Determination of moisture content in a deformable soil using time-domain reflectometry (TDR)

Time-domain reflectometry (TDR) is being used increasingly for measuring th e moisture content of porous media. However, successful application for mea suring water in soil has been limited to non-deformable soils, and it would be a valuable extension of the technique if it could be used for soils tha t shrink on drying. We have recently investigated its application to soils rich in clay and organic matter and peats. Here we propose a method for det ermining moisture content in deformable soils based on the relation between the dielectric constant, K, and the volumetric moisture content, Theta, me asured by TDR. Parallel TDR probes with a length of 15 cm and a spacing of 2 cm were place d horizontally in soil cores with a diameter of 20 cm and height of 10 cm t aken from a forest. The soil is very porous with large proportions of both silt and clay. The sample weight and travel time of the electromagnetic wav e guided by parallel TDR probes were simultaneously measured as a function of time, from saturation to oven-dryness during which the core samples shra nk considerably. Vertical and horizontal components of shrinkage were also measured to take the air-exposed region of TDR probe into account in the de termination of K. The effect of deformation on volumetric moisture content was formulated for two different expressions, namely actual volumetric mois ture content (AVMC) and fictitious (uncorrected) volumetric moisture conten t (FVMC). The effects of air-exposure and expressions of volumetric moistur e content on the relation between K and Theta were examined by fitting the observations with a third-order polynomial. Neglecting the travel time in t he air-exposed part or use of the FVMC underestimated the Theta for a given K. The difference was more pronounced between AVMC and FVMC than between t wo different dielectric constants, i.e. accounting for air-exposure, K-ac, and not accounting for air-exposure, K-au. When the existing empirical mode ls were compared with the fitted results, most underestimated the relation based on the AVMC. This indicates that published empirical models do not re flect the effect of deformation on the determination of Theta in our forest soil. Correct use of the Theta expression has more impact on determining m oisture content of a deformable soil than the accommodation of travel time through the air-exposed region of TDR probe.