Errors in Heat Flux Measurement by Flux Plates of Contrasting Design and Thermal Conductivity

Abstract

<p>The thermal conductivity (λ) of soils may vary by a factor of about 4 for a range of field soil water contents. Measurement of soil heat flux (<em>G</em>) using a heat flux plate with a fixed λ distorts heat flow through the plates and in the adjacent soil. The objectives of this research were to quantify heat flow distortion errors for soil heat flux plates of widely contrasting designs and to evaluate the accuracy of a previously reported correction. Six types of commercially available heat flux plates with varying thickness, face area, and thermal conductivity (λ_m) were evaluated. Steady-state laboratory experiments at flux densities from 20 to 175 W m⁻² were completed in a large box filled with dry or saturated sand having λ of 0.36 and 2.25 W m⁻¹K⁻¹ A field experiment compared <em>G</em> measured with pairs of four plate types buried at 6 cm in a clay soil with <em>G</em> determined using the gradient technique. The flux plates underestimated <em>G</em> in the dry sand by 2.4 to 38.5% and by 13.1 to 73.2% in saturated sand while in moist clay plate performance ranged from a 6.2% overestimate to a 71.4% underestimate. Application of the correction generally improved agreement between plate estimates and independent <em>G</em>measurements, especially when λ > λ_m, although most plate estimates were still significantly lower than the actual <em>G</em> Limitations of the correction procedure indicate that renewed effort should be placed on innovative sensor designs that avoid or minimize heat flow distortion and/or provide direct, in situ calibration capability.</p>