Applicability of multiple length TDR probes to measure water distributions in an Andisol under different tillage systems in Japan

Time domain reflectometry (TDR) has become increasingly popular worldwide for measuring soil moisture. The objective of this study was to show if the TDR technique with multiple length probes could be used to evaluate the distributions of water content in root zones under both minimum and conventional tillage conditions. TDR calibrations for an Andisol (Hydric Pachic Melanudands) with three different bulk densities were tested, and field measurements were conducted. A single as well as three separate calibration functions were examined for the measured data with three different bulk densities. The water contents measured by TDR and gravimetric sampling were compared to each other. Even though the single calibration function tended to overestimate at a moisture range larger than 0.50 m3 m−3, the standard deviation (S.D.) of the difference between the TDR and gravimetric values was ±0.035 m3 m−3. This value was only ±0.002 m3 m−3 larger than the S.D. obtained using the separate calibration functions. We found the single calibration function to be accurate enough for describing the relationship between volumetric water content and apparent dielectric constant for this specific soil with different bulk densities. The TDR measurements with multiple length probes were conducted intermittently under both minimum and conventional tillage in a field over time. The TDR technique easily determined water distributions, and characterized the soil moisture regime resulting from different tillage systems. The TDR measurement with multiple length probes can provide useful information on the relationship between crop yield and soil environment and for improving soil water management under various conditions.