Modeling Concepts - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 260

Again, the parameters required for estimating the water budget are the river water
inflow ( ), the atmospheric precipitation ( ) and evaporation ( ), the under-
ground inflow ( ), and the marine water inflow ( ). The ( ) and ( ) are
usually measured directly by standard hydrometeorological monitoring programs,
and some empirical approaches are used to estimate the remaining terms. Some
practical recommendations are provided below.

Q riv

Q prc Q evp

Q grd Q inflow Q riv Q prc

6.3.2.2.1 Surface Evaporation Budget
The evaporation from a lagoon surface area can be estimated as follows. 5,6 Under
equilibrium conditions, when the difference between water and air temperatures
(

∆ t 1 ) is 2 to 4 ° C, the evaporation in mm day −1 is

Q =⋅ −⋅ 014 1 072 . ( . u ) ( ⋅ e − e ) (6.19a)

evp 2 0 2

where u 2 [m s −1 ] is the average wind velocity at a 2-m height, e 0 [mm Hg] is the
absolute humidity for surface water temperature, and e 2 [mm Hg] is the absolute
humidity for air temperature at a 2-m height above the water surface.
Under nonequilibrium conditions, the evaporation is

Q = 0 104 . ⋅ ( k + u ) ( ⋅ e − e ),

evp 0 2 0 2

(6.19b)

2

k = 1.378 + 0.195 ⋅ ∆ t − 0.003 ⋅ ∆ t

0 1 1

6.3.2.2.2 Ocean-Lagoon Exchange Budget
Tides and wind surges are the main driving forces at the ocean-lagoon boundaries
and water level variations can be used to calculate the corresponding ocean-lagoon
fluxes. As a first approximation, the time-dependent, spatially averaged lagoon levels
can be used. In this case, any influence of surface inclinations inside the lagoon on
the exchange budget is filtered out and only seawater flux that penetrates deeply
into the lagoon will affect the exchange. The remaining sea influx, which gives rise
to short period inflow and outflow movements near the entrance, is not considered
in this approach.
The spatially averaged and time-dependent lagoon levels are normally
obtained from averaging all levels measured at stations around the lagoon. These
stations should be selected to represent the lagoon area proportionally, depending on
its shape.
A more rigorous way to calculate a net water exchange through the ocean-lagoon
boundary is to subtract, at each monitoring time step

h t
avg

()

t , the other local components
of the water budget such as the river inflow Q riv , the precipitation Q prc , and the
evaporation Q evp . First, the level variation time series

∆

∆ h = h − h are

( t + 05 . ∆ t ) ( t + ∆ t ) () t

avg avg avg

to be constructed from the spatially averaged level time series h t

avg

over the entire

()