Field Evaluation of the Dual-Probe Heat-Pulse Method for Measuring Soil Water Content

Abstract

The dual-probe heat-pulse (DPHP) method is useful for measuring water content (θ) and change in water content (Δθ) 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 θ and Δθ at 3-h intervals for 3 mo. In addition, θ was estimated by coupling Δθ measurements with independent measurements of θ obtained by soil sampling at sensor installation. Additional soil samples were collected from each location during the monitoring period to provide independent measurements of θ. Regression of DPHP and independent θ 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 θ from Δθ 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 Δθ approach combined with use of the empirical calibration appears to have practical utility.