A review of time domain reflectometry (TDR) applications in porous media

Time domain reflectometry (TDR) is the most widely used non-destructive method to determine the water content of soils and other porous media. TDR equipment can be automated and multiplexed to acquire accurate and rapid waveforms (return signal) without safety concerns associated with radioactive methods (e.g., neutron probe and Gamma-ray probe). Two key steps are required for TDR applications: (1) Obtain and analyze TDR waveforms using travel-time and signal attenuation analysis to determine dielectric permittivity and electrical conductivity, respectively. (2) Calibrate to determine a new- or apply an existing (e.g., Topp et al. (1980)) relationship between the derived soil dielectric permittivity and the volumetric water content of the porous medium of interest. A majority of researchers and practitioners focus on step two and additionally develop new mathematical models to get better estimates of water content. Although there are reviews of TDR principles and applications in soil science, there is a lack of information on how TDR can disclose critical information in porous media beyond average soil water content. Therefore, we present a newly expanded review of TDR applications in porous media including soils, plants, snow, food stuffs, and concrete. We begin by reviewing TDR basics, including principles, probe design, commercially available equipment, and graphical and numerical methods as well as available software for waveform analysis. Applications of TDR to estimate volumetric water content in various types of porous media, the latest techniques available to derive spatial variability of soil water distributions along a single TDR probe are included, followed by TDR waveform based analyses to estimate electrical conductivity (EC), wetting/drying and freezing/thawing fronts, and snow depth. The combination of TDR measurements coupled with other methods (e.g., gypsum/ceramics and heat pulse method) to determine a wide range of soil physical properties (e.g., soil water retention curve, thermal properties, and hydraulic conductivity) and fluxes (e.g., soil heat flux, liquid water flux, and vapor flux) are also included. The study concludes with a discussion of limitations and future perspectives on various TDR applications.