The influence of time domain reflectometry rod induced flow disruption on measured water content during steady state unit gradient flow

Intrusive measurement techniques require placing a sensor within the sample, possibly changing the conditions under which the measurement is collected and thereby affecting the quality of the measurement. In this study we consider time domain reflectometry (TDR), which is an intrusive water content measurement method. TDR rods are impermeable, and thus water is forced to flow around the rods. In an unsaturated medium this changes the water content distribution in the vicinity of the rods, with the water content increased at the tops of the rods and decreased at the bottoms of the rods. TDR has nonuniform spatial sensitivity, with much higher sensitivity immediately adjacent to rods, in the regions that experience the greatest change in water content due to this flow disruption. Furthermore, the spatial sensitivity of TDR depends on the water content distribution within the sample volume. This raises the possibility that flow disruptions caused by TDR rods may affect the TDR-measured water content. In this study we are specifically interested in the effects of flow disruption due to TDR rods. Therefore we consider steady state unit gradient unsaturated flow in a homogeneous medium to eliminate spatial heterogeneity of water content due to soil heterogeneity and transient flow conditions. For common TDR probe designs in the wide range of soils examined, flow disruption gives rise to a water content measurement error that is less than 0.005 cm3 cm-3. This is smaller than the reported accuracy of the TDR method of 0.02 cm3 cm-3. As a result, we conclude that it is appropriate to ignore flow disruption caused by commonly used TDR probes when assessing sources of TDR measurement error.