Performance evaluation of TDT soil water content and watermark soil water potential sensors

Varble, Jordan L., author
Chávez, José L., author
Andales, Allan A., author
Butters, Greg L., author
Trout, Thomas J., author
U.S. Committee on Irrigation and Drainage, publisher
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This study evaluated the performance of digitized Time Domain Transmissometry (TDT) soil water content sensors (Acclima, Inc., Meridian, ID) and resistance-based soil water potential sensors (Watermark 200, Irrometer Company, Inc., Riverside, CA) in two soils. The evaluation was performed by comparing volumetric water content (θv) data collected in the laboratory and in fields near Greeley, CO, with values measured by the sensors. Calibration equations of θv were then developed based on the laboratory and field data. Statistical targets to determine accuracy of the equations were ±0.015 m³ m⁻³ mean bias error and a root mean square error of less than 0.020 m³ m⁻³. Under laboratory and field conditions, the factory-based calibrations of θv did not consistently achieve the required accuracy for either sensor. Field tests indicated that using the calibration equation developed in the laboratory to correct data obtained by TDT and Watermark sensors in the field at Site A (sandy clay loam) was not consistently accurate. Using the laboratory equations developed for the Watermark sensors at Site B (loamy sand) accurately measured θv. Field tests found that a linear calibration of the TDT sensors (and a logarithmic calibration for the Watermark sensors) could accurately correct the factory calibration of θv in the range of permanent wilting point (PWP) to field capacity (FC). Furthermore, the van Genuchten (1980) equation was not significantly more accurate than the logarithmic equation, and the additional work of deriving the former equation did not seem worthwhile, within the range of soil water contents analyzed.
Presented at Emerging challenges and opportunities for irrigation managers: energy, efficiency and infrastructure: a USCID water management conference held on April 26-29, 2011 in Albuquerque, New Mexico.
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