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(a)
35
z
d
=5cm
z
d
=10cm
z
d
=20cm
z
d
=50cm
30
25
20
15
10
5
0
3
6
9
12
15
18
21
24
Time (hour UTC)
(b)
35
z
d
=5cm
z
d
=10cm
z
d
=20cm
z
d
=50cm
30
25
20
15
10
5
0
3
6
9
12
15
18
21
24
Time (hour UTC)
Figure 2.18
Diurnal cycle of the soil temperature of a bare soil plot at four depths at
Haarweg Meteorological station on a cloudy day (May 22, 2007, left) and on a sunny
day (May 23, 2007, right). Note that the lowest level is actually below grass, but the
difference will be insigniicant.
2.3.1 Bare Soil
To get an impression of some important features of the soil temperature, the diurnal
cycle of the soil temperature at four depths is shown in
Figure 2.18
for two consec-
utive days: a cloudy and a sunny day. Temperatures have been measured at 5 cm,
10 cm, 20 cm and 50 cm below the (bare) soil surface. First the four curves within one
plot are compared. A few important features are visible:
•
The shape of the temperature curve for May 23 is similar to a sine-wave, although the
curve is not exactly symmetric (steep increase in the morning, slow decrease in the after-
noon).
The amplitude of the diurnal variation decreases with depth. On May 22 the amplitudes
•
at the four depths are 4 K, 2.5 K, 1 K and 0 K, whereas on the sunny day the amplitudes
are 10 K, 7 K, 3 K and 0 K, respectively.
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