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

Abstract

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 (G) 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 −1 K −1 A field experiment compared G measured with pairs of four plate types buried at 6 cm in a clay soil with G determined using the gradient technique. The flux plates underestimated G 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 Gmeasurements, especially when λ > λ m , although most plate estimates were still significantly lower than the actual G 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. The thermal conductivity () of soils may vary by a factor of about

A novel approach to evaluate soil heat flux calculation: An analytical review of nine methods

“…Another source of error in heat flux plate measurements is liquid water and vapor flow disruption since plates can block liquid water and vapor flow [Sauer et al, 2003;Ochsner et al, 2006]. If a heat flux plate is installed near the soil surface (e.g., at 2 cm depth), the plate might overestimate the soil heat flux by more than 20%, as demonstrated by Ochsner et al [2006] and Peng et al [2015].…”

“…The reliability of this approach is highly dependent on accurate measurements from the SHF plate and accurate estimation of S G . The plate method is simple to implement, but a number of studies have demonstrated that the performance of the plate method is likely to suffer from several errors, including heat flow distortion and liquid water and vapor flow disruption [Sauer et al, 2003;Ochsner et al, 2006]. Fortunately, the use of self-calibrating SHF plates (HFP01SC, Huskeflux Thermal Sensors) can substantially reduce errors caused by heat flow distortion and improve the accuracy of G z measurements [Ochsner et al, 2006[Ochsner et al, , 2007Peng et al, 2015].…”

Estimation of ground heat flux from soil temperature over a bare soil

“…In spite of the simplicity and durability, evidence suggests that the soil heat flux plate is prone to errors during the estimation of G z in field, especially errors from heat flow distortion which may be induced by the differences of thermal conductivity between the plate and the surrounding soil (Sauer et al 2003;Peng et al 2015). Due to the varied near-surface soil moisture, the thermal conductivity of the soil surrounding the plate was varying (Lu et al 2007) and not always matched with that of the plate (Philip 1961).…”

A Numerical Model to Estimate the Soil Thermal Conductivity Using Field Experimental Data

“…Ochsner et al [15] concluded that the heat flux plate underestimates soil heat flux because of the low plate thermal conductivity, thermal contact resistance, and latent heat transfer effects. However, this is the main method to obtain the experimental soil heat flux and widely used in studies of surface energy balance closure [27][28][29].…”