Environmental Engineering Reference
In-Depth Information
The wind-speed profile is also more complex, owing to the presence of the crop. Its
precise form depends upon the nature of the crop and the prevailing wind speeds. By day,
there is normally a sudden decrease in speed as far as the middle canopy. Below that
level, most crops have fewer leaves, enabling the wind to blow through the crop. So we
get a slightly windier zone before the final decrease towards the soil surface. This effect
can be felt behind a hedgerow or windbreak, where the stems are not so effective at
reducing wind speed as the leafier branches at higher levels.
Figure 8.6 Typical profiles of net radiation, wind speed,
temperature and humidity above and within a plant canopy.
Black lines show the night-time profiles and blue lines those
for daytime. Q H is sensible heat flux and Q E is latent heat
flux.
MOISTURE IN THE VEGETATED LAYER
Daytime humidity levels usually show a progressive decrease from the soil surface,
through the crop, into the atmosphere (Figure 8.6). Moisture is evaporated from the soil
and transpired by the plant leaves, so that the main moisture sources are within the crop.
As wind speeds are low, much of the moisture remains within the vegetation, but that in
the upper layers may be carried away by convection and turbulence to mix with the drier
air above. At night the shape of the humidity profile is more complicated. Cooling may
give rise to dewfall on the upper leaves, producing an inverted profile for a short
distance, but normally humidity differences are relatively weak throughout the crop.
Within a plant canopy, moisture exchanges are extensive and of vital importance to
the well-being of the crop. In reality these processes are highly complex, but we can get
an idea of the exchanges by constructing a simple model of the water balance. Figure 8.7
shows the inputs and outputs of moisture we might expect with an ideal crop. The major
input of most climatic regimes is precipitation in the form of either rain or snow, but hail,
dewfall, frost and fog can add small amounts. Some of this moisture is intercepted by the
leaves. Depending upon the intensity and duration of the precipitation and the nature of
the leaf, the water may drop off the leaves, or be directly evaporated without ever having
Search WWH ::




Custom Search