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

to evaluate the level of accuracy and precision that can be achieved with the DPHP method. Tarara and Ham The dual-probe heat-pulse (DPHP) method provides a means of (1997) conducted a laboratory experiment in which estimating volumetric soil water content ( ) and change in volumetric water content ( ) from measurements of volumetric heat capacity. DPHP-based estimates were compared with values The purpose of this investigation was to characterize the accuracy and obtained by the gravimetric method. They found that precision that can be achieved in measuring and with the DPHP the two methods agreed to within 0.03 and 0.04 m3 m 3 method. Tempe pressure cells fitted with DPHP sensors were used for two different soil materials over a water content to conduct desorption experiments in which DPHP-based estimates range of 0.10 to 0.45 m3 m 3. Estimates of for the of and were compared with values estimated by the gravimetric two methods agreed to within 0.01 m3 m 3 for both soils. method. For water contents corresponding to soil water pressure poSong et al. (1998, 1999) evaluated the DPHP method in tentials below 100 kPa, comparisons were made by packing the greenhouse experiments involving containers that were pressure cells with soil wetted to known water contents. The investigafitted with multiple DPHP sensors. Their results showed tion was conducted with seven soil materials representing a wide range that the DPHP method measured average to within of physical properties for mineral soils. The DPHP sensors slightly overestimated at low water contents, but it was shown that the bias 0.02 to 0.03 m3 m 3 and average to within 0.01 m3 m 3. could be removed by using an empirical calibration equation, Campbell et al. (2002) and Noborio et al. (1996) also 1.09 DPHP 0.045. This relationship appears to be general inasmuch evaluated the DPHP method for measuring and . as it was shown to be applicable for all seven soil materials and for Campbell et al. (2002) inserted DPHP sensors in undiswater contents ranging from 0.02 to 0.59 m3 m 3. The general calibraturbed peat cores obtained from two peat bogs in New tion equation was also shown to be effective in removing bias in Zealand. The DPHP-based estimates were compared estimates. Pooled regression analysis (all soil materials) showed that with values obtained by the gravimetric method for a can be measured with a root mean square error (RMSE) of 0.022 water content range of approximately 0.15 to 0.90 m3 m3 m 3. Greater precision can be achieved with measurements m 3. Regression analysis with estimates obtained by (RMSE 0.012 m3 m 3); however, the results indicated a decrease the two methods showed the DPHP method to be unbiin precision with increasing magnitude of . ased. The analysis also revealed that excellent precision was achieved with the DPHP method; however, precision was not quantified using a regression RMSE. The T DPHP method provides an effective means of evaluation performed by Noborio et al. (1996) revealed measuring soil water content and changes in soil poor precision in estimates obtained by the DPHP water content (Campbell et al., 1991; Bristow et al., method, but it is likely that their estimates were ad1993; Noborio et al., 1996; Tarara and Ham, 1997; Song versely affected by excessive deflection of the probes et al., 1998; Bristow, 1998; Song et al., 1999; Bristow of their DPHP sensor. et al., 2001; Campbell et al., 2002). The method, first The objective of this experiment was to provide a proposed by Campbell et al. (1991), utilizes a sensor to thorough assessment of the accuracy and precision of obtain measurements of the soil volumetric heat capacthe DPHP method for measuring and . To maximize ity. Volumetric water content ( ) and change in voluthe applicability of the results, experiments were conmetric water content ( ) are then estimated from the ducted with soil materials having a range of physical linear relationship between heat capacity and water properties. Specifically, soil materials were chosen to content. yield a range of textures, organic matter contents, bulk Dual-probe heat-pulse sensors have been widely used densities, and specific heats. The experiment was patfor routine and measurement in field experiments terned after the laboratory experiment of Tarara and (Bremer et al., 1998; Ham and Knapp, 1998; Bremer Ham (1997); however, improved techniques were used and Ham, 1999; Bremer et al., 2001; Campbell et al., in the design and construction of both the DPHP sensors 2002); however, few investigations have been conducted and the data acquisition and control system (DACS). J.M. Basinger, Plant and Soil Science Department, Texas Tech University, Lubbock, TX 79409; G.J. Kluitenberg, J.M. Ham, and M.B. KirkTHEORY ham, Department of Agronomy, Kansas State University, Manhattan, Campbell et al. (1991) proposed a sensor with two KS 66505; J.M. Frank, Rocky Mountain Research Station, U.S. Forest Service, 240 W. Prospect Rd., Fort Collins, CO 80526; P.L. Barnes, parallel, cylindrical probes. One probe contained a therDepartment of Biological and Agricultural Engineering, Kansas State mocouple and the other contained enamel-coated resisUniversity, Manhattan, KS 66505. Contribution no. 03-204-J from tance wire that was used to introduce a heat impulse. By the Kansas Agric. Exp. Stn., Manhattan, KS. Received 7 Dec. 2002. assuming that the sensor approximates instantaneous *Corresponding author ([email protected]). Abbreviations: DACS, data acquisition and control system; DPHP, Published in Vadose Zone Journal 2:389–399 (2003).  Soil Science Society of America dual-probe heat-pulse; PVC, polyvinyl chloride; RMSE, root mean square error. 677 S. Segoe Rd., Madison, WI 53711 USA