Geoscience Reference
In-Depth Information
H
2
O
40
400
−
L
p
F
p
L
e
E
30
300
Wm
−
2
Wm
−
2
20
CO
2
200
10
100
0
0
8
10
12
14
16
18
Time (EST)
L
p
F
p
(circles and left-hand ordinate scale), into a stand of maize in W m
−
2
estimated by means of the energy budget method. The experiment took place near Ithaca, NY,
on August 13, 1970. The
Fig. 2.27 CO
2
flux density,
−
L
p
F
p
values shown were around 8% of the estimated latent heat flux
L
e
E
(triangles and right-hand ordinate scale), which can be seen to follow a similar diurnal
variation. The time is Eastern Standard Time. The vertical bars are error estimates of the flux
values. (After Sinclair, 1971.)
−
of remotely sensed surrogates for La, such as the normalized difference vegetation index
(
N
DVI
), and other measures of surface greenness; for instance, with measurements over the
same types of vegetation, Kustas
et al.
(1993) derived
c
R
=
0
.
40
−
0
.
33
N
DVI
.
The daytime variations of the major energy fluxes at land surfaces are often quite simi-
lar, exhibiting some kind of self-preservation, which keeps them proportional to each other
through the day (see Section 4.3.4). Nevertheless, both (2.85) and (2.86) are oversimpli-
fications, since
G
is related not to one but to all terms in Equation (2.72); therefore, such
simple relationships should be calibrated anew for each given problem, and the values of the
constants can be considered accurate only for certain specific conditions. One point in their
favor is that, much more so than the other major fluxes in the energy budget
R
n
,
H
and
L
e
E
,
the soil heat flux
G
tends to be highly variable in space (see Kustas
et al.
, 2000), so that
a dense network of measurements would be needed to obtain a meaningful areal average.
Therefore, expressions like (2.85) and (2.86) can be useful to obtain averages over larger
areas, especially, when used with remotely sensed observations. In the past, attempts have
also been made to determine
G
on the basis of analytical solutions of the linearized heat
flow equation with effective parameters for the thermal conductivity and the specific heat
of the soil profile (see Brutsaert, 1982, p. 151). However, also this approach can produce
only rough estimates.
2.6.3
Minor terms in the energy budget
Although they may be quite important under certain conditions, the energy absorption by
photosynthesis, the energy advection, and the rate of change of energy storage are usually
relatively small in most applications in hydrology.
The flux of CO
2
is usually neglected, although under favorable conditions, say on a
sunny summer day, it can be of the order of 5% of the global radiation, and up to 8% to
