A numerically based analysis of the sensitivity of conventional and alternative time domain reflectometry probes

Conventional time domain reflectometry (TDR) probes are comprised of two or three parallel metal rods. Other probes have been designed for water conte nt profiling [Hook ef al., 1992; Ferre et al., 1998b; Redman and DeRyck, 19 94], surface water content measurement [White and Zegelin, 1992; Selker et nl., 1993], or measurement in electrically conductive media. We use the num erical approach of Knight et nl. [1997] to predict the responses of variant s of these probes when surrounded by materials with different relative diel ectric permittivities. These predictions are compared with published calibr ation curves and analytical solutions where available. Conventional rods ar e shown to be most sensitive to changes in the water content of the medium. The Hook et al. [1992] probe shows the highest sensitivity of the alternat ive designs; both surface probes can be used to measure the water content a t the soil surface nonintrusively with similar sensitivities. All of the al ternative probes have sensitivities that vary with the soil water content, leading to incorrect averaging of the water content if the water content va ries along the probes. However, those probes that place nonmetallic compone nts in series with the soil have more pronounced errors than those that pla ce these materials and the soil more nearly in parallel.