Choosing the Right Water Potential Sensor
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If you're not measuring water potential, or not measuring it correctly, your data could be telling you the wrong thing. Water content measurements can only tell you so much, and inferring water potential from water content is inaccurate at best, and completely misleading in worst-case scenarios. In this 30-minute webinar, METER research scientist Leo Rivera discusses the good, the bad, and the ugly sides of measuring soil water potential. He'll walk you through the considerations and choices you need to take into account to select the perfect water potential sensor for your needs. Discover the challenges, limitations, and advantages of new sensor tech, and learn how to collect the most accurate measurements for your particular application. Learn about: - The large variety of technology available on the market - The most recent trends and technologies - Installation considerations and the tools available to make install better - The limitations of using water content to infer water potential - Our most recent research projects and findings
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Choosing the Right Water Potential Sensor
- 2. CHOOSING THE RIGHT WATER POTENTIAL SENSOR IN 2023 Leo Rivera METER Group, Inc. USA
- 3. EXTENSIVE VS. INTENSIVE PROPERTIES Two variables necessary to describe state of matter or energy in environment Extensive variable – describes the extent or amount of matter or energy Intensive variable – describes the intensity or quality of matter or energy Extensive Intensive Volume Density Water Content Water Potential Heat Content Temperature
- 4. WATER POTENTIAL DEFINITION Energy required, per quantity of water, to transport an infinitesimal quantity of water from the sample to a reference pool of pure, free water
- 5. SOIL FINGERPRINT SOIL WATER CHARACTERISTIC CURVES EXTENSIVE INTENSIVE
- 6. TOTAL WATER POTENTIAL SUM OF COMPONENTS ψT = ψm + ψg + ψo + ψp ψT – Total water potential ψm – matric potential - adsorption to surfaces ψg – gravitational potential - position ψo – osmotic potential - solutes ψp – pressure potential - hydrostatic or pneumatic
- 7. WATER POTENTIAL MEASUREMENTS PAST & PRESENT http://www.environmentalbiophysics.org/the-history-and-future-of-water-potential/
- 9. WATER POTENTIAL MEASUREMENTS FIELD SENSORS Solid equilibration methods • Electrical resistance • Capacitance • Thermal conductivity Liquid equilibration methods • Tensiometer Vapor equilibration methods • Thermocouple psychrometer
- 10. FIELD SENSORS LIQUID EQUILIBRATION Equilibrates water under tension with soil water through a porous cup Pressure sensor directly measures suction of water
- 11. FIELD SENSORS LIQUID EQUILIBRATION TENSIOMETERS Advances in sensor capabilities • Digital pressure transducers • Minimize water column • External refilling • Measures + and - pore pressures • Extended measuring range* Ceramic Cup Digital Pressure Transducer External Refilling Tubes
- 12. FIELD SENSORS HIGH CAPACITY TENSIOMETERS Sensors capable of directly measuring very low water potentials Sensors consist of • High air entry ceramic/filter • Pressure transducer • Protective housing Sensors are degassed under high pressures Not commercially available Pressure transducer
- 13. FIELD SENSORS LIQUID EQUILIBRATION PROS & CONS Advantages • Highest accuracy of any sensor in wet range • Positive pore pressure measurements Disadvantages • Limited to water potential from 0 to -90 kPa • Significant maintenance requirements
- 14. 0 10 20 30 40 50 60 1 10 100 1000 10000 100000 1000000 θ Ψ FIELD SENSORS SOLID MATRIX SENSORS Measure water content in a consistent matrix Output apparent water potential
- 15. FIELD SENSORS SOLID MATRIX SENSORS THERMAL Standard matrix equilibrates with soil Water content of matrix is measured by heat pulse induced temperature rise Sensors calibrated to output water potential based on ceramic characteristics Flint, Alan & Campbell, Gaylon & Ellett, Kevin & Calissendorff, C.. (2002). Calibration and Temperature Correction of Heat Dissipation Matric Potential Sensors. Soil Science Society of America Journal - SSSAJ. 66. 10.2136/sssaj2002.1439.
- 16. FIELD SENSORS SOLID MATRIX SENSORS CAPACITANCE Standard matrix equilibrates with soil Water content of matrix is measured by capacitance technology Sensors calibrated to output water potential based on ceramic characteristics
- 18. FIELD SENSORS SOLID MATRIX SENSORS PROS & CONS Advantages • No maintenance required • Large sensing range • Covers plant available range • Good accuracy for calibrated sensors Disadvantages • Accuracy dependent on calibration • Limited wet end performance (0 to -10 kPa) • Temp sensitivity on very dry end
- 19. FIELD SENSORS INSTALLATION QUALITY Water potential data dependent on installation Keys • Sensor to soil contact • Minimize site disturbance • Limit preferential flow
- 20. LABORATORY METHODS SOIL WATER CHARACTERISTIC CURVE ADVANCEMENTS Evaporation Method Dewpoint Techniques
- 21. LABORATORY METHODS EXPANDING OUR UNDERSTANDING OF SWCC’S Vapor Sorption Analyzer
- 22. LABORATORY METHODS ADDITIONAL IMPLICATIONS OF SWCC’S Shrink swell capacity Cation exchange capacity Soil specific surface area Rivera, Leo & Cobos, Douglas & Campbell, Colin & Morgan, Cristine. (2013). Quantifying Shrink Swell Capacity of Soil Using Soil Moisture Isotherms.. 10.1201/b17034-131.
- 23. LABORATORY METHODS GROWING BODY OF WORK References 1. Wang et al. (2022). “Soil water isotherm equations for particle surface sorption and interlamellar sorption.” VZJ 2. Zhang and Lu (2019). “Augmented Brunauer–Emmett–Teller equation for water adsorption on soils.” VZJ 3. Luo et al. (2023). “Determination of cation exchange capacity of soil by water vapor sorption.” JGGE 4. Luo et al. (2023). “Determination of pore size distribution of soil by water vapor sorption.” JGGE 5. Luo et al. (2022). “Determination of soil sorptive potential by soil water isotherm.” JGGE 6. Lu et al. (2022). “Water adsorption-induced pore water pressure in soil.” JGGE 7. Wang et al. (2022). “Soil swelling potential: Theory, determination, and validation.” JGGE 8. Lu and Kaya (2013). “A drying cake method for measuring suction-stress characteristic curve, soil–water-retention curve, and hydraulic conductivity function.” GTJ 9. Lu and Dong (2017). “Correlation between soil-shrinkage curve and water-retention characteristics.” JGGE 10. Zhang et al. (2021). “Unified elastic modulus characteristic curve equation.” JGGE 11. Zhang and Lu (2019). “Unified effective stress equation for soil.” JEM
- 24. FUTURE TESTS EFFECTS OF A CONFINING LOAD Confining loads or constant pressures can impact soil hydraulic properties How can we characterize these impacts with conventional tools?
- 25. CONCLUSION Variety of tools available to measure water potential • Keep in mind measurement ranges and applications • Installation is key Need to continue advancing the capabilities of these measurements • Sensor capabilities and models
- 26. QUESTIONS Leo Rivera, MS Director of Scientific Outreach METER Group, Inc. 2365 NE Hopkins Ct, Pullman, WA USA 99163 T: +1 509 332 2756 E: leo.rivera@metergroup.com W: www.metergroup.com