Geoscience Reference
In-Depth Information
500
500
Moist crop - day
Moist crop - night
400
400
Latent heat
dominant
300
300
200
200
100
100
R
n
A
l
E HGS
t
0
0
R
n
A
l
E HGS
t
Forest reflects
less solar so
R
n
is greater
−
100
−
100
500
500
Moist forest - day
Moist forest - night
400
400
Latent heat
Less dominant
Little
soil heat
storage
300
300
200
200
100
100
R
n
A
l
E HGS
t
0
0
R
n
A
E HGS
t
l
−
100
−
100
Figure 4.4
Representative daytime and nighttime surface energy budget for short crop and forest when there is plenty of
water available in the soil.
heat flux dominates outgoing sensible heat during the day for wet soil. The outgo-
ing net radiation flux at night, which is entirely longwave radiation, is supported
partly by energy returning to the surface as soil heat flux and partly by an inward
flux of sensible heat flux, the latter being the more important contribution in the
dry soil example.
Figure 4.4 shows representative daytime and nighttime surface energy budgets for
a short crop and for forest. In each case there is plenty of water available to the veg-
etation in the soil. Again, downward solar radiation,
S
, is assumed to be 350 W m
−2
for the daytime examples, nighttime net radiation flux is assumed to be
75 W m
−2
,
and nighttime evaporation is set to zero. For short annual crops (including
grass), both of the storage terms,
S
t
and
P
are negligible, but in the case of forest
physical energy storage can be significant. In Fig. 4.4, both the short and forest
vegetation are assumed to fully cover the ground and as a result the soil heat flux
is small in both cases. But
G
is particularly low for forest vegetation because the
leaf cover is greater. Forests usually reflect less solar radiation than short annual
crops because the top of the canopy is rougher, consequently the net radiation
input tends to be higher during the day. For well-watered crops, most of the avail-
able energy is used for evaporation during the day and the outgoing latent heat
−
