Improved Interpretation of Water Content Reflectometer Measurements in Soils

calculated permittivity values for the sand compare reasonably well with Topp’s equation. The permittivity in the sandy loam to silt loam Of these, transmission line oscillators are particularly soils is overestimated by as much as 104 dimensionless permittivity interesting because they operate in the time domain, units. The overestimated permittivity values are due to dielectric dis- like TDR, but do not require a separate pulse and sampersion and ionic conductivity, brought about by the low effective plingunit.Transmissionlineoscillatorsgenerateconsecfrequency in the electromagnetic pulse of the sensors as compared utivevoltage pulsesfrominside theprobe headwhereby with standard TDR. The performance of the reflectometers may be the arrival of the reflected pulse triggers the next pulse. improved by increasing the frequency of operation of the sensors from The output is a square wave with a frequency that is 175 MHz to 1 GHz. At higher frequencies, the sensors become proportional to the number of reflections per second. less sensitive to ionic conductivity. Furthermore, dielectric dispersion These sensors can be read directly by a datalogger, rebecomes less of an issue at higher frequencies, thereby increasing the sulting in a low price per unit. Transmission line oscillaapplicability of existing permitivity-water content relationships such as Topp’s equation.

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