Evaluation of the Performance of TDR and Capacitance Techniques for Soil Moisture Measurement

Though researchers have employed various techniques (gravimetric, electromagnetic, neutron scattering, heat pulse, microwave, and optical remote sensing techniques) for soil moisture measurement, dielectric-based techniques (Time Domain Reflectometry [TDR] and capacitance technique [CT]) have gained much more popularity, mainly due to revolutionary developments in the fields of electronics and data communication systems. However, the suitability and relative performance of these techniques for moisture measurement of soils is a point of debate. Hence, in order to address this issue, extensive studies were conducted on soils of entirely different characteristics, compacted at various compaction states (dry densities and water contents) by employing TDR and capacitance probes. Subsequently, the dielectric constant of the soil and its bulk electrical conductivity were obtained using these probes and compared against each other and those computed from Topp's equation, which is a well-established relationship between the dielectric constant of the soil and its volumetric moisture content. An attempt was also made to correlate Ka values obtained from the dielectric techniques and Topp's equation with those of the Time Propagation (TP) Mixing model, which incorporates the properties of the soil matrix as well. It has been observed that the Ka-TDR matches well with the Ka-Topp and Ka-TP, while the best match has been observed between Ka-TDR and Ka-Topp as compared to the Ka-CT. As such, the study demonstrates clearly that Topp's equation, which ignores the soil-specific parameters, is capable of determining the soil moisture content appropriately. This study proposes an empirical equation that relates dielectric constants obtained from Topp's equation to those obtained from TDR, CT, and the TP Mixing model. Such a relationship can be further utilized for estimating the volumetric soil moisture content