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

The dual-probe heat-pulse (DPHP) method provides a means of estimating volumetric soil water content (theta) and change in volumetric water content (Delta theta) from measurements of volumetric heat capacity. The purpose of this investigation was to characterize the accuracy and precision that can be achieved in measuring theta and Delta theta with the DPHP method. Tempe pressure cells fitted with DPHP sensors were used to conduct desorption experiments in which DPHP-based estimates of theta and Delta theta 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 theta at low water contents, but it was shown that the bias could be removed by using an empirical calibration equation, Delta theta 1.09 theta(DPHP) - 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 m(3) m(-3). The general calibration equation was also shown to be effective in removing bias in Delta theta estimates. Pooled regression analysis (all soil materials) showed that theta can be measured with a root mean square error (RMSE) of 0.022 m(3) m(-3). Greater precision can be achieved with Delta theta measurements (RMSE = 0.012 m(3) m(-3)); however, the results indicated a decrease in precision with increasing magnitude of Delta theta.