Author(s): D J Timlin, Ya A Pachepsky
A calibration of time domain reflectometry (TDR) probes to measure soil water in a particular soil is desirable since no consistent relationship between water content and apparent dielectric constant (Ka) has been found. We compared three procedures to compute Ka from TDR traces: a manual method that uses a plot of the wave trace, a derivative-based computer algorithm, and fitting a simulated TDR trace to a measured one with a simple multiple reflection model. We added a model of inertia to the multiple reflection model to approximate the rise time of the cable tester. The TDR traces were measured in situ with a Tektronix 1502B cable tester (Tektronix, Inc., Wilsonville, OR) on samples from two soils with contrasting textures, Beltsville silt loam (fine-loamy, mixed, mesic Typic Fragiudult) and Rumford loamy sand (coarse-loamy, siliceous, thermic Typic Normudult). Ten-centimeter probes with three rods were used. The water contents were measured gravimetrically with soil cores. Calibration with apparent dielectric constants obtained from the derivative-based algorithm had the smallest standard error for both soils, and the manual method was better than the wave simulation method. The intercepts and slopes of the calibration equations for the manual and derivative methods were not significantly different from each other for both soils. The wave simulation method, which gives physically meaningful values for Ka, and characteristic impedances can be a useful tool for theoretical studies.