Amplitude and Phase Characteristics of Diurnal Soil-Temperature Waves

Amplitude Differences with Depth

Daily temperature-wave amplitude (half the peak-to-peak swing) normally decreases sharply as depth increases. This attenuation reveals key thermal properties:

• Shallow layers (≈ 5–10 cm)

  • Exposed directly to surface heating and cooling.

  • Exhibit the largest amplitude, warming rapidly by day and cooling quickly at night.

• Deep layers (≈ 70–90 cm)

  • Receive energy only after it travels through the overlying soil.

  • Show a much smaller amplitude because the soil mass dampens the heat wave.

Interpretation

• Rapid amplitude decay ⇒ high heat-storage capacity (large C:sub:`v`) and/or low–moderate thermal diffusivity.

• Amplitude that persists deeply ⇒ higher thermal diffusivity, often due to higher thermal conductivity (k) or lower heat capacity (dry or sandy soils).

Phase (Lag) Between Diurnal Peaks

The daily time lag (phase shift) between temperature peaks at two depths reflects how quickly heat moves downward.

• Short lag

  • Indicates high thermal diffusivity and high k.

  • Typical of sandy or dry soils where heat propagates efficiently.

• Long lag

  • Reflects low thermal diffusivity and low k.

  • Common in moist, clay-rich, or organic soils that transmit heat slowly.

Summary of Indicators

Relation between field observations and inferred heat-transfer properties

Observation	Likely interpretation
Rapid decrease in amplitude with depth	High volumetric heat capacity; moderate–low thermal diffusivity
Gradual amplitude decrease persists to depth	Higher thermal conductivity; lower heat capacity
Short lag time between successive peaks	Higher thermal diffusivity; efficient downward heat transfer (sandy, dry soils)
Long lag time between successive peaks	Lower thermal diffusivity; sluggish transfer (moist clay or organic-rich soils)