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SPECIAL SECTION: SOIL WATER SENSING

Time Domain Reflectometry Laboratory Calibration in Travel Time, Bulk Electrical Conductivity, and Effective Frequency

Soil and Water Management Research Unit, Conservation & Production Research Laboratory, USDA-ARS, Bushland, TX 79012
^* Corresponding author (srevett{at}cprl.ars.usda.gov)

Received 21 March 2005.

Accurate soil water content measurements to considerable depth are required for investigations of crop water use, water use efficiency, irrigation efficiency, and the hydraulic properties of soils. Although the soil moisture neutron probe has served this need well, it cannot be used unattended. Newer methods, which respond to the electrical properties of soils, typically allow data logging and unattended operation, but with uncertain precision, accuracy, and volume of sensitivity. In laboratory columns of three soils, we calibrated a conventional time domain reflectometry (TDR) system for use as a reference system for a companion study of water content sensors that are used in access tubes. Measurements were made before, during, and after wetting to saturation in triplicate repacked columns of three soils: (i) a silty clay loam (30% clay, 53% silt), (ii) a clay (48% clay, 39% silt), and (iii) a calcic clay loam (35% clay, 40% silt) containing 50% CaCO₃. Each 75-cm-deep, 55-cm-diameter column was weighed continuously to 50-g precision on a platform scale. Conventional TDR measurements of water content and thermocouple measurements of temperature were made at eight depths in each column every 30 min. Accuracy of the TDR system was judged by the root mean squared difference (RMSD) between column mean water contents determined by mass balance and those determined using the Topp equation as a standard calibration. Smaller values of the RMSD metric indicated more accurate standard calibration. Although the TDR system exhibited RMSD <0.03 m³ m^–3 using the Topp equation, there were differences in accuracy between the three soils, and there was some temperature dependency at the saturated end, although not at the dry end. This paralleled the temperature dependency of the soil bulk electrical conductivity (BEC). Incorporation of bulk electrical conductivity and effective frequency of the TDR measurement into the calibration model reduced the calibration RMSE to <0.01 m³ m^–3 and practically eliminated temperature effects. Because the temperature effects on the TDR measurement are embedded in the BEC and effective frequency, a measurement of temperature is not needed to apply the calibration to these soils.

Abbreviations: BEC, bulk electrical conductivity • EM, electromagnetic • IAEA, International Atomic Energy Agency • NIST, U.S. National Institute of Standards and Technology • NMM, neutron moisture meter • RMSD, root mean squared difference • TDR, time domain reflectometry

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