Laboratory Evaluation of the Dual‐Probe Heat‐Pulse Method for Measuring Soil Water Content

Abstract

The dual‐probe heat‐pulse (DPHP) method provides a means of estimating volumetric soil water content (θ) and change in volumetric water content (Δθ) from measurements of volumetric heat capacity. The purpose of this investigation was to characterize the accuracy and precision that can be achieved in measuring θ and Δθ with the DPHP method. Tempe pressure cells fitted with DPHP sensors were used to conduct desorption experiments in which DPHP‐based estimates of θ and Δθ were compared with values estimated by the gravimetric method. For water contents corresponding to soil water pressure potentials below −100 kPa, comparisons were made by packing the pressure cells with soil wetted to known water contents. The investigation was conducted with seven soil materials representing a wide range of physical properties for mineral soils. The DPHP sensors slightly overestimated θ at low water contents, but it was shown that the bias could be removed by using an empirical calibration equation, θ = 1.09 θDPHP − 0.045. This relationship appears to be general inasmuch as it was shown to be applicable for all seven soil materials and for water contents ranging from 0.02 to 0.59 m3 m−3 The general calibration equation was also shown to be effective in removing bias in Δθ estimates. Pooled regression analysis (all soil materials) showed that θ can be measured with a root mean square error (RMSE) of 0.022 m3 m−3 Greater precision can be achieved with Δθ measurements (RMSE = 0.012 m3 m−3); however, the results indicated a decrease in precision with increasing magnitude of Δθ.

Probe Body and Thermal Contact Conductivity Affect Error of Heat Pulse Method Based on Infinite Line Source Approximation

“…The plug is then mounted on the bottom plate of a PVC container (10 cm high and 10 cm in diameter for DP with 3.175-mm heater needle, 5 cm high and 7.9 cm in diameter for DP with 1.27-mm heater needle). Basinger et al (2003) provide additional details regarding the fabricating of the DP units.…”

“…THE DP METHOD (Campbell et al, 1991) is widely used for measuring the .soil thermal conductivity /^(W m~ K" ), the thermal difFusivity tv, and the volumetric heat capacity C (J m"-^ K" ) of soil (Bristovi', 1994). The DP method tends to overestimate the soil specific heat capacity c (J kg" * K~ ' ) (Song et al, 1998;Ren et al, 2003;Basinger et al, 2003). The evalúes from the DP method, are between 881 and 973 J kg"' K"' (Ren et al, 2003), and between 864 and 939 J kg"' K"' (Campbell et al, 1991).…”

A low-power, low-cost soil-moisture sensor using dual-probe heat-pulse technique

“…Since the sensor network is implemented in an open field, we have used solar power for recharging the primary power source of the system, which is a Li-ion battery. [6][7][8][9][10][11][12]. As shown in the figure, power consumption by the device built in this work is the second lowest.…”

A Dual‐Probe Heat‐Pulse Sensor with Rigid Probes for Improved Soil Water Content Measurement

“…Specifically, while the measured volumetric heat capacity and water content are well estimated for saturated soil conditions, they are overestimated for dry soil conditions, with a typical linear estimation response for intermediate water contents Heitman et al, 2003;Ochsner et al, 2003;Ren et al, 2003;Tarara and Ham, 1997). In some studies, such as that of Basinger et al (2003), where considerable attention was given to keeping the probe spacing constant, the DPHP sensor showed similar disparities for a wide range of soil types. Their comparison of DPHP-measured and gravimetrically evaluated volumetric water content showed a linear response with an average offset and slope of 4.1% and 0.92, respectively, for a wide range of soil types and water contents.…”