Zulueta
Rommel C.
Zulueta
Rommel C.
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ArticleA robust calibration method for continental-scale soil water content measurements(American Society of Agronomy, 2018-05-24) Roberti, Joshua A. ; Ayres, Edward ; Loescher, Henry W. ; Tang, Jianwu ; Starr, Gregory ; Durden, David J. ; Smith, Derek E. ; de la Reguera, Elizabeth ; Morkeski, Kate ; McKlveen, Margot ; Benstead, Heidi ; SanClements, Michael D. ; Lee, Robert H. ; Gebremedhin, Gebremedhin ; Zulueta, Rommel C.Technological advances have allowed in situ monitoring of soil water content in an automated manner. These advances, along with an increase in large-scale networks monitoring soil water content, stress the need for a robust calibration framework that ensures that soil water content measurements are accurate and reliable. We have developed an approach to make consistent and comparable soil water content sensor calibrations across a continental-scale network in a production framework that incorporates a thorough accounting of uncertainties. More than 150 soil blocks of varying characteristics from 33 locations across the United States were used to generate soil-specific calibration coefficients for a capacitance sensor. We found that the manufacturer’s nominal calibration coefficients poorly fit the data for nearly all soil types. This resulted in negative (91% of samples) and positive (5% of samples) biases and a mean root mean square error (RMSE) of 0.123 cm3 cm−3 (1σ) relative to reference standard measurements. We derived soil-specific coefficients, and when used with the manufacturer’s nominal function, the biases were corrected and the mean RMSE dropped to ±0.017 cm3 cm−3 (±1σ). A logistic calibration function further reduced the mean RMSE to ±0.016 cm3 cm−3 (±1σ) and increased the range of soil moistures to which the calibration applied by 18% compared with the manufacturer’s function. However, the uncertainty of the reference standard was notable (±0.022 cm3 cm−3), and when propagated in quadrature with RMSE estimates, the combined uncertainty of the calibrated volumetric soil water content values increased to ±0.028 cm3 cm−3 regardless of the calibration function used.