Field evaluation of the dual-probe heat-pulse method for measuring soil water content

The dual-probe heat-pulse (DPHP) method is useful for measuring water content (theta) and change in water content (delta theta) near the soil surface. The method has been evaluated in laboratory and greenhouse experiments, but not in a field setting. Our objective was to test the DPHP method under field conditions and for a range of soil properties. Twenty-five DPHP sensors and five monitoring stations were constructed and installed at five locations in northeastern Kansas to measure theta and delta theta at 3-h intervals for 3 mo. In addition, theta was estimated by coupling delta theta measurements with independent measurements of theta obtained by soil sampling at sensor installation. Additional soil samples were collected from each location during the monitoring period to provide independent measurements of theta. Regression of DPHP and independent theta measurements revealed slight bias but substantial offset error (about 0.1 m3 m-3) in the DPHP method. The offset error could not be fully attributed to bias in any single input parameter, but could have been caused by a combination of biased parameters. Estimates of theta from delta theta measurements also revealed slight bias, but offset error was considerably smaller. Use of a published empirical calibration for DPHP sensors almost completely eliminated this bias and further reduced the offset error to approximately 0.01 m3 m-3. Thus, the delta theta approach combined with use of the empirical calibration appears to have practical utility.