In Situ Calibration of Time Domain Reflectometry Sensors in Multiple Soils

Abstract

As part of a larger field study, time domain reflectometry (TDR) sensors were installed to monitor volumetric water content (VWC) change with time at multiple sites across North America. Currently there are many manufactures of TDR equipment and these vendors often provide standard calibration curves that are intended to relate msec return time to volumetric water content (VWC). Some research has suggested that TDR does not require calibration to adequately describe volumetric water content in soil. However, for the method to be most useful, a reasonable estimate of volumetric water content should be made. In this study, an in situ method was used to develop a calibration curve based on soils with many different physical properties. Additionally, the effect of salinity on the calibration curve was examined as well as the difference between horizontal and vertical insertion of sensors. Results from this work indicate that the new in situ calibration curve greatly increases the ability of the sensors to accurately describe VWC across a wide variety of soils when compared to the manufacturer provided relationship. As suggested by the literature, effects from salinity were observed to enhance VWC measurements in soil where electrical conductivity was greater than 1 dS m−1. Based on the limited evaluation of vertical and horizontally installed sensors, vertically installed sensors produced data equally useful to horizontally installed sensors. A new in situ calibration curve has been introduced and greatly improves the use and applicability of TDR across a wide range of soils.