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

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 [ Glycine max (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 m 3 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 r 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 m 3 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 m 3 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 m 3 m −3 This procedure requires no additional soil sampling and is recommended for field applications of the DPHP technique.