Environmental Engineering Reference
In-Depth Information
100
28·6 25·9 23·8 25·9 29·9 33·0 33·3 34·0 31·9 34·2
Semi-desert
2
23·0 19·9 23·1 28·4 33·5 37·0 36·7 37·8 33·9 29·4
25
24·5 21·4 23·5 28·1 33·4 35·6 35·3 36·5 33·5 32·2
50
26·2 22·6 23·9 28·2 33·4 35·0 34·7 36·3 33·0 32·9
100
28·6 23·9 25·1 27·8 32·9 34·6 33·9 35·8 32·8 33·1
Source : After Goltsberg (1969).
its upper surface, the angle between the leaf and the sun's rays, and the colour of the leaf.
Through absorption the temperature of the leaf rises and, consequently, the amount of
long-wave radiation emitted also increases. Some radiation is transferred downwards
towards the soil, and some flows upwards. With a large number of leaves the sun's rays
are increasingly obstructed, so the 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 numerous 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 macroclimate in which they
grow. It is now possible to determine the types of plants growing and to check their
health by aircraft or satellite photography. 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.
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 8.6). 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 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
much smaller because surface heating and cooling are greatly reduced through shading by
the leaves.
WIND IN THE VEGETATED LAYER
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