Soil Temperature and Heat Flow

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Soil Temperature and Heat Flow

• Mario Perez-Bidegain
• 4/4/03

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Introduction

• Temperature affects soil physical, chemical, and
  biological processes
• Soil temperature varies in response to energy
  exchange processes that take place at the soil
  surface
• Semiquantitative theory
• Water flowheat flow interaction

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Modes of Energy Transfer

• Radiation emission of energy in the form of
  electromagnetic waves from all bodies above 0 K.
• Stefan-Boltzmann law
• Jt? ? T4
• ? emissivity coefficient
• ? constant 5.67 10-8 W m-2 K-4
• T Temperature in Kelvins

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Modes of Energy Transfer

• Wiens law
• ?m 2900/T ? m micrometer
• T Kelvins
• Plancks law
• E? c1 ?-5/ eC2/?T-1
• C1 3.7407 10-16 W m-2
• C2 1.44 10-3 m K

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Modes of Energy Transfer

• Convection involves the movement of a
  heat-carrying mass
• Conduction is the propagation of heat within a
  body by internal molecular motion
• Latent heat transfer

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Conduction of Heat in Soil

• Fouriers law (Steady state)
• qh -? ?T (1)
• qh thermal flux
• ? thermal conductivity
• ?T spatial gradient of temperature

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Conduction of Heat in Soil
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Conduction of Heat in Soil

• Non-steady or transient conditions (no sources or
  sink of heat)
• Principle of Energy Conservation
• ?cm?T/?t -?. qh (2)
• ? Mass density
• cm Specific heat capacity per unit mass, or
  specific heat
• ?cm (C) Specific heat capacity per unit volume

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Conduction of Heat in Soil

• Combining (1) and (2)
• Second law of heat conduction
• ?cm?T/?t -? (??T)

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Conduction of Heat in Soil

• Account for possible sinks or sources of energy
• Thermal diffusivity (DT)
• DT ? /C

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Conduction of Heat in Soil

• Thermal diffusivity (DT) DT ? /C
• qh -DTC ?T/ ?x
• If DT is constant
• ?T/?t DTd2T/dx2

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Conduction of Heat in Soil

• Thermal properties of soil
• C Volumetric heat capacity
• ? Thermal conducivity
• DT Thermal diffusivity

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Volumetric Heat Capacity of Soils

• Change in heat content of a unit bulk volume of
  soil per unit change in temperature. It denotes
  C.
• How to estimate C?
• de Vries (1975)
• C? fsCs fwCw faCa
• f vol fraction
• s solid w water aair

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Volumetric Heat Capacity of Soils
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Volumetric Heat Capacity of Soils

• C fmCm foCo fwCw
• m mineral o O.M. w water
• C0.48fm 0.60fo fw

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Summary