Time domain reflectometry: development of a model to determine volumetric water content in organic substrates.

摘要

Time domain reflectometry (TDR) technique has been widely used to determine volumetric water content ( theta v) in soils during the last two decades. The TDR utilizes the apparent dielectric constant (Ka) for the estimation of theta v. The Ka- theta v relationship has been developed using empirical relationships as proposed by Topp et al. (1980) for mineral soils and Paquet et al. (1993) for organic substrates. Topp's equation is inefficient to determine theta v in organic substrates and Paquet's equation is valid up to an apparent dielectric constant of 55. However, when substrates are saturated or near saturation, Ka exceeds 55 for most soilless mixes. This study aims to develop an empirical model to determine with accuracy the volumetric water content of organic substrates from measurements of water dielectric constant. In this study, thirty different organic substrates were investigated. Substrates were potted in PVC cylinders (200 mm high with a 200 mm i.d.) and then subjected to saturation. After saturation, the cylinders were first drained by gravity and then further drained by placing them on tension tables. Several measurements of Ka and theta v were performed on each cylinder during the draining process. A polynomial cubic curve was fitted to the observed data (R2=0,97). In comparison with existing models (Topp and Paquet), the proposed model is more efficient in determining the volumetric water content of organic substrates. Since this model has been developed only from organic substrates, it is best adapted for soilless mixes. By applying the proposed model, it is possible to determine theta v at water dielectric constant higher than 55 (near saturation point of organic substrates), which is frequently observed in soilless mixes.