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SOIL PHYSICS

A Partial Cylindrical Thermo-Time Domain Reflectometry Sensor

^a Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108
^b USDA-ARS, Soil and Water Management Research Unit, St. Paul, MN 55108

^* Corresponding author (ochsner{at}umn.edu).

Thermo-time domain reflectometry (T-TDR) sensors can be used to measure soil thermal properties and water content, and to obtain indirect estimates of bulk density and air-filled porosity; however, the small size and sensitivity to needle deflection of the conventional T-TDR sensor limit its accuracy, precision, and durability. The objective of this study is to change the size and geometry of this sensor to improve accuracy and precision and to better withstand the stress of field use. The new partial cylindrical design features opposing curved heaters with a central temperature-sensing needle and is more than double the size of conventional T-TDR sensors. The partial cylindrical design virtually eliminates sensitivity to needle deflection. Laboratory testing indicated that the new sensor was capable of accurately measuring soil water content and thermal properties. Comparison with literature values showed that this sensor performed as well as, or better than, conventional sensors in all areas except volumetric heat capacity estimates. Qualitative assessment indicated that this type of sensor should be less prone to distortion during field use.

Abbreviations: T-TDR, thermo-time domain reflectometry