Geoscience Reference
In-Depth Information
Figure 12.7
Energy balance components for a
melting snow cover at Bad Lake, Saskatchewan
(51°N) on 10 April 1974.
Source
: Granger and Male. Modified by Oke (1987).
By permission of Routledge and Methuen & Co,
London, and T.R. Oke.
Table 12.1
Rates of energy dispersal (W m
-2
) at noon in a
20-cm stand of grass (in higher mid-latitudes
on a June day).
Net radiation at the top of the crop
550
Physical heat storage in leaves
6
Biochemical heat storage (i.e. growth processes)
22
Received at soil surface
200
Figure 12.8 shows the diurnal and annual energy
balances of a field of short grass near Copenhagen
(56°N). For an average twenty-four-hour period in June,
about 58 per cent of the incoming radiation is used in
evapotranspiration. In December the small net outgoing
radiation (i.e.
R
n
negative) is composed of 55 per cent
heat supplied by the soil and 45 per cent sensible heat
transfer from the air to the grass.
We can generalize the microclimate of short growing
crops according to T. R. Oke (see Figure 12.9):
1
Temperature
. In early afternoon, there is a temper-
ature maximum just below the vegetation crown,
where the maximum energy absorption is occurring.
The temperature is lower near the soil surface,
where heat flows into the soil. At night, the crop
cools mainly by long-wave emission and by some
continued transpiration, producing a temperature
minimum at about two-thirds the height of the crop.
Under calm conditions, a temperature inversion may
form just above the crop.
Figure 12.8
Energy fluxes over short grass near Copenhagen
(56°N). (A) Totals for a day in June (seventeen hours daylight;
maximum solar altitude 58°) and December (seven hours daylight;
maximum solar altitude 11°). Units are W m
-2
. (B) Seasonal curves
of net radiation (
R
n
), latent heat (
LE
), sensible heat (
H
) and ground-
heat flux (
G
).
Source
: Data from Miller (1965); and after Sellers (1965).
