Geoscience Reference
In-Depth Information
positioned to be in close contact with the soil. But the presence of a thermometer
alters the surface energy balance and therefore the surface temperature that is
being measured. Fine thermocouple thermometers positioned on the soil surface
disturb the surface energy balance less, but the issue of how to obtain an adequate
sample to determine the area-average temperature remains. In principle, the
average temperature can be measured over areas of a few square meters of soil by
positioning a radiometer above it and deducing the surface temperature from the
measured thermal radiation emitted. However, soil surfaces are not blackbody
emitters so soil emissivity has to be assumed and poor knowledge of this can
systematically bias such a radiometric measurement by several degrees.
The soil surface temperature is determined by the surface energy balance. As a
result, for dry exposed soils in fairly calm conditions and with clear skies, the ampli-
tude of the daily cycle in soil surface temperature can be very large, perhaps greater
than 30°C. This is noticeably larger, by perhaps a factor of two, than the daily cycle
in air temperature measured at 2 m. Incoming radiant energy plays a major role in
the surface energy balance that determines the surface temperature of bare soil and
the timing of the daily cycle in soil temperature, therefore, tends to follow that in
net radiation, albeit with some lag (typically less than an hour). For a wet soil surface,
much of the energy outgoing to the atmosphere is as latent heat and the sensible heat
flux is less. There is, therefore, less need for a large difference in temperature between
the soil surface and the overlying air to support the sensible heat flux. Consequently,
the amplitude of the daily cycle in soil surface temperature is much smaller. But solar
radiation is still the dominant term in available energy for wet bare soil so the timing
of the daily cycle in soil temperature still tends to follow that in net radiation.
When there is vegetation overlying the soil, the magnitude and timing of the
daily cycle in soil surface temperature differs from that for bare soil. Much of
the incoming radiant energy is captured by the vegetation canopy and returned to
the atmosphere before it reaches the soil. The presence of vegetation also enhances
turbulent mixing in the air near the ground thus reducing the difference in
temperature between air adjacent to the soil surface and that in the atmosphere
above. Since there is much less solar radiation reaching the soil surface, the solar
cycle has less influence on surface energy balance and soil surface temperature, the
latter being thus more similar to near-surface air temperature. Consequently, the
daily cycle in vegetation-covered soil surface temperature has reduced amplitude
relative to that for bare soil in the same meteorological conditions, and the timing
of the cycle tends to follow that in air temperature and so typically lags the solar
radiation cycle by several hours.
Subsurface soil temperatures
The temperature of soil below the surface is somewhat easier to measure than
soil surface temperature providing the thermometers are carefully inserted with
minimum disturbance to the soil structure. Commonly, small thermometers are
