Soil Moisture Content
Name: Peter d'E.
How does moisture content in soil affect the soil
Water has a very different thermal conductivity than
most soil particles and air (the thermal properties of the soil
are determined by these three). The thermal conductivity
of water is much greater than that of air, so the higher
the soil moisture content the greater the thermal conductivity.
The greater the soil moisture content, the more the soil
thermal conductivity is like that of water. Therefore, a
saturated soil has a conductivity near that of water.
However, just because the soil moisture content is high,
doesn't mean that the soil will warm up faster in the Sun
than a dry soil. Evaporation of the water will remove much
of the Sun's energy before the soil will have a chance to warm.
Therefore, dry soils do warm up faster from sunlight and cool
faster at night. This is assuming that there isn't a vegetation
cover over the soil. Most wet soils evaporate the water, keeping
the soil from warming as fast during the day, and cool more
slowly at night because of their greater heat capacity (because of
the higher water content).
Soils that are better at holding water in them (reducing
evaporation), such as clays and peat, are the exception to the above;
they may not evaporate as much water and therefore do heat up in
the sun, and do not loose as much energy at night. Peat bogs are
often very warm, although part of that energy comes from rapidly
occurring rotting of organic matter. Wet clays can also become
very warm in the Sun.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012