SOIL-MOISTURE CALIBRATION OF A TDR MULTILEVEL PROBE IN GRAVELY SOILS

A recently commercially available probe design employs remote diode sh orting and differential waveform detection to create a segmented time domain reflectometry (TDR) probe using two parallel rods, The objectiv e of this study was to investigate the accuracy of the multilevel prob e in a laboratory soil column and in a gravely soil in the field, We o utlined two procedures for calibrating the TDR probe, One technique us es physically based parameters, whereas the second type of calibration applies a curve-fitting process relating volumetric water content to measured travel times in the bulk soil, We show that instrument-relate d water content errors of the TDR are less than 0.010 cm(3) cm(-3), If the physically based calibration approach is used, probe and segment- specific calibration yields an accuracy of about 0.015 cm(3) cm(-3) or less, However, we also show that excellent calibration results are ob tained by simple calibration using segment-specific corrected bulk soi l travel times, in which case a standard error of estimate of volumetr ic water content of about 0.015 cm(3) cm(-3) is obtained for both labo ratory and field measurements, If TDR probes are combined and each dep th interval is calibrated separately, as is usually done for neutron p robe calibration, the standard error of estimate is 0.020 cm(3) cm(-3) or less, with R-2 values ranging from 0.63 to 0.92.