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

Time domain reflectometry (TDR) has become increasingly popular worldwide f or 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 dis tributions of water content in root zones under both minimum and convention al tillage conditions. TDR calibrations for an Andisol (Hydric Pachic Melan udands) with three different bulk densities were tested, and field measurem ents were conducted. A single as well as three separate calibration functio ns 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 overes timate at a moisture range larger than 0.50 m(3) m(-3), the standard deviat ion (S.D.) of the difference between the TDR and gravimetric values was +/- 0.035 m(3) m(-3). This value was only +/-0.002 m(3) 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 mult iple length probes were conducted intermittently under both minimum and con ventional 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 pr obes can provide useful information on the relationship between crop yield and soil environment and for improving soil water management under various conditions. (C) 2001 Elsevier Science B.V. All rights reserved.