Performance of TDR calibration models as affected by soil texture

Time domain reflectometry (TDR) is widely used to measure the soil water co ntents. The effect of soil composition on TDR calibration has to be quantif ied to diminish the need in re-calibration of TDR. Our objectives were: (a) to evaluate the ability of published models to fit TDR calibration data fo r the soils of different texture, and (b) to explore using a piecewise-cons tant distribution to approximate the continuous dependence of the dielectri c constant on the position of water within soil pore space. We calibrated T DR in sand-kaolin mixtures, dernovo-podzolic soil, gray forest soil, and le ached chernozem. Distinct changes in slopes of the calibration curves were observed for the fine-textured samples in the range of volumetric water con tents from 0.13 to 0.27. The ratio of the lack-of-fit mean square error and the mean square pure error was used as a statistic for the error compariso n to conclude whether a model provided a satisfactory fit of data. Publishe d models that did not have parameters to adjust fitted well data for coarse -textured mixtures and soils but failed to fit data for fine textured mixtu res and soils. The piecewise-constant distribution model with three adjusta ble parameters fitted well all data. Parameters of this model were related to clay content in the samples. (C) 1999 Elsevier Science B.V. All rights r eserved.