Field Calibration of a Time-Domain Reflectometry Sensor for Water Content Measurement in Soils with Low Content of Coarse Fragments

Comparison of soil water sensors for irrigation scheduling requires an accurate reference measurement. An Acclima TDR-310H soil water content sensor was calibrated and validated for sand, loamy sand, clay loam, sandy clay loam, and sandy loam soils. In 2021, sensor readings and soil samples were collected in the same sensor volume at depths of 15 and 46 cm at three irrigated field sites with five stations each. Gravimetric water content, bulk density, and volumetric water content (θv) were determined in the lab. The root mean square error was 0.032 cm3 cm−3, which was within acceptable limits. The sensor underestimated θv with a mean bias error of −0.025 cm3 cm−3. The linear field calibration equation was θv-Sample = 0.9498 θv-TDR + 0.0357 with R2 = 0.8984. t-tests showed that the 95% confidence intervals (CIs) were 0.82 < slope < 1.08 and 0.0072 < intercept < 0.0642. Since the field calibration result was different from the factory calibration (intercept CI not straddling 0), use of the field calibration is recommended. A validation dataset collected in 2023 confirmed the calibration equation. The calibrated sensor can be used to evaluate other θv sensors for irrigation scheduling in the soil types in this study.