Calibration of water content reflectometer sensors with a large soil sample

In situ measurement of soil water content is of fundamental importance in vadose zone processes. While time domain reflectometry (TDR) is a universally accepted technique, having been developed since the 1980s, water content reflectometer (WCR) is still a relatively new technique and does not hold a similar background. The main goal of this study was to establish an accurate calibration curve for WCR sensors to be installed in an artificial hillslope designed to study the triggering of shallow landslides. Therefore, high accuracy calibration at the high end of the water content range and for large soil volumes was necessary. An experimental device was specifically designed to provide a calibration procedure for a soil control volume commensurate with the application scale of the probes in the hillslope. A large box container (60 by 50 by 60 cm) was used, inside which three WCR probes and three tensiometers were arranged. A drip emitter produced assigned flow rates, while a load cell at the base measured the time evolution of the infiltrated water mass. Water content was evaluated at approximately steady-state flow conditions for infiltration and drainage experiments, performed with varying degrees of compaction to detect the effect of the porosity on the calibration curve. The experimental results suggest a calibration relationship linearly depending not only on the WCR output signal but also on the porosity. In addition, the calibration curve provided by the manufacturer significantly underestimates the soil water content, with a range of predicted volumetric water content values between 3 and 35%, compared with actual values varying between 19 and 61%.