Volume 34, Issue 1 (January 2011)
Effects of Temperature on Measurements of Soil Water Content with Time Domain Reflectometry
This paper analyses the effects of temperature on the quantification of soil water content by means of time domain reflectometry (TDR) method. For this purpose, the dielectric constant of soil specimens with known water content was measured at different temperatures in a range of 20–80°C. The soil types used in the present study were fine sand, sand-bentonite mixture (SBM), and sandy loam. For each soil type, the dielectric constant of at least three specimens having identical dry density but varying initial water content was measured at temperatures ranging from 20 to 80°C. The results obtained agree with previous studies showing that there are two competing phenomena during the measurement of soil water content by means of TDR: (i) The soil bulk dielectric constant increases with increasing temperature due to the release of bound water from soil solid particles and (ii) the soil bulk dielectric constant decreases with increasing temperature due to the temperature effect of free water molecules. Moreover, it has an existing equilibrium water content at which both competing phenomena compensate each other. However, for the SBM no equilibrium water content was found, but the dielectric constant increased significantly with the temperature for the whole water content range. This can be explained by the significant clay content and the high specific surface area, which leads to a dominating effect of the increase of soil bulk dielectric constant with increasing temperature.