Geoscience Reference
In-Depth Information
500
500
Dry soil - day
Dry soil - night
400
400
No latent heat
(
H
=
A
)
300
300
200
200
100
100
R
n
A
l
E
HGS
t
0
0
R
n
A
l
E
HGS
t
−
100
−
100
Wet soil
reflects less
solar so
R
n
is
greater
500
500
Moist soil - day
Moist soil - night
400
400
Latent heat
dominant
More soil heat
storage for wet
soil
300
300
200
200
100
100
R
n
A
l
E
HGS
t
0
0
R
n
A
l
E
HGS
t
100
−
−
100
Figure 4.3
Representative daytime and nighttime surface energy budget for dry and wet soil.
Figure 4.3 shows representative values of (vertical) energy fluxes for the energy
budgets of dry soil and wet soil in daytime and nighttime conditions. In these
example cases the downward solar radiation,
S
, was assumed to be 350 W m
−2
for
the daytime example, and the net longwave flux (equal to the net radiation flux at
night) was assumed to be -75 W m
−2
. Evaporation is usually small at night and the
nighttime latent heat flux is arbitrarily set to zero in this figure. It is further
assumed that the net advected energy,
A
d
, is zero and the biochemical storage,
P
,
and the physical storage,
S
t
, are assumed negligible because there is no vegetation
present.
The greater thermal conductivity of moist soil means that the soil heat flux is
greater for the wet soil case than the dry soil case, i.e., for the wet soil
G
is greater
when energy is conducted into the soil during the day and out again at night. Dry
soil also reflects more incoming solar radiation than wet soil so the daytime net
radiation flux is somewhat less for the dry soil example. The most obvious differ-
ence between these dry and wet soil examples is in the way the available energy is
partitioned between latent and sensible heat. In the dry soil example when there is
no water available, there is no daytime latent heat flux. However, outgoing latent
