Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2005
Publication Date: 11/20/2005
Citation: Kelleners, T.J., Seyfried, M.S., Blonquist, J.M., Bilskie, J., and Chandler, D.G. 2005. Improved interpretation of water content reflectomenter measurements in soils. Soil Science Society of American Journal 69:1684-1690. Interpretive Summary: Measurement of soil water content is useful for a variety of applications including irrigation management, pollution abatement and analysis of snow resources. In the 1980’s time domain reflectometry, which relates measured soil electrical properties to water content, was established as the best technology for electronic monitoring of soil water content. Due to the high cost and difficulties in data analysis, a variety of less expensive, more easily used soil water sensors based on the same measurement principles, have been developed for commercial applications. It has been found, however, that the performance of these instruments is often much worse than that of time domain reflectometry. Improved accuracy, via calibrations, and improved instrument design, require a better understanding of what the instruments actually measure and why their performance is often relatively poor. In this study we investigated one popular example of these sensors, the water content reflectometer (WCR). We demonstrated, using principles of the sensor operation, how WCR measurements can be directly related to soil electric properties. This improved understanding of how the WCR works will enable improved, more general, calibration of the instrument as well as improved interpretation of data currently collected with the instrument. This work also provides a basis for evaluating future soil water measurement sensors.
Technical Abstract: Water content reflectometers use time domain reflectometry (TDR) to estimate the apparent permittivity of soil, which in turn can be related to the soil water content. The objective of this study is to develop a physical model for water content reflectometers. The length of the sensor rods and the delay time of the circuitry in the probe head are the two unknown parameters. The two parameters are determined both analytically, using sensor readings in air and deionized water, and by optimization, using air and non-conductive fluids. The calibrated parameters are used to calculate the apparent permittivity as a function of water content for sensor readings in five soils, ranging from sand to silt loam. Calculated permittivity values are compared with Topp's permittivity-water content relationship. Results show that the calculated permittivity values for the sand compare reasonably well with Topp's equation. The permittivity in the sandy loam to silt loam soils is overestimated by as much as 104 dimensionless permittivity units. The overestimated permittivity values are due to dielectric dispersion and ionic conductivity, brought about by the low effective frequency in the electromagnetic pulse of the sensors as compared with standard TDR. The performance of the reflectometers may be improved by increasing the frequency of operation of the sensors from <175 MHz to >1 GHz. At higher frequencies, the sensors become less sensitive to ionic conductivity. Furthermore, dielectric dispersion becomes less of an issue at higher frequencies, thereby increasing the applicability of existing permitivity-water content relationships such as Topp's equation.