Empirical two-point α-mixing model for calibrating ECH2O EC-5 soil moisture sensor in sands

Considerable interest has developed in recent years in the use of sensors that can be automated to remotely collect soil moisture data in hydrologic, environmental, and agricultural systems. Once the sensors become cost effective, it is expected that large numbers of these sensors will be deployed in large field systems. Our current research focuses on evaluating the reliability of sensors and development of effective calibration methods that can be rapidly implemented. One of these sensors, the recently improved ECH2O soil moisture sensor, has received significant attention in many field and laboratory applications. Focusing on the EC-5 sensor, a simple and robust calibration method is proposed. The sensor-to-sensor variability in the readings (ADC counts) among 30 EC-5 sensors was relatively small but not negligible. A large number of ADC counts were taken under various volumetric water contents (θ) using four test sands. A two-point α-mixing model was proposed and fitted, as well as linear and quadratic models, to the ADC-θ data. Unlike for conventional TDR measurements, the effect of sensor characteristics is lumped into the empirical parameter α in the two-point α-mixing model. The value of α was fitted to be 2.5, yielding a nearly identical calibration curve to the quadratic model. Errors in θ associated with the sensor-to-sensor variability for the two-point α-mixing model were ± 0.005 cm3 cm-3 for dry sand and ± 0.028 cm3 cm-3 for saturated sand. In the validation experiments, the highest accuracy in water content estimation was achieved when the sensor-specific ADCdry and ADCsat were used in the two-point α-mixing model. Assuming that α = 2.5 is valid for most mineral soils, the two- point α-mixing model only requires the measurement of two extreme ADC counts in dry and saturated soils. Sensor-specific ADCdry and ADCsat counts are readily measured in most cases. Therefore, the two-point α-mixing model (with α = 2.5) can be considered as a quick, easy, and robust method for calibrating ECH2O EC-5 sensors. Although further investigation is needed, the two-point α-mixing model may also be applied to calibrating other sensors.