Use of the Dual-Probe Heat-Pulse Technique to Monitor Soil Water Content in the Vadose Zone

Abstract

<p>The dual-probe heat-pulse (DPHP) technique is emerging as a useful technique for measuring soil volumetric water content (θ). However, few published data are available regarding the performance of the DPHP technique under field conditions. The objective of this study is to evaluate the effectiveness of the DPHP technique for measuring θ under field conditions. We used 24 DPHP sensors to monitor θ in a soybean [<em>Glycine max</em> (L.) Merr.] field during the 2001 and 2002 growing seasons. The DPHP sensors demonstrated durability in field conditions and clear sensitivity to temporal and spatial variations of θ at the scale of measurement. The mean θ measured by the DPHP sensors (θ_DPHP) was on average 0.040 m³ m⁻³ larger than the mean θ measured by soil sampling (θ_SS). The response of the DPHP sensors was linear. Regressions of θ_DPHP vs. θ_SS yielded <em>r</em> ² values of 0.949 and 0.843 at depths of 7.5 and 37.5 cm. The DPHP technique showed good resolution with RMSE values for the regression of 0.009 and 0.011 m³ m⁻³ at the two measurement depths. The slopes of the regressions were 0.75 rather than 1.0. Errors in θ_SS are a likely cause of this low slope. We shifted all the θ values for each sensor up or down by a constant value to make the first θ measurement from each sensor equal θ determined from soil sampling near that sensor at the time of installation. This simple matching point procedure improved the accuracy of the DPHP technique, resulting in a −0.024 m³ m⁻³ average difference between θ_DPHP and θ_SS Also, the matching point procedure markedly reduced the variability between sensors, reducing the average SD from 0.063 to 0.026 m³ m⁻³ This procedure requires no additional soil sampling and is recommended for field applications of the DPHP technique.</p>