Temperature
variation with depth in soil. The features to note are that the temperature
extremes occur at the surface where radiant energy exchange occurs, and
that the diurnal variation decreases rapidly with depth in the soil. Figure
2.5 also shows these relationships and gives additional insight into soil
temperature variations. Here, temperatures measured at three depths are
shown.
Note that the diurnal variation is approximately sinusoidal, that the amplitude decreases rapidly with depth in the soil, and that the time of maximum andminimum shifts with depth. At the surface, the time ofmaximum
temperature is around 14:OO hours, as it is in the air. At deeper depths the
times of the maxima and minima lag solar noon even farther, and at 30 to
40 cm, the maximum is about 12 hours after the maximum at the surface.
We derive equations for heat flow and soil temperature later when we
discuss conductive heat transfer. For the moment, we just give the equation
which models temperatures in the soil when the temperature at the surface
is known. This model assumes uniform soil properties throughout the
soil profile and a sinusoidally varying surface temperature. Given these
assumptions, the following equation gives the temperature as a function
of depth and time:
1
0
6
12
18
24
30
36
42
48
Time (hrs)
FIGURE 2.5. Hypothetical temperature variations in a uniform soil at the surface
and two depths showing the attenuation of diurnal variations and the shift in
maxima and minima with depth.