Derived functions of time domain reflectometry for soil moisture measurement

This paper gives a systematic framework for evaluating the time domain reflectometry (TDR) response of soil. TDR measures the soil composite dielectric constant ( K a ); thus any factor that has an influence on K a measurement can affect soil moisture determination. On the basis of known dielectric constants of air, soil, water, and ice, as well as the volumetric fraction of each phase in a bulk soil, functional relationships between volumetric water content (θ υ , cm 3 /cm −3 ) and K a are derived in the form of θ v = aK a 0.5 + b . In general, the TDR functions derived from this study are in good agreement with the “universal equation” developed through experiments by Topp et al . [1980] and other available TDR calibration data. This paper demonstrates that (1) soil solid and porosity have little effect on dielectric constant measurement; (2) temperature has a minor influence except for very wet soils; and (3) surface area is an important factor affecting the water content measurement.