Submitted to: Vadose Zone Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/26/2005
Publication Date: 11/16/2005
Citation: Evett, S.R., Howell, T.A., Tolk, J.A. 2005. Time domain reflectometry laboratory calibration in travel time, bulk electrical conductivity, and effective frequency. Vadose Zone Journal. 4:1020-1029. Interpretive Summary: Accurate soil water content measurements are essential for determination of crop water use rates, and for studies to increase the efficiency of irrigation applications and to increase crop water use efficiency. These are key elements of saving the nation’s water supply for beneficial use. New methods, which measure the electrical properties of soil in order to estimate the water content, are of uncertain accuracy. In the context of a larger laboratory and field study of these methods, we calibrated a method called time domain reflectometry (TDR) for use as a reference method against which the others would be compared. To ensure that the results would be applicable over a broad region, we used soil types that are typical of silty and clayey agricultural soils of the Great Plains as well as a soil that was typical of the caliche layer of subsoil underlying these soils. We included new parameters in the calibration equation to reduce influences of changes in soil temperature, conductivity (salinity) and clay type and content. The resulting calibrated measurement system was six times less sensitive to temperature variations than was the standard calibration, and a single calibration equation worked for all three soils with accuracy that was three times as good as that of the standard calibration. The resulting measurement system is of an accuracy that will allow it to be used as a reference system against which others can be compared with confidence.
Technical Abstract: 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. Newer methods, based on electromagnetic (EM) measurements, typically allow data logging and unattended operation, but with uncertain precision and accuracy. We compared and calibrated four EM devices that operate in access tubes, using conventional time domain reflectometry (TDR) as a reference method. This paper describes calibration of the reference TDR system. Measurements were made before, during and after wetting to saturation in triplicate re-packed columns of three soils: a silty clay loam, a clay, and a calcic clay loam containing 50% CaCO3. Each 55-cm diameter column was weighed continuously to 50-g precision. Conventional TDR measurements of water content and thermocouple measurements of temperature were made at eight depths in each column every 30 min. The TDR system was accurate to <0.03 m**3/m**3 using the Topp equation; but there were differences in accuracy between the three soils, and there was some temperature dependency at the saturated end, though not at the dry end. This paralleled the temperature dependency of the soil bulk electrical conductivity (BEC) and the effective frequency. Incorporation of bulk electrical conductivity and effective frequency of the TDR measurement into the calibration model improved the calibration error to <0.01 m**3/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.