Geoscience Reference
In-Depth Information
where
H
B
is the radiance or heat loss rate per unit surface area (Wm
−2
) and
T
s
is the
water surface temperature (
C).
Evaporative heat loss
(
H
L
, Wm
−2
) depends on the water density (
°
), on the
specific evaporative heat or latent heat of the water (
L
w
is the heat energy required
to evaporate a given mass of water, Jkg
−1
), and on a rate of evaporation (E, m s
−1
),
which in turn depends on the wind and water vapor pressure gradient between the
water and atmosphere. It is usually estimated by the following formula:
ρ
HL
=⋅
ρ
⋅
(
a
+⋅
b u
) (
⋅
e
−
e
)
(6.29)
L
w
w
s
a
where
a
is an empirical coefficient representing the effect of vertical convection,
which occurs even in the absence of the horizontal wind velocity,
e
s
is the saturated
vapor pressure at the water surface temperature (mb), and
e
a
is the vapor pressure
at the air temperature (mb).
Sensible heat transfer
, or the transport of heat due to convection and conduction,
can be estimated by an approach based on the Bowen ratio, which has been observed
to be valid over an extended range of conditions. Practical means of estimating the
sensible heat flux
H
s
(Wm
−2
) is
P
P
HL
=⋅
ρ
⋅
(
a
+⋅
b u
)
⋅
C
⋅
a
⋅
(
TT
−
)
(6.30)
S
W
w
b
s
a
where
C
b
is a coefficient (0.61 mb
C
−1
),
P
a
is the atmospheric pressure (mb),
P
is
a reference pressure at the mean sea level,
T
s
is the water surface temperature, and
T
a
is the air temperature. See Martin and McCutcheon
4b
for a more detailed descrip-
tion of the heat budget terms.
°
6.3.3
P
RE
-E
STIMATION
OF
S
PATIAL
AND
T
EMPORAL
S
CALES
It should be emphasized that although hydrodynamic numerical, physical, and other
types of models can expand our knowledge about the hydrodynamics of a study area,
the dominant physical processes must be known beforehand, i.e., before the model is
applied. A numerical model, which incorporates the physics of these processes, can
then be chosen to provide a quantitative description of these processes. Knowledge of
the following temporal and spatial scales can contribute to the choice of a hydrody-
namic model.
6.3.3.1
Flushing Time
Without loss of generalities, the flushing time is the time required for a measurable
volume of the water in a lagoon to be replaced by waters from river run-off,
precipitation, and water exchange with the adjacent coastal marine waters. In a case
where the trial volume of water equals the total lagoon volume, it will be an integral
flushing time, which is also a measure of the self-cleaning capability of a polluted
lagoon. This messure is commonly used in the assessment of rehabilitation schemes
for lagoon ecosystems that are under stress from pollution. However, even when all
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