Single‐Probe Heat Pulse Method for Soil Water Content Determination: Comparison of Methods

Abstract

Core Ideas Six methods to obtain soil water content by single probe are presented and compared. Errors of estimated soil water content by six methods increase with water content. Heating duration affects soil water content estimation errors. Four of the six methods are probe dependent but can be easily calibrated. Each method works for some soils, and combining different methods is a solution. The estimation of soil thermal conductivity (λ) using the single‐probe heat pulse (SPHP) method is well known, but estimation of soil water content (θ) using the SPHP is poorly understood. In this study, we examined six methods—λ, normalized cumulative temperature increase (TNcum), normalized maximum temperature increase (TNmax), and the reciprocals of each—for θ estimation using the SPHP. The temperature response curves of four soils at different θ were measured following 600‐s heat pulses with heating strengths of about 6 W m−1, from which λ, TNcum, and TNmax values were determined. The maximum measurement errors of these three methods were 0.11 m3 m−3 for the coarse sand and 0.01 m3 m−3 for the fine sand, sandy loam, and silty clay, except for 0.05 m3 m−3 for the fine sand by the λ(θ) method. The predicted θ from all of the λ, TNcum, and TNmax methods agreed well with that from the oven‐dry method for all soils with the exception of the TNcum(θ) and TNmax(θ) methods for the coarse sand for θ > 0.20 m3 m−3. The measurement errors and θ predictions of the 1/λ(θ) method were similar to that of the TNcum(θ) and TNmax(θ) methods, and that of the 1/TNcum(θ) and 1/TNmax(θ) methods were similar to that of the λ(θ) method. Because each of the six methods worked well for only some soils, improved estimations were obtained when the λ(θ) method was combined with the 1/TNcum(θ) [or 1/TNmax(θ)] method for coarse‐textured soils and the 1/λ(θ) method was combined with the TNcum(θ) [or TNmax(θ)] method for fine‐textured soils.

Signal Processing for In-Situ Detection of Effective Heat Pulse Probe Spacing Radius as the Basis of a Self-Calibrating Heat Pulse Probe

“…The mechanical construction and design of the HPP used for this paper has been reported and rationalized in other papers (Li et al, 2016;Si, 2008, 2010). The needles of length 3.0 cm = were constructed from stainless-steel tubing (1.28 mm OD and 0.84 mm ID) and filled with thermally-conductive epoxy.…”

“…For 2 /  n tr , where n r is the radius of the needle and  is the thermal diffusivity of the medium, the last term in Eq. (1) can be neglected 90 (Bristow et al, 1994;Li et al, 2016). In this paper, Eq.…”

“…The precision sense resistor was chosen as 0.01 = s R (0.1%, ±15ppm/°C) to reduce the voltage drop over this circuit element and to ensure that the output voltage could be accurately adjusted. This resistance is smaller than the 1  current sense resistor nominally used in other HPP experiments (Bristow et 310 al., 1994;Li et al, 2016;Si, 2011, 2008;Valente et al, 2006;Zhang et al, 2011). Moreover, the precision sense resistor had a Kelvin terminal connection for precision and was physically large to reduce self-heating by current flow.…”

“…The Heat Pulse Probe (HPP) is widely used to determine thermal conductivity (Abu-Hamdeh, 2001;Abu-Hamdeh and Reeder, 2000;Jin et al, 2017;Li et al, 2016;Liu et al, 2007;Ochsner and Baker, 2008;Penner, 1970;Yun and Santamarina, 2008), thermal diffusivity and heat capacity (Bristow, 1998;Ham and Benson, 2004;Kluitenberg et al, 1993;Liu et al, 2007;Ochsner 25 et al, 2001;Zhang et al, 2014) of soil. HPPs have been used to measure the thermal conductivity (Morin et al, 2010;Sturm and Johnson, 1992) and density of snow (Liu and Si, 2008); a review is presented by Kinar and Pomeroy (2015).…”

“…(1) with a starting value of the determined thermal conductivity is used to estimate parameters for application of the SP forward model. (Li et al, 2016) that is implemented in the context of this paper.…”