Calibration and validation of soil water reflectometers

Abstract

Abstract In this study, we tested soil water reflectometers represented by the CS65x series soil water reflectometers. The objectives were to (a) quantify the sensing volume of the CS65x sensors, (b) evaluate common univariate and multivariate calibration models, and (c) assess the implications of using uncalibrated sensors. The support volume was determined by analyzing the response of the bulk dielectric permittivity to a changing amount of surrounding media in the radial and axial directions of the sensor. Calibration models were tested using a dataset of ∼300 soil samples spanning nine soil textural classes. A time series of rootzone soil water storage recorded in two contrasting soils and two intensive (n ∼300) spatial surveys of near‐surface soil moisture were used to quantify the implications of using uncalibrated sensors. The electromagnetic signal of CS655 was concentrated within a ∼3.5‐cm radius around the center of the sensor rods. Univariate and multivariate calibration equations reduced the mean absolute error (MAE) in soil moisture observations by ∼40% compared with factory default equations. Uncalibrated sensors did not change the pattern of soil moisture temporal dynamics but resulted in a strong bias in soil water storage in a silty clay loam soil and altered the soil moisture spatial patterns obtained in moderate to wet field conditions in a silty clay soil. The CS655 sensor with the Kargas and Soulis calibration equation (MAE = 0.026 cm3 cm−3) appears to be an acceptable candidate for deployment in soil moisture monitoring networks seeking to avoid site‐specific sensor calibrations.