Calibrating Access‐tube Time Domain Reflectometry Soil Water Measurements in Deep Heterogeneous Soils

Abstract

Hydrologic characterization of the vadose zone requires accurate measurements of soil water in a variety of soil textures. Time domain reflectometry (TDR) instruments are commonly used to obtain soil moisture. Although there are advantages to using TDR, certain types of instruments pose challenges, including proper installation and calibration, which must be overcome. The principal objective of this study was to develop a new method for obtaining and calibrating access‐tube TDR measurements in deep heterogeneous soils. Using two regression techniques, physical and chemical property‐based calibrations were developed to predict soil water and explain large differences between TDR and gravimetric soil water values. Differences between TDR soil water and gravimetric soil water were almost entirely controlled by physical and chemical soil properties; indicating that our access‐tube TDR probe likely produced an accurate measurement of ‘free’ but not bound water content. The bound fraction is controlled by physical and chemical properties and is a nearly static parameter at the depths and time scales considered in this study. Changes in TDR measurements after the initial calibration represent changes in the ‘free’ water content. Our calibration equations used easily obtained parameters to predict accurate soil water values in a wide range of soil textures. Approximately 95% of the soil water values predicted using our calibration equations had residuals of less than ± 10% soil water. We also developed a cost‐effective method for installing access tubes which minimized problems related to air gaps and soil structure disturbances in deep soil profiles.