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Field Calibration and Monitoring of Soil-Water Content with Fiberglass Electrical Resistance Sensors

USDA-ARS Northwest Watershed Center, 800 Park Blvd., Plaza IV, Boise, ID 83712

*Corresponding author.

ABSTRACT

Electrical resistance sensors, combined with data acquisition systems, offer a relatively inexpensive means of continuously monitoring soil-water content () at barely accessible remote sites. Fiberglass resistance sensors respond across the range of but require calibration. With field calibration, site-specific soil conditions are implicitly accounted for, but calibration results have not been presented in the literature. The objectives of this study were to determine the accuracy and precision of field-calibrated fiberglass resistance sensors and to demonstrate their application to monitoring at remote sites. Time domain reflectometry (TDR) was used for calibration. Sixteen individual sensor-TDR calibrations and one overall calibration combining all measurements showed a strong log-linear relationship between TDR-measured and sensor-measured resistance. Individual calibration 80% confidence intervals ranged from 0.02 to 0.045 m³ m^–3. Calibration statistics did not appreciably drift during the study. These results, and subsequent measurements, were unaffected by soil freezing, indicating that the sensors respond to liquid water content. The overall calibration 80% confidence interval was 0.065 m³ m^–3, due largely to high variability among sensors. However, changes in could be estimated with reasonably accuracy. Most (73%) of the resistance measurements made the year after calibration were within ±0.05 m³ m^–3 of the TDR-measured value. Sensor response time was shown to be within the 1-h measurement interval. In this study, field-calibrated fiberglass resistance sensors provided reasonably accurate estimates of at a high level of spatial and temporal resolution.

Received for publication August 31, 1992.

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