Use and calibration of a wall-assimilated TDR probe for various soils

Time domain reflectometry (TDR) is a simple method for measuring soil water content. The TDR measures the average of the dielectric permittivity of the soil along and around a probe, leading to an oval sampling volume. Using Topp's equation, one can translate the dielectric permittivity to water content. In some cases the presence of TDR in-situ is not desired as it can influence the flow regime and create biased measurement above or underneath the probe that in turn alter the process under investigation. We present here an assimilated three rod TDR probe, embedded in a Perspex wall under laboratory conditions. The measurement of such configuration averages of the dielectric permittivity of the Perspex wall and the dynamic dielectric permittivity of the soil. The shape of the sampling volume is oval around the probe, where its exact size and shape depends on the rods configuration. A laboratory experiment was conducted, comparing assimilated TDR measurement with that of a regular TDR probe. For this experiment a Perspex box was build with dimensions of 10x10x20 cm. In one of its walls three half circular slots of 15 cm long and a diameter of 0.3 cm where carved with 1.8 cm in between the slots. In each slot a stainless steel rod was assimilated. The TDR electrode was connected to TDR100 system. Five different soils were explored (sand, pure quarts, Hamra (red loamy sand), loess, and clayey soil). Each soil was packed manually in the customized measurement chamber. A measurement was then taken by the assimilated electrode and then by the "traditional" reference electrode. The waves where saved, and analyzed later using WinTDR. For each soil different water content and three repetitions starting at 5% volumetric water content and up to saturation were tested. A linear relation between the water content as measured by the assimilated probe and the free probe was found for all the lighter soils (clayey soil is analyzed at time of abstract submission) and the same relation for the dielectric permittivity. The measurement of the assimilated probe was biased toward the a lower value as it made an average of the changing water content in the soil from one side and the Perspex wall from the other, which value stayed constant ( the Perspex dielectric permittivity is 3 F/m). It should be noted that all four soils showed similar relation and the values fitted on one curve for all the light soils. Therefore, it is concluded that such a measurement can be a reliable tool and that by careful control of exact location the embedded probe can be used as a reliable minimally invasive measurement tool. This wall-assimilated TDR probe is currently being used in experimental work carried out in Helle Shaw chambers, enable direct measurements of water content in different sections of the soil-packed chamber.