COMPARISON OF 3 METHODS TO OBTAIN THE APPARENT DIELECTRIC-CONSTANT FROM TIME-DOMAIN REFLECTOMETRY WAVE TRACES

A calibration of time domain reflectometry (TDR) probes to measure soi l water in a particular soil is desirable since no consistent relation ship between water content and apparent dielectric constant (K-a) has been found. We compared three procedures to compute K-a from TDR trace s: a manual method that uses a plot of the wave trace, a derivative-ba sed computer algorithm, and fitting a simulated TDR trace to a measure d one with a simple multiple reflection model. We added a model of ine rtia to the multiple reflection model to approximate the rise time of the cable tester. The TDR traces were measured in situ with a Tektroni x 1502B cable tester (Tektronix, Inc., Wilsonville, OR) on samples fro m two soils with contrasting textures, Beltsville silt loam (fine-loam y, mixed, mesic Typic Fragiudult) and Rumford loamy sand (coarse-loamy , siliceous, thermic Typic Normudult). Ten-centimeter probes with thre e rods were used. The water contents were measured gravimetrically wit h soil cores. Calibration with apparent dielectric constants obtained from the derivative-based algorithm had the smallest standard error fo r both soils, and the manual method was better than the wave simulatio n method. The intercepts and slopes of the calibration equations for t he manual and derivative methods were not significantly different from each other for both soils. The wave simulation method, which gives ph ysically meaningful values for K-a, and characteristic impedances can be a useful tool for theoretical studies.