INTEGRATING TOOLS FOR SURFACE WATER QUALITY MANAGEMENT - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 50

3.3. HYDRODYNAMIC - WATER QUALITY MODELLING APPROACHES FOR CATCHMENT -
SHALLOW LAKE SYSTEMS
Models are invaluable tools for resource management. Models help resource managers develop a
shared conceptual understanding of complex natural systems, allow testing of management scenarios,
predict outcomes of high risk and high cost environmental manipulations, and set priorities. Using
mathematical models is a key for better understanding of natural systems specially water systems and
complex water resources problems. Nowadays, relative to the present advances in computational
sciences, hardware and software, improvement in rivers, catchments and lakes modeling has been only
modest since the last decades. One of the reasons can be attributed to that in most of catchments
modeling development has been undertaken by environmental scientist and engineers with limited
computer science training. Most of the models they develop rarely make use of the power and
efficiency of modern software engineering methods. Due to this, most catchments models are difficult
to understand, use and adapt, leading to frustration amongst the users of such models and their
developers (Argent et al., 2001). Environmental modelling is a specialist field, and different modelling
approaches are specialist areas in themselves. Different philosophies abound; there are experts who
advocate systems approaches using conceptual models and others who dismiss these as being too
uncertain and based on opinion rather than fact. Even when the approach is agreed, experts may be at
odds over which modelling product is superior.
Sometimes the needs of the resource manager appear to be lost in the technical debate between
modellers who support fundamentally different approaches. The tensions between resource
management and science are explored by Cullen (1990) and his findings are especially applicable to
environmental modeling which can be expensive and highly technical. The quality of a water body as
a part of the ecosystem, is reflected in its physical condition which strongly influences the chemical
and biological processes that occur in water. Water quality interactions are by necessity simplified
descriptions of an aquatic ecosystem that is extremely complex.
An essential tool, which can be used by water quality researchers and managers in developing
management plans for rivers and watersheds, is mathematical or water quality modelling. Water
quality modelling has had a long history (Streeter and Phelps, 1925, Velz, 1938, O'Connor, 1960, 962,
1967, Chapra, 1996). The increased computational power of modern computers, as well as the need
for watershed or basin-wide planning, has resulted in the expansion of mathematical modelling codes
beyond simple steady-state, one dimensional models of pollutants to complex, time-variable, three-
dimensional models of nutrients / eutrophication and toxic materials. These models are often coupled
to hydrodynamic and watershed or urban runoff models. Mathematical models can be used to predict
changes in ambient water quality due to changes in discharges of wastewater. Predicting the water
quality impacts of a single discharge can often be done quickly and sufficiently accurately with a
simple model. Watershed water quality planning usually requires a model with a broader geographic
scale, more data, and a more complex model structure. A water quality model is a set of mathematical
equations that represent the physical, chemical, and biological characteristics and processes of a water
body in a way that approximates reality. Water quality models can be used to simulate water quality
changes that could be expected to result from changes in pollutant loads and different water quality
management strategies. These simulations, called "scenarios," allow us to predict positive or negative
changes on water quality.
3.3.1. Hydrodynamic - Water Quality Modelling Tools
Flow of water in a river or stream is described by the continuity and momentum equations. The latter
is known as the Navier- Stokes or Reynolds equation. The actual form of a hydrodynamic model
depends on assumptions made on characterizing turbulence. Methods vary from the use of eddy
viscosity as known parameters to the application of the so called k- et al ., 1988;
Rodi, 1993). Complex models are available, (Abbott, 1979; Naot and Rodi, 1982) but for water quality