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Published online 3 October 2006
Published in Vadose Zone J 5:1073-1075 (2006)
DOI: 10.2136/vzj2006.0062
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Response to "Comments on ‘TDR Laboratory Calibration in Travel Time, Bulk Electrical Conductivity, and Effective Frequency’"

Steven R. Evett, J.A. Tolk and T.A. Howell

Soil and Water Management Research Unit, USDA-ARS, P.O. Drawer 10, 2300 Experiment Station Road, Bushland, TX 79012

Figure 1
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  		Fig. 1. Plot of a waveform and its first derivative from a Tektronix 1502C TDR cable tester set to begin at –0.5 m (inside the cable tester). The voltage step is shown to be injected just before the zero point (BNC connector on instrument front panel). At 3 m from the instrument, a TDR probe is connected to the cable. The relative voltage levels, VI, Vmin, V0, and VF are used in calculations of the bulk electrical conductivity of the medium in which the probe is inserted, and for determining the probe characteristic impedance. Waveform positions for determining values of these parameters are described numerically in Evett (2000a, 2000b, 2000c) where V02 was used for V0.

 

Figure 2
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  		Fig. 2. Probe impedance calculated using Eq. [3] as a function of cable length and the resulting cell constant values. Data from Evett et al. (2005) for 6.4 to 10 m, and new data for 2 and 3 m of cable.

 

Figure 3
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  		Fig. 3. Bulk electrical conductivities calculated: (i) using the methods of Evett et al. (2005), (ii) using the method of Evett et al. (2005) but with a constant characteristic probe impedance of 244 {Omega}, and (iii) using the methods of Evett et al. (2005) but with VR rather than V0 in the calculation of the reflection coefficient.

 




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