MAQ22806

• Soil


  • Extremely variable in the horizontal
  • Variable in the vertical in terms of properties and organic matter content
  • Variable water content
• We are looking at energy transport in the soil, which is a complex system
• Solving the diffusion equation for a simple boundary condition

  1. Impose a sine wave at the surface

  2. Result is a sine wave of the temperature with reduced amplitude and phase shift as you go deeper into the soil
• Damping depth


  Central parameter that comes out of solving the diffusion equation
• In the first part of the lecture on soil heat flux

  1. Look at soil thermal properties and factors determining them

  2. Look at effect of rain on heat transport in soil

  3. Understand how heat transport in soil is measured and what corrections are needed
• Composition of soil matrix

  • Quartz minerals
  • Clay minerals
  • Organic matter
• Thermal properties of soil materials

  • Large variation in thermal conductivity and diffusivity from quartz to organic matter
  • Smaller variation in specific heat
• Thermal properties of air

  • Thermal conductivity is an order of magnitude lower than water, making air an insulator
• Calculating soil thermal properties from composition

  1. Can calculate density, specific heat, and volumetric heat capacity

  2. Cannot directly calculate thermal conductivity and diffusivity as they depend on arrangement of particles
• Volumetric heat capacity


  Depends strongly on soil moisture content, with large increase as soil goes from dry to wet
• Thermal conductivity of soil

  • Depends on properties of materials and their arrangement, especially number and size of contact points between particles
  • In dry soil, heat can only be transported through quartz particles and their contact points
• Conductivity


  The ability of a material to conduct heat
• Conductivity is different for quartz, clay, water, and air
• Factors affecting conductivity


  • Arrangement of the soil particles
  • Number of contact points
  • Size of contact points
• Air in the soil is an insulator
• Heat is normally conducted through the contact points between soil particles and the presence of water
• Heat transport in dry soil

  1. Heat has to be transported through the soil particles

  2. Then through the contact points between the soil particles downwards
• Conductivity of materials in dry soil

  • Quartz has higher conductivity than air
• Effect of adding water to soil

  1. Water accumulates close to the contact points between soil particles

  2. Increases the area of contact points between particles
• Adding water to dry soil

  Instantly increases the conductivity of the soil
• Conductivity of water is larger than that of air, but smaller than that of quartz
• Composition of soil matrix has a large effect on thermal properties
• Calculating soil diffusivity


  Diffusivity = Conductivity / Volumetric heat capacity
• Maximum diffusivity occurs at an optimum soil moisture content, which varies for different soil types
• Soil heat flux plates are used to measure soil heat flux
• Soil heat flux plate

  • Plate of material with a thermopile to measure temperature difference
  • Allows calculation of heat flux through the plate
• Thermal properties of the soil heat flux plate may differ from the surrounding soil
• Heat flux measured at the plate depth may differ from the surface heat flux due to heat storage in the soil layer above the plate
• Correcting measured heat flux for storage effects

  1. Use calorimetric or harmonic method

  2. Requires measuring temperature change in soil layer above plate
• Rainfall and infiltration


  Can transport heat into or out of the soil, affecting surface temperature
• Thermal properties of soil are complex, depending on matrix material, porosity, structure, and moisture content
• Measuring soil heat flux requires accounting for flux divergence between surface and measurement depth
• Rainfall infiltration can significantly affect soil heat transport