Precision and accuracy of three alternative instruments for measuring soil water content in two forest soils of the Pacific Northwest

We compared the accuracy and precision of three devices for measuring soil water content in both natural and repacked soils and evaluated their temperature sensitivity. Calibrations were developed for a capacitance instrument (ECH2O), a time domain reflectometry cable tester (CT), and a water content reflectometer (WCR) in soils collected from the Wind River and H.J. Andrews Experimental Forests. We compared these calibrations with equations suggested by manufacturers or commonly used in the literature and found the standard equations predicted soil moisture content 0%-11.5% lower (p < 0.0001) than new calibrations. Each new calibration equation adequately predicted soil moisture from the output for each instrument regardless of location or soil type. Prediction intervals varied, with errors of 4.5%, 3.5%, and 7.1% for the ECH2O, CT, and WCR, respectively. Only the ECH2O system was significantly influenced by temperature for the range sampled: as temperature increased by 1 °C, the soil moisture estimate decreased by 0.1%. Overall, the ECH2O performed nearly as well as the CT, and thanks to its lower cost, small differences in performance might be offset by deployment of a greater number of probes in field sampling. Despite its higher cost, the WCR did not perform as well as the other two systems. Resume : Les auteurs ont compare l'exactitude et la precision de trois systemes de mesure du contenu en eau du sol dans des sols naturels et reconstitues et ils ont evalue leur sensibilite a la temperature. Des calibrages ont ete effectues pour un instrument a capacitance (ECH2O), un analyseur de câble (AC) qui fonctionne en reflectometrie dans le do- maine temporel et un reflectometre du contenu en eau (RCE) dans des sols preleves dans les forets experimentales Wind River et H.J. Andrews. Ils ont compare ces calibrages avec les equations suggerees par les manufacturiers ou couramment utilisees dans la litterature, et ils ont trouve que les equations standard predisent un contenu en eau du sol de 0%a1 1,5 %p lus faible (p < 0,0001) que celui des nouveaux calibrages. Chaque nouvelle equation de calibrage predit adequatement l'humidite du sol a partir de la lecture de chaque instrument peu importe la localisation ou le type de sol. Les intervalles de prediction varient avec des erreurs respectives de 4,5 %, 3,5 % et 7,1 % pour l'ECH2O, l'AC et le RCE. Seul le systeme ECH2O est influence par la temperature dans les limites de l'echantillonnage; la mesure de l'humidite du sol diminue de 0,1 % pour une augmentation de la temperature de 1 °C. Globalement, la performance de l'ECH2O est presque aussi bonne que celle de l'AC et, grâce a son cout plus faible, les petites differences de perfor- mance peuvent etre compensees par l'utilisation d'un plus grand nombre de sondes pour l'echantillonnage sur le ter- rain. En depit de son cout plus eleve, le RCE n'a pas performe aussi bien que les deux autres systemes.

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