The Pursuit of Precise Water Potential in Soil and Plants: Rethinking Thermocouple Psychrometry.

Scarce water resources are being stretched to irrigate an ever-increasing number of moisture sensitive crops in arid environments. Deficit irrigation practices require that growers walk the fine line between maximizing irrigation water use efficiency and crop failure. Accurately measuring the water potential in these crops can be accomplished with stem and soil psychrometers. Peltier style thermocouple psychrometers (TCP) have been used by researchers for decades as a means to accurately measure water potential beyond the range of the tensiometer. Notwithstanding TCP’s potential accuracy, its use beyond the laboratory and research realms has never materialized. Various hurdles have impeded TCP use outside the laboratory, to wit, the high cost of instrumentation, the lack of a turn-key solution, thermal gradients hurting accuracy, difficulty interpreting data, sensor calibration, and a perception that the method is outdated. Recent advances in instrumentation and methodologies have shown how to overcome many of these hurdles.
Stem and leaf TCPs that are well insulated, sealed, and shielded from thermal radiation have been shown to improve stability in measurements. Further enhancements have been made with optimized Peltier cooling time, fast burst measurements, and increased voltage resolution that limit the remaining effects of thermal and vapor gradients, Joule heating, voltage offsets, and data interpretation. We believe that these advances can be used to create turn-key solutions that growers can use to maximize water use efficiency and yields when water is severely limited.