Modeling Concepts - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 256

for model calibration by field data. Presenting both the upper and bottom layer T-S
data, as in Figure 6.12B, reveals periods of vertical stratification and how they
compare with the seasonal T-S variations. For example, it may not be appropriate
to use a vertically integrated model when the vertical T-S variations are comparable
to the seasonal T-S variations in both the upper and bottom layers ( Figure 6.12C ).
Furthermore, since the temporal evolution of the T-S curve in these layers is
significantly different, this may point to some independent forcing factors in each
layer. This can be noticed in highly stratified estuarine lagoons that could express
high variability from year to year ( Figure 6.12.D) .
The quantitative parameters introduced in this section define a multidimensional
coordinate system from which lagoon types can be objectively defined using prob-
ability functions derived from similar or different lagoon combinations around the
world.
In conclusion, the morphometric and hydrological features presented above may
not quantitatively classify a lagoon as one type or another, and in that respect,
Kjerfve's classification remains the most useful one in a qualitative sense. However,
a combination of selected quantitative parameters may provide insights into the
behavior of a lagoon, or in the absence of data, it may allow us to apply findings
about other lagoons with similar dimensions to the lagoon under study and, even-
tually, to choose an appropriate model.

6.3.2 D ESCRIPTION OF F ORCING F ACTORS

The state of any lagoon can be described at any point in time ( t ) by a number of
variables. Some of these variables apply to the lagoon as a whole, such as its water
volume V lag , its average depth H avg , and its free surface area S lag . They are time-
dependent functions of the lagoon water-level variations in time. Other variables, such
as the depth H ( x, y, t ) and the free surface variations h ( x, y, t, ), have two-dimensional
spatial and time-dependent distributions, while others have three-dimensional space
and time-dependent distributions, such as the three velocity components U i ( x, y, z, t )
and various dissolved and suspended substances concentrations C k ( x, y, z, t ) including
temperature T ( x, y, z, t ) which is considered as a separate admixture in general terms.
The relationships between these model variables are prescribed by the governing model
equations, whereas all external driving forces causing the temporal and spatial vari-
ability of these variables are described as boundary conditions.

6.3.2.1 General Hierarchy of Driving Forces

Conceptually, a lagoon can be considered as a system acted upon by external forces
and internal factors ( Figure 6.13 ). This introduces the concept of different types of
boundaries for the lagoon system. These are physical boundaries of the lagoon such
as coastal line, bottom, and free surface; conventional physical processes boundaries
where, for example, air-water or water-sediment interactions occur; and conven-
tional internal boundaries for internal exchange parameterization (e.g., biological
sink/source of mass and energy, or subgrid dissipation). As such, the lagoon becomes
a separate system environment driven by external driving forces.