The influence of tdr-rod-induced flow disruption on measured water content during steady-state flow

Abstract

Intrusive measurement techniques require placing a sensor within the sample, possibly changing the conditions under which the measurement is collected. Time domain reflectometry (TDR) 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 probe. TDR is most sensitive in the region that experiences the greatest change in water content due to flow disruption. A numerical approach was used to examine the changes in water content in the vicinity ofTDR rods during steady-state unsaturated flow. 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 /cm3. This is smaller than the reported accuracy of the TDR method ofO.Ol cm3/cm3 (Topp, 1982). However, custom TDR probes with small separations and larger diameters or probes attached to impermeable probe bodies could lead to significant errors in water content measurement during steady-state unsaturated flow.