Geoscience Reference
In-Depth Information
(a)
(b)
Local time (hr)
Local time (hr)
04
00
08
20
12 16
04 00 08
20
12 16
0
0
10
10
20
20
30
30
Figure 6.1 Profiles of soil
temperature measured at
selected hours during the day
beneath (a) a bare soil surface
and (b) a crop of potatoes.
(Redrawn from Monteith and
Unsworth, 1990, published
with permission.)
40
40
Bare soil
Potatoes
50
50
10
15
20
25
30
35
10
15
20
25
30
35
Soil temperature (
C)
Soil temperature (
C)
used which are inserted horizontally into the vertical edge of a cautiously dug soil
pit, so that they sample soil at least 0.1 m away from the edge of the pit. If the soil
is uniform, heat flow in the soil tends to average out some of the spatial heteroge-
neity in temperature present at the surface, so horizontal variations in measured
subsurface temperatures are less extreme.
Subsurface soil temperatures are determined by heat flow into and out of the
soil in response to changing surface temperature. Consequently, the magnitude
and timing of the daily cycle in subsurface temperatures are necessarily different
for dry soil and wet soils, and for vegetation-covered soils. Not surprisingly, the
magnitude of the cycle in subsurface temperatures is greater for bare soil than for
soil covered by vegetation, and is later relative to the cycle in the solar radiation,
depending on depth. Figure 6.1 shows profiles of soil temperature measured at
selected hours of the day in bare soil and in the soil beneath a nearby crop of
potatoes. On this day, the range of variation in the surface temperature measured
beneath the potato crop is around 15°C and less than the 22°C range for the bare
soil. The range of subsurface temperatures measured at the same depth in the two
profiles reflects this difference in the surface temperature cycle. In both cases, the
magnitude of diurnal variation progressively decreases with depth. This last
feature is evident in Fig. 6.2, which also more clearly shows how the phase-lag of
the cycle in subsurface soil temperature increases with greater depth.
Thermal properties of soil
The thermal properties of soil determine how the magnitude and phase of the soil
heat flux and subsurface soil temperatures respond to changes in the soil surface
temperature. All the relevant soil properties are strongly dependent on the
moisture content of the soil. This is because air-filled pores present in dry soil are
progressively filled with water as the moisture content of soil increases, and the
Search WWH ::




Custom Search