Geoscience Reference
In-Depth Information
forest. For the well-watered crop (a) daytime latent heat flux typically consumes
70-90% of net radiation, as in this example, and the sensible and soil heat fluxes
can be of similar magnitude. Notice that this example data shows evaporation con-
tinuing into the evening with the energy in excess of net radiation primarily being
provided by a downward sensible heat flux. The behavior shown is also broadly
typical for forest vegetation, with the outgoing daytime fluxes of latent and sensi-
ble heat more similar than for the crop. In the figure showing the forest (b), the soil
heat flux and physical energy storage terms are plotted as a sum. In practice the
physical storage term is likely to be the greater of these two.
Evaporative fraction and Bowen ratio
The ratio of the latent heat flux to the sum of the sensible heat flux and latent heat
flux is called the e
vaporative fraction, E
F
. If the instantaneous values of sensible and
latent heat are
H
and
E
, respectively, the instantaneous value of the evaporative
fraction is therefore given by:
λ
E
E
HE
l
=
(4.4)
(
)
F
+
l
Similarly, if
th
e tim
e av
erage values of
se
nsible and latent heat over a specific time
period are
H
and
E
respectively,
E
, the evaporative fraction of the average
fluxes over this same period is given by:
λ
E
l
E
=
(4.5)
F
(
)
HE
+
l
It is important to recognize that, because the ratio
E
F
may change rapidly with
time, the time-average of instantaneous values of
E
F
does
not
reliably give the daily
evaporative fraction of the average fluxes, the evaporative fraction for a day must
be calculated from the all-day average fluxes.
The ratio of the sensible heat flux to the latent heat flux is called the
Bowen
Ratio
,
β
. If the instantaneous values of sensible and latent heat are
H
and
λ
E
,
respectively, the instantaneous value of the Bowen Ratio is therefore given by:
H
E
(4.6)
b
=
l
Similarly, if the t
i
me-ave
rag
e values of sensible and latent heat over a specified
time period ar
e
H
and
E
λ
respectively, the time-average Bowen Ratio over this
b
same period,
, is given by:
H
E
(4.7)
b
=
l
