Modeling Concepts - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 258

sources and sinks: for example, sewage outlets (point sources), non-point
sources or diffusive pollution, withdrawals, groundwater seapage (usually
non-point and extended in space), etc.
• Processes responsible for heat exchange: These are direct solar radiation,
direct turbulent heat fluxes through the air-water boundary caused by
temperature gradients, and indirect heat fluxes due to evaporation. Advec-
tive transports of water from the sea, rivers, and the atmosphere also
contribute to heat gains and losses in the lagoon due to temperature
differences in lagoon and incoming waters.
• Processes responsible for mechanical momentum transport: Wind and
tidal variations are the main factors here. Wind acts directly on the upper
water mass, setting up a water level surge that in turn defines a pressure
gradient that is responsible for the movement of the total water body.
Any inflow water flux, irrespective of its origin (wind or tidally induced
flow, river, or artificial source) provides a positive gain of momentum in
the lagoon system. Negative inputs are provided by bottom and internal
frictions.

The hierarchy of these forces is as follows: mass and heat fluxes at the boundaries
define the state and time evolution of the lagoon system whereas momentum fluxes
are responsible for internal transformations of mass and heat, their redistribution,
and their exchanges between different parts of the lagoon system. A precise identi-
fication of all mass and heat sources/sinks for a lagoon under study requires a good
description of the external driving forces. More specifically, the following parameters
must be identified:

• The number of sources/sinks
• The name of the sources/sinks
• The position of sources/sinks in geographical coordinates (latitude, lon-
gitude) and in the computational grid coordinates
• The average water volume discharge [m 3 s −1 ] in each source/sink
• The average concentrations of chemical substances under study [kg m −3 ]
at each source

• The average discharges of all studied chemical substances [kg s −1 or t year −1 ]

Additional parameters on air-water interactions and lagoon-ocean exchanges,
on sediment erosion and deposition and on human-made changes must also be
properly identified.
A good starting point is to estimate mean annual characteristics of all forcing
factors. This provides information on the nature (sink or source) and range of the
factors involved and first-order estimates of water, heat, sediment, and chemical
budgets for the lagoon system.
The absence of information for some parameters will prompt additional field
measurements. If these measurements cannot be performed, the estimations should
be obtained through a numerical modeling approach. For example, the magnitude
of the sea-lagoon water exchange and its temporal (seasonal or synoptic) variations,