A fast TDR-inversion technique for the reconstruction of spatial soil moisture content

Abstract

Abstract. Spatial moisture distribution in natural soil or other material is a valuably information for many applications. Standard measurement techniques give only mean or punctual results. Therefore a new inversion algorithm has been developed to derive moisture profiles along single TDR sensor-probes. The algorithm uses the full information content of TDR reflection data measured from one or both sides of an embedded probe. The system consisting of sensor probe and surrounded soil can be interpreted as a nonuniform transmission-line. The algorithm is based on the telegraph equations for nonuniform transmission-lines and an optimization approach to reconstruct the distribution of the capacitance and effective conductance along the transmission-line with high spatial resolution. The capacitance distribution can be converted into permittivity and water content by means of a capacitance model and dielectric mixing rules. Numerical investigations have been carried out to verify the accuracy of the inversion algorithm. Single-and double-sided time-domain reflection data were used to determine the capacitance and effective conductance profiles of lossless and lossy materials. The results show that singlesided reflection data are sufficient for lossless (or low-loss) cases. In case of lossy material two independent reflection measurements are required to reconstruct a reliable capacitance profile. The inclusion of an additional effective conductivity profile leads to an improved capacitance profile. The algorithm converges very fast and yields a capacitance profile within a sufficiently short time. The additional transformation to the water content requires no significant calculation time.

Determination of Hydraulic Conductivity Based on (Soil) - Moisture Content of Fine Grained Soils

“…Their crystal structure is responsible for a number of their characteristic chemical properties such as cation exchange capacity or high sorption capacity (Pusch, 2002). Hydraulic conductivity of swellable clayey and clay-enriched silty soils strongly depends on the density, the type of the adsorbed ions and the salinity of the percolating water (Scheffer, 1992). For example it is the high swelling properties that provide sodium bentonite's unique sealing qualities.…”

“…The pF-curve shows a characteristic course for each different soil (Scheffer, 1992). It depends on the particle size distribution, the formation and crosslinking of the pores and also of their size.…”

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full‐scale levee model

“…In order to reconstruct a water content profile from TDR measurements, a procedure has been developed consisting of three steps. This procedure is described in detail by Schlaeger [2005]. In the first step the spatially distributed transmission line parameters C(x) and G(x) are reconstructed with two independent measurements from both ends of the flat ribbon cable.…”

“…The maximal length depends on the surrounding material. For low-loss clean sand the typical maximal length is approximately 5 m. For lossy materials like clay soil or concrete the maximum length should be reduced to approximately 3 m. The two-parameter reconstruction used in the algorithm of Schlaeger [2005] explicitly takes into account losses by dissipation as one effect of the electrical conductivity.…”

“…[4] A recently developed reconstruction algorithm [Schlaeger, 2002[Schlaeger, , 2005 allows the fast computation of soil moisture profiles along elongated and electrically insulated probes from TDR signals, using either one-or two-ended measurements. The application of this method leads to an appreciable reduction in the number of probes required accompanied by a higher spatial resolution of the resulting moisture profiles.…”