Geoscience Reference
In-Depth Information
amount of sunlight reaching the ground may be small.
The actual quantity depends upon the type and number
of leaves (or
leaf area index
) and the crop height.
Because of its agricultural importance there have
been innumerable studies of the climate within crops.
Agronomists and plant physiologists use the information
in order to increase yields from plants best suited to the
micro- and macro-climate in which they grow. It is now
possible to determine the types of plants growing and to
check their health by aircraft photography or satellite
imagery. The nature of the radiation reflected and emitted
from leaves varies from one species to another, and from
healthy to unhealthy plants, owing to alterations in the
distribution of pigments in the leaves.
canopy top and gets warmer down towards the soil surface.
If the crop has a low density, with large gaps between
plants, the air cooled by contact with the radiating leaves
becomes denser and sinks towards the ground to give the
minimum temperatures there. In the soil, temperature
changes are smaller because surface heating and cooling
are greatly reduced through shading by the leaves.
Wind in the vegetated layer
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 wind speed. By day, there
is normally a sudden decrease in speed as far as the middle
canopy. Below that level, some crops have fewer leaves,
enabling the wind to blow more easily through the crop.
So, for this type of crop, we get a slightly windier zone
before the final decrease towards the soil surface.
Temperatures in the vegetated layer
If we look at mean profiles of wind speed, temperature and
humidity within a plant crop, there is some similarity with
those found above a bare soil surface (
Figure 8.5
).
In this
instance the main heat exchange zone is found slightly
below the canopy top rather than at the soil surface. As a
result, daytime temperatures reach their maximum values
within the canopy. The actual location represents a balance
between the reduction in sunlight intensity as it penetrates
into the crop and the decrease in wind speed and
turbulence which would help to remove the heated air. At
night, under clear skies and with light winds, long-wave
radiation continues to flow from the leaf surfaces, but only
that from the upper leaves is able to escape from the plant
system. At lower levels in the crop, radiation is trapped and
re-emitted, maintaining warmer temperatures. Thus the
temperature profile has a minimum value below the
Moisture in the vegetated layer
Daytime humidity levels usually show a progressive
decrease from the soil surface, through the crop, into the
atmosphere (
Figure 8.5
).
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.
Net radiation
Wind speed
Temperature
Humidity
Q
*
u
T
q
N
D
N
D
N
D
N
D
Q
H
Q
E
Figure 8.5
Typical profiles of net radiation, wind speed, temperature and humidity above and within a plant canopy. Blue lines
show the night-time profiles and orange lines those for daytime.
