A LABORATORY CALIBRATION OF TIME-DOMAIN REFLECTOMETRY FOR SOIL-WATER MEASUREMENT INCLUDING EFFECTS OF BULK-DENSITY AND TEXTURE

Water content was determined gravimetrically and apparent dielectric constant was measured by time domain reflectometry (TDR) for a total of 189 samples of soil from the plough layer and the subsoil at five Danish locations. The soil types ranged from a coarse sandy soil to a sandy clay loam. Water contents varied from air dry to near water saturation. Samples were packed at dry bulk densities around 1.35 x 10(3) or 1.55 x 10(3) kg m(-3). A third-order polynomial relationship between the volumetric water content and the apparent dielectric constant was found suitable for calibration. The relationship differed from earlier results; the deviations could be explained partly by differences in texture. Inclusion of linear terms for dry bulk density, clay content and organic matter content in the calibration equation each yielded an improvement in the correlation; this improvement, although small compared with the uncertainties in the measured dielectric constant and in the gravimetrically determined water content, was statistically significant. Even after inclusion of density, clay and organic matter content in the calibration equation, statistically significant differences between the ten soil types remained; this suggests that rather complex interactions between the soil components affect the electric properties of the soil.