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Development of Thermo-Time Domain Reflectometry for Vadose Zone Measurements

^a Inst. of Geographic Sci. and Natural Resources Research, Chinese Academy of Sci., Beijing, China 100101
^b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

Correspondence: ^* Corresponding author (rhorton{at}iastate.edu).

Received for publication 14 March 2003. Soil and environmental studies in the vadose zone are often restricted by the lack of equipment that can measure the variation of soil physical parameters in space and time. In this paper we demonstrate that thermo-time domain reflectometry (thermo-TDR) can be used for a wide range of soil physical measurements. The thermo-TDR probe combines TDR and heat-pulse technologies into a single probe. It allows measurements of volumetric soil water content (), temperature (T), electrical conductivity (EC), thermal conductivity (), thermal diffusivity (), and volumetric heat capacity (c) at the same sampling positions simultaneously. Furthermore, other soil physical parameters, such as bulk density (_b), air-filled porosity (n_a), and degree of saturation (S), can be determined from their relationships with c and . We examined the performance of the thermo-TDR using both published data and laboratory measurements on packed columns and intact cores from six soils of varying texture. The results show that the thermo-TDR provides reliable measurements of , EC, c, , n_a, and S, but relatively large errors exist in _b. The average standard error between thermo-TDR measurements and gravimetric measurements is 0.026 m³ m^-3 for , 0.050 m³ m^-3 for n_a, 0.069 for S, and 0.134 Mg m^-3 for _b. The standard error between thermo-TDR measurements and theoretical predictions of c is 0.134 MJ m^-3 K^-1. These promising findings coupled with the characteristics of small probe size and easy automation make the thermo-TDR sensor an ideal tool for studying coupled flow processes in the vadose zone. Further improvements in probe design, waveform interpretation, and determination of effective probe length are also noted as steps toward improving accuracy and precision of future thermo-TDR measurements.

Abbreviations: EC, electrical conductivity • TDR, time domain reflectometry

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