Open access publications in the SLU publication database

Abstract

In the monitoring of soil water Time-Domain Reflectometry (TDR) has gained widespread use. TDR has proved to be useful both in determination of soil water content and soil bulk electrical conductivity. These measurements are, however, complex and there are many sources of errors to consider. The purpose of this investigation is therefore to identify errors, the causes of these errors and to suggest improvements. This was achieved by a literature study as well as by two experiments, one conducted in the field and one conducted in the laboratory. Four TDR-systems were tested. The results show that errors can be classified in two groups, errors which is influencing the determination of the dielectric constant, Ka, and errors affecting the conversion of Ka to volumetric water content, θν. The former type can be further divided into errors which concern the quality of the trace and errors influencing the evaluation of the trace. Unbalanced probes and long cables were identified as contributing to uncertainties. Errors from conversion of Ka to θν were considered when the systems were calibrated. One of the programs tested allows convenient one point calibration with a trace offset parameter. The advantage of re-evaluation of measurements with individual settings also permits increased accuracy of measurements.