Field Calibration of PR2 Capacitance Probe in Pullman Clay-Loam Soil of Southern High Plains

Core Ideas: A noncalibrated PR2 capacitance probe showed significant deviations from actual soil water content. Calibration improved the accuracy and precision of soil moisture monitoring with the PR2. Calibration was necessary for using the PR2 probe for research-quality soil water measurements. Multi-depth capacitance sensors are popular to monitor soil water content for scheduling irrigation thanks to their ease of operation and maintenance. The PR2/6 Profile Probe (Delta-T Devices) measures soil moisture by using either the manufacturer's built-in equation or a user-calibrated equation if the soil is high in clay or organic matter. The objectives were to evaluate the performance of the PR2/6 Profile Probe in a perennial grassland and to develop a site-specific equation to correct probe measurements in Pullman (fine, mixed, superactive, thermic Torrertic Paleustolls) clay loam soils. Permittivity recorded by the profile probe was regressed on the gravimetrically measured soil volumetric water content (VWC). Parameters were optimized to obtain least RMSEs. The default equation provided by the manufacturer estimated VWC with average RMSE and r² values of 0.053 m³ m^–3 and 0.71, respectively. New calibration coefficients were effective in explaining 91% of the variability in soil VWC measurements and reducing RMSE to 0.017 m³ m^–3. The results indicate that site-specific calibration of the capacitance probe is necessary to attain research-quality accuracy and precision when applied to grasslands in Pullman clay loam and associated clay loam soils.