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.