Field calibration of water content reflectometers

Field monitoring of volumetric soil water content (VWC) is critical for a variety of applications. Recently developed electronic soil water sensors provide a relatively inexpensive monitoring option. However, the calibration of these sensors is more sensitive to variations in soil properties than for time domain reflectometry (TDK), which is generally regarded as the best electronic means of VWC measurement and which has a relatively robust calibration. Field calibration incorporates the effects of within-profile and between-site soil variations and individual variability on sensor response. The objective of this study was to evaluate the effectiveness of using TDR to field-calibrate the Campbell Scientific water content reflectometer (WCR), or CS-615, which is an example of a newly developed sensor in widespread use. We found that (i) there was a strong, linear correlation between the WCR-measured period and TDK-measured VWC; (ii) the WCR calibration varied with soil type; (iii) calibration of individual sensors resulted in excellent agreement between TDR and the WCR measurements; and (iv) calibration resulted in improved description of soil water dynamics and improved precision of VWC estimates.