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DIVISION S-1 - SOIL PHYSICS

Time Domain Reflectometry Sensitivity to Lateral Variations in Bulk Soil Electrical Conductivity

^a Dep. of Environmental Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^b Dep. of Communication Technology, Aalborg Univ., Fredrik Bajersvej 7, DK-9000 Aalborg, Denmark

* Corresponding author (i5hn{at}civil.auc.dk)

It has been assumed, but not proved, that the spatial weighting and sample volume of Time domain reflectometry (TDR) regarding bulk soil electrical conductivity (EC_b) equal those determined for the relative dielectric permittivity (K). In this study, two types of experiments were carried out: (i) sensitivity experiments where the cumulative vertical weighting as a function of distance between the probe rods and the soil surface were determined for both K and EC_b at two different soil water contents and (ii) solute diffusion experiments to evaluate the discrepancies between actual and TDR-measured relative EC_b in the case where a steep gradient in EC_b passes the probe. Three-rod TDR probes and a sandy loam soil were used in both experiments. The sensitivity experiments confirmed that TDR weights K and EC_b equally in the transverse plane. Hence, previously established relationships to calculate the weighting function and sample area of a given TDR-probe geometry apply for both K and EC_b. The diffusion experiments showed that if a steep vertical solute gradient passes a horizontally inserted TDR probe, the TDR-measured EC_b profile will be more spread than the actual profile passing the probe. This phenomenon of artificial (TDR-induced) diffusion is caused by (i) TDR probes not representing a point measurement, and (ii) the nonlinear weighting of K and EC_b in the transverse plane. As the steepness of the solute concentration profile diminishes the TDR-induced diffusion decreases rapidly and becomes negligible for most applications. The effect of TDR-probe geometry on the discrepancy between actual and TDR-measured relative concentration was examined theoretically for selected two- and three-rod probe configurations.

Abbreviations: DC, direct current • D_p, solute diffusion coefficient • EC, relative electrical conductivity • EC_b, bulk soil electical conductivity • EM, electromagnetic • h, height • G(x, y), relative sensitivity function • K_a, apparent relative dielectric permittivity • K, relative dielectric permittivity • TDR, time domain reflectometry • W(h, B, D), cumulative vertical weighting function • w(x, y), spatial weighting function • w(y), vertical weighting function • Relative EC_b, TDR-measured relative EC_b distrubution • , volumetric soil water content • , the dimensionless radius