Improved interpretation of water content reflectometer measurements in soils

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.