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

et al. (1991), and has since been utilized by several researchers (Tarara and Ham, 1997; Ren et al., 1999; The dual-probe heat-pulse (DPHP) technique is emerging as a Song et al., 1999; Campbell et al., 2002). These studies useful technique for measuring soil volumetric water content ( ). have shown that the DPHP technique can provide accuHowever, few published data are available regarding the performance of the DPHP technique under field conditions. The objective of this rate measurements of and change in ( ) in laborastudy is to evaluate the effectiveness of the DPHP technique for tory and greenhouse settings. measuring under field conditions. We used 24 DPHP sensors to However, evaluations of the effectiveness of the monitor in a soybean [Glycine max (L.) Merr.] field during the DPHP technique in the field have been limited. In one 2001 and 2002 growing seasons. The DPHP sensors demonstrated field study, Tarara and Ham (1997) compared meadurability in field conditions and clear sensitivity to temporal and surements from three DPHP sensors with measurespatial variations of at the scale of measurement. The mean ments made with a attenuation meter and found that measured by the DPHP sensors ( DPHP) was on average 0.040 m3 m 3 the two methods agreed to within 0.05 m3 m 3. In anlarger than the mean measured by soil sampling ( SS). The response other field study, Campbell et al. (2002) compared of the DPHP sensors was linear. Regressions of DPHP vs. SS yielded measurements from 10 DPHP sensors in a peat bog with r2 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 regresmeasurements from four water content reflectometer sion of 0.009 and 0.011 m3 m 3 at the two measurement depths. The sensors (CS615, Campbell Scientific Inc., Logan, UT).1 slopes of the regressions were 0.75 rather than 1.0. Errors in SS are They reported similar temporal patterns of but differa likely cause of this low slope. We shifted all the values for each ent mean values between the two types of sensors at sensor up or down by a constant value to make the first measurement the 5-cm depth. At the 30-cm depth, they reported simifrom each sensor equal determined from soil sampling near that lar mean values of but different temporal patterns of sensor at the time of installation. This simple matching point procedure between the two types of sensors. In total, we can improved the accuracy of the DPHP technique, resulting in a 0.024 find only two figures in two papers comparing DPHP m3 m 3 average difference between DPHP and SS. Also, the matching measurements with independent measurements in point procedure markedly reduced the variability between sensors, the field. More extensive field comparisons between reducing the average SD from 0.063 to 0.026 m3 m 3. This procedure requires no additional soil sampling and is recommended for field the DPHP technique and other accepted techniques for applications of the DPHP technique. measuring are needed to clearly define the effectiveness of the DPHP technique under field conditions. The objective of this study is to evaluate the effectiveness of the DPHP technique for measuring under field conM of in the vadose zone are often needed by researchers who study components of ditions. the terrestrial hydrologic cycle or who study the many THEORY biological, physical, and chemical processes that are influenced by . Measurements of are also often utilized Dual-probe heat-pulse sensors can be used to measure soil by irrigation managers in agriculture and horticulture. volumetric heat capacity (C), which is directly related to . A number of useful direct and indirect techniques for A brief heat pulse emitted from the heating needle of the measuring are available, each having characteristic DPHP sensor is transferred through the soil, resulting in a small temperature increase ( T) approximately 6 mm away strengths and weaknesses (Topp and Ferré, 2002). This at the sensing needle of the sensor. The maximum value of study focuses on the DPHP technique, an indirect techthis temperature increase ( Tm) is inversely related to C nique that enables automated, nondestructive measure(Campbell et al., 1991): ments of on a small volume of soil. The DPHP technique for measuring was first suggested by Campbell C q/( er 2 Tm) [1] where q is the heat output per unit length of the heater (J T.E. Ochsner, USDA-ARS, Soil and Water Management Research m ), e is the base of the natural logarithms, and r is the Unit, St. Paul, MN 55108; R. Horton, Dep. of Agronomy, Iowa State distance between the heating and sensing needles (m). VoluUniv., Ames, IA 50011; T. Ren, Inst. of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, metric heat capacity is related to by China 100101. Journal paper of the Iowa Agriculture and Home Economics Exp. Stn., Ames, IA, Project No. 3287. Supported by the 1 Mention of products and suppliers is for the convenience of the Soybean Research and Development Council, the Agronomy Dep. reader and implies no endorsement on the part of the authors or the Endowment Funds, the Hatch Act, and the State of Iowa. Received USDA-ARS. 14 Mar. 2003. Special Section—Advances in Measurement and MoniAbbreviations: , volumetric water content; DPHP, mean soil volumettoring Methods. *Corresponding author ([email protected]). ric water content measured by the dual-probe heat-pulse technique; SS, mean soil volumetric water content measured by soil sampling; Published in Vadose Zone Journal 2:572–579 (2003).  Soil Science Society of America AWG, American Wire Gauge; C, soil volumetric heat capacity; DPHP, dual-probe heat-pulse. 677 S. Segoe Rd., Madison, WI 53711 USA