Geoscience Reference
In-Depth Information
Figure 12.9
Temperature and wind-
velocity profiles within and above
a metre-high stand of barley at
Rothamsted, southern England, on
23 July 1963 at 01:00 to 02:00 hours
and 13:00 to 14:00 hours.
Source
: After Long
et al
. (1964).
emitted by respiration. The maximum sink and
source of CO
2
is at about two-thirds the crop height.
Finally, we examine the conditions accompanying
the growth of irrigated crops. Figure 12.10 illustrates
the energy relationships in a 1-m high stand of irrigated
Sudan grass at Tempe, Arizona, on 20 July 1962. The
air temperature varied between 25 and 45°C. By day,
evapotranspiration in the dry air was near its potential
and
LE
(anomalously high due to a local temperature
inversion) exceeded
R
n
, the deficiency being made up
by a transfer of sensible heat from the air (
H
negative).
Evaporation continued during the night due to a
moderate wind (7 m s
-1
) sustained by the continued heat
flow from the air. Thus evapotranspiration leads to
comparatively low diurnal temperatures within irrigated
desert crops. Where the surface is inundated with water,
as in a rice paddy-field, the energy balance components
and thus the local climate take on something of the
character of water bodies (see B, this chapter). In
the afternoon and at night the water becomes the most
important heat source and turbulent losses to the atmos-
phere are mainly in the form of the latent heat.
Figure 12.10
Energy flows involved in the energy diurnal balance
of irrigated Sudan grass at Tempe, Arizona, on 20 July 1962.
Source
: After Sellers (1965).
2 Forests
The vertical structure of a forest, which depends on
the species composition, the ecological associations
and the age of the stand, largely determines the forest
microclimate. The climatic influence of a forest may
be explained in terms of the geometry of the forest,
including morphological characteristics, size, cover, age
and stratification. Morphological characteristics include
amount of branching (bifurcation), the periodicity
of growth (i.e. evergreen or deciduous), together with
the size, density and texture of the leaves. Tree size is
2
Wind speed
. This is at a minimum in the upper crop
canopy, where the foliage is most dense. There is a
slight increase below and a marked increase above.
3
Water vapour
. The maximum diurnal evapotran-
spiration rate and supply of water vapour occurs at
about two-thirds the crop height, where the canopy
is most dense.
4
Carbon dioxide
. By day, CO
2
is absorbed through
photosynthesis of growing plants and at night is
