Laboratory calibration and field precision messurement of FDR tubular moisture sensor
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Abstract:
Dynamic monitoring of soil moisture is of great significance for guiding timely and appropriate agricultural irrigation. Currently, the water content measurement instruments used in domestic field experiments are mainly imported from instrument companies in Australia, the Netherlands, and other countries. However, these instruments do not open the bottom-level sensor calibration program to ordinary users, resulting in significant differences in water monitoring accuracy when used in different soil types. Frequency domain reflectometry (FDR) is the most widely used soil moisture measurement method in China. In response to the monitoring needs of soil moisture in agricultural irrigation fields, a ring-shaped soil moisture sensor based on the FDR method was designed and developed. The function relationship between soil volume moisture content and FDR sensor circuit oscillation frequency was established by preparing standard soil samples in the laboratory, and the accuracy of the sensor was verified through field measurement data. Seven treatments were set up in the indoor experiment, with soil volume moisture content of 13.21%, 16.28%, 20.46%, 27.45%, 31.20%, 34.54%, and 39.80%, respectively. FDR sensors were installed at two depths of 5 cm and 15 cm from the soil surface, and the oscillation frequency SF of the sensor and the soil volume moisture content θv obtained by the drying method were fitted using the exponential function relationship θV=aSBF to determine the coefficients a and b corresponding to the sensor oscillation circuit. The field validation experiment was conducted at the irrigation test station in Baoding. FDR sensors were buried at depths of 15 cm and 60 cm in the irrigation district for soil moisture observation, and conventional observations were conducted using the soil drying method at the same time. The laboratory calibration results showed that at a depth of 15 cm, the fit between the oscillation frequency SF and the soil volume moisture content θv was good, and the R2 value of the fitting curve was 0.876 4. In contrast, at a depth of 5 cm, the data measurement was unstable due to the small distance between the FDR sensor and the soil surface, and the electromagnetic wave was interfered with by the surface air, resulting in slightly worse fitting results. The field validation experiment results showed that the FDR soil moisture sensor calibrated with a and b coefficients prepared in the laboratory with soil samples can accurately reflect the actual soil moisture content ranging from the crop wilting coefficient to the soil saturation moisture content, and can be used for guidance of precision irrigation. In response to the needs of agricultural irrigation, a ring-shaped soil moisture sensor based on the FDR principle was developed, and its accuracy was tested in the laboratory and field. The laboratory soil column test of the sensor found that the accuracy of the sensor is affected by the burial depth in the soil. When the burial depth of the sensor is 15 cm from the soil sample surface, the R2 value of the fitting curve between the FDR sensor measurement data and the volume moisture content obtained by the drying method is 0.876 4, and the fitting effect is better. At a depth of 5 cm from the soil sample surface, the fitting curve effect between FDR data and drying method data is not ideal, mainly because the FDR sensor is too close to the soil surface, and the electromagnetic wave is interfered with by the surface air, resulting in unstable data measurement. To ensure the reliability of the data, it is recommended that the ring-shaped soil moisture sensor be used for soil moisture monitoring below 15 cm from the soil surface in practical applications. Field calibration is usually more cumbersome than laboratory calibration, and requires much more time than laboratory calibration. The FDR sensor developed is calibrated with soil samples prepared from the field measurement area, and its observed soil volume moisture content and the error range of the soil drying method are [0.5%, 1.1%], which meets the accuracy requirements of the "Automatic Soil Moisture Observation Instrument" (QX/T 567-2020). Therefore, the method of laboratory calibration by taking soil samples in the measurement field can be used to improve the calibration efficiency.
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