Geoscience Reference
In-Depth Information
200
C
VEGETATED SURFACES
R
n
From the viewpoint of energy regime and plant
canopy microclimate, it is useful to consider short
crops and forests separately.
150
100
Δ
M
50
1 Short green crops
Short green crops, up to a meter or so high,
supplied with sufficient water and exposed to
similar solar radiation conditions, all have a
similar net radiation (
Rn
) balance. This is largely
due to the small range of albedos, 20-30 percent
for short green crops compared with 9-18 percent
for forests. Canopy structure appears to be the
primary reason for this albedo difference.
General figures for rates of energy dispersal
at noon on a June day in a 20cm high stand of
grass in the higher mid-latitudes are shown in
Table 12.1
.
Figure 12.8
shows the diurnal and annual
energy balances of a field of short grass near
Copenhagen (56°N). For an average 24-hour
period in June, about 58 percent of the incoming
radiation is used in evapotranspiration. In
December the small net outgoing radiation (i.e.,
Rn
negative) is composed of 55 percent heat
supplied by the soil and 45 percent sensible heat
transfer from the air to the grass.
We can generalize the microclimate of short
growing crops according to T. R. Oke (see
Figure
12.9
):
LE
0
H
Δ
S
-50
-100
00
04
08
12
16
20
24
Hour
Figure 12.7
Energy balance components for a
melting snow cover at Bad Lake, Saskatchewan
(51°N) on 10 April 1974.
Source: Granger and Male. Modified by Oke (1987). By
permission of Routledge and Methuen, London, and T. R.
Oke.
(
Figure 12.7
). In this example of snow-melt at
Bad Lake, Saskatchewan on 10 April 1974, the
value of
Rn
was kept low by the high albedo of the
snow (0.65). As the air was always warmer than
the melting snow, there was a flow of sensible heat
from the air at all times (i.e.,
H
negative). Prior to
noon, almost all the net radiation went into snow
heat storage, causing melting, which peaked in
the afternoon (Δ
M
maximum). Net radiation
accounted for about 68 percent of the snow-melt
and convection (
H
+
LE
) for 31 percent. Snow
melts earlier in the boreal forests than on the
tundra, and as the albedo of the uncovered spruce
forest tends to be lower than that of the tundra, the
net radiation of the forest can be significantly
greater than for the tundra. Thus, south of the
arctic tree-line the boreal forest acts as a major
heat source.
1
Temperature
. In early afternoon, there is a
temperature maximum just below the vegeta-
tion crown, where the maximum energy
absorption is occurring. The temperature is
lower near the soil surface, where heat flows
into the soil. At night, the crop cools mainly
by longwave emission and by some con-
tinued transpiration, producing a tempera-
ture minimum at about two-thirds the
height of the crop. Under calm conditions, a
temperature inversion may form just above
the crop.
Table 12.1
Rates of energy dispersal (W m
-2
) at noon in a
20cm stand of grass (in higher mid-latitudes on a June day)
Net radiation at the top of the crop
550
Physical heat storage in leaves
6
Biochemical heat storage (i.e. growth processes)
22
Received at soil surface
200
