Abstract
Soil water content is a key variable for understanding and modelling ecohydrological processes. Low-cost electromagnetic sensors are increasingly being used to characterize the spatio-temporal dynamics of soil water content, despite the reduced accuracy of such sensors as compared to reference electromagnetic soil water content sensing methods such as time domain reflectometry. Here, we present an effective calibration method to improve the measurement accuracy of low-cost soil water content sensors taking the recently developed SMT100 sensor (Truebner GmbH, Neustadt, Germany) as an example. We calibrated the sensor output of more than 700 SMT100 sensors to permittivity using a standard procedure based on five reference media with a known apparent dielectric permittivity (1 < Ka < 34.8). Our results showed that a sensor-specific calibration improved the accuracy of the calibration compared to single “universal” calibration. The associated additional effort in calibrating each sensor individually is relaxed by a dedicated calibration setup that enables the calibration of large numbers of sensors in limited time while minimizing errors in the calibration process.
Highlights
Knowledge of soil water content (SWC) is essential, as it represents a key variable in many hydrological, climatological, environmental and ecohydrological processes
This indicates that change in permittivity with changing temperature is solely related to the temperature sensitivity of the dielectric permittivity of water and that the sensor electronics are showing no temperature effect
We present an effective calibration procedure for electromagnetic soil water content sensors taking the low-cost SMT100 sensor as an example
Summary
Knowledge of soil water content (SWC) is essential, as it represents a key variable in many hydrological, climatological, environmental and ecohydrological processes. Electromagnetic (EM) methods, such as time domain reflectometry (TDR) (e.g., [12]), time domain transmission (TDT) (e.g., [13]), and capacitance [14] and impedance sensors (e.g., [15,16]), are most commonly used for soil water content measurements at the point scale. As the dielectric permittivity of liquid water is much higher than the dielectric permittivity of other soil components, SWC is the principal factor governing EM wave propagation in the soil. TDR and TDT sensors measure the propagation velocity of EM waves along open and closed transmission
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