Thus, for water quality
studies often the equation of steady, gradually variable flow is employed (which may be further
simplified to the so-called Manning equation. Unsteady models, which are based on the continuity and
momentum equations, include the kinematic, diffusive, and dynamic wave approaches. The difference
stems from simplifications of the latter equation: dynamic wave models solve the full equation,
diffusive models exclude the acceleration terms, while kinematic models disregard also the pressure
gradient term that is essential for the description of backwater effects. (Routing methods used widely
in hydrology usually correspond to the last approach, (Mahmood and Yevjevich, 1975). The
hydrodynamic equations are generally solved by efficient finite difference methods (Mahmood and
Yevjevich, 1975). For water quality issues the acceleration terms in the momentum equation rarely
play a significant role and the typical time scales are amplified by conversion processes.
In general, three types of models are significant in the investigation of the environmental impact of
water bodies:
1) Hydrodynamic models which describe the velocity and salinity distributions within the study
area.
2) Water quality models which predict physical characteristics and chemical constituent
concentrations of the waiter at various locations within the study area.
3) Ecological models which predict the interactions between water quality and the aquatic
community.
3.3.2. Data Requirements for Mathematical Modelling of Water Quality
As it might be expected, the data requirements for different models increase with the level of
complexity and scope of application. All models require data on flows and water temperatures. Static,
deterministic models require point estimates of these data and often use worst case “design flow”
estimates to capture the behavior of pollutants under the worst plausible circumstances. For most
management purposes, the worst case will be high summer temperatures, which intensify problems
with dissolved oxygen and algal growth, and low flows, which lead to high concentrations of BOD
and other pollutants. Dynamic models will need time-series data on flows, temperatures, and other
parameters. In addition to hydraulic data, models require base case concentrations of the water quality
parameters of interest (dissolved oxygen, mercury, and so on). These are required both to calibrate the
models to existing conditions and to provide a base against which to assess the effects of management
alternatives. The models also need discharges or loads of the pollutants under consideration from the
sources (e.g., industrial plants) being studied. The types and amounts of data needed for a given
application are specific to the management or research question at hand.
The information derived from hydrodynamic models forms the basic part of the database for water
quality and ecological models, this data in turn will become necessary for water quality models that
would then be part of the database for ecological models. Hence, it is essential that these foundation
modeling activities be accomplished with adequate accuracy. The various models described require
input data which may be classified as:
Data that describe the initial conditions of the system.
Data that describe the "boundary conditions" of the system These data include system
geometry and the quantity and constituent concentration of freshwater inflows.
Other data necessary for the calibration of the models, including a description of the
hydrography of the study area.