CHANGES IN SOIL-STRUCTURE CAUSED BY THE INSTALLATION OF TIME-DOMAIN REFLECTOMETRY PROBES AND THEIR INFLUENCE ON THE MEASUREMENT OF SOIL-MOISTURE

The time domain reflectometry technique for measuring the volumetric s oil water content is highly sensitive to installation effects because the sampling volume is more heavily weighted close to the transmission line elements. To investigate changes in soil structure caused by the installation and its influence on the measurement, field and laborato ry experiments covering different probe dimensions, soil types, and in stallation techniques were performed. The results indicate that merely pushing the probes into the soil entails a significant reduction of t he measured water content up to 0.10 cm(3)/cm(3). The effect is strong est close to saturation since the large pores are affected most. The d egree of compression differed strongly according to the varying compre ssibility of the investigated soils. It was smaller for the smaller pr obe indicating a more favorable relationship between sampling volume a nd compression. The compression effect could be minimized using a dril l to remove the soil for the probes. X ray-computed tomography was use d to visualize the installation effects and to measure the density dis tribution around the probes.