Geoscience Reference
In-Depth Information
6
Soil Temperature
and Heat Flux
Introduction
The flow of energy into temporary storage in the soil that underlies the interface
between terrestrial surfaces and the atmosphere can be a substantial component
(
25%) of the midday surface energy budget for bare soil surfaces. It can also be a
significant component of the daytime surface energy budget for vegetated surfaces,
depending on the extent to which the plants shade the soil. The surface temperature
of the soil is determined by the need to continuously maintain a balance between
the energy fluxes of radiation, latent heat, and sensible heat into the overlying air
and the flow of energy by thermal conduction into the soil. Consequently, the
surface temperature of soil is a dependent variable. However, energy flow within
the soil and associated changes in below ground soil temperature are determined
by changes in the surface temperature of the soil. In this sense, soil surface
temperature is the forcing variable that determines thermal behavior in the soil.
The present chapter considers soil temperatures and heat flow in soil from the
perspective that soil surface temperature is a forcing variable that varies with time.
∼
Soil surface temperature
Measuring area-average soil surface temperature is difficult because it varies
greatly from place to place and can change quickly with time. The surface
temperature of typically rough bare soil surfaces can vary over short distances
during daylight hours depending on the relative orientation between the local soil
surface and the solar beam. Spatial variability in plant shading complicates this
relationship for soil covered by vegetation. Attempts have been made to measure
soil surface temperature using conventional mercury bulb thermometers
