Environmental Engineering Reference
In-Depth Information
This is illustrated in Figure 8.5, where the energy balances over a wet and a dry soil are
contrasted, and in Table 8.1, where the thermal differences above such surfaces are
demonstrated.
MICROCLIMATE ABOVE A VEGETATED SURFACE
The nature of microclimatic conditions and processes becomes far more complex when
vegetation cover is present, for not all the energy is absorbed at a single surface. Some is
absorbed by the top of the vegetation, some penetrates into the plants, and some may
even reach the soil surface. The amount that gets through to the soil depends upon the
height of the crop, the density of the leaves and the angle of the sun's rays. As the size of
the plants increases, so does the degree of microclimatic modification.
Let us look at some of the detailed effects of plants on the microclimate by
considering conditions around a single leaf. The amount of short-wave radiation absorbed
by a leaf depends upon the quantity of radiation reaching
Figure 8.5
Net radiation (left) and energy budget (right) for
an oasis and a desert site. Q
H
is sensible heat flux, Q
E
is
latent heat flux and Q
G
is soil heat flux. In the oasis Q
E
is
sustained by Q
H
from the warmer atmosphere and can
become similar to the net radiation value.
Table 8.1 Twenty-four-hour diurnal temperature variation in July
Height above surface (m)
Hour of day
1
5
7
9
11
13
15
17
19
21
Irrigated oasis
2
21·4 18·9 20·7 25·4 30·5 33·2 33·9 33·7 30·0 26·4
25
23·8 20·8 21·8 25·3 30·2 33·0 33·9 34·3 30·9 29·7
50
26·2 22·6 22·5 25·5 30·0 33·1 33·5 34·5 31·6 32·7
