Tensiometers Then and Now Elizabeth Scherling BAE 558 Semester Project Spring 2005

Outline Theory Components Historical Devices –Applications Modern Devices –Applications Calibration Maintenance Challenges Future Tensiometers

Theory Tensiometers directly measure soil water matric potential Forces responsible for capillary action and water retention –van der Waals –Electrostatic dipole –Osmotic forces –Surface tension

Components I Thin-walled porous cup Rigid body Pressure gauge –Mechanical –Electrical: pressure transducer –Hydraulic: manometer De-aired water

Components II Porous CupBodyPressure Gauge

Historical Devices I B. E. Livingston 1908 Earliest account of a tensiometer-like device

Historical Devices II H. E. Pulling and B. E. Livingston 1915 Measured what they called the “water supplying power of the soil”

Historical Devices III C. J. Lynde and H. A. Dupre 1913 First hanging column design Measured what they called the “capillary lift of soil”

Historical Applications Irrigation control –Auto-irrigation for potted plants Capillary pressure measurement

Modern Devices I J. M. Hubbell and J. B. Sisson 1998 Advanced tensiometer design

Modern Devices II B. Lebeau, S. Barrington, and R. Bonnell 2003 Micro-tensiometer

Modern Applications Irrigation Control –Greenhouse –Farm –Field Vadose Zone Studies Transport Research Ultimately: Tensiometers have the potential to save water and reduce infiltration contamination

Calibration The tensiometer must be subjected to known positive and negative pressures. –U-tube manometer –Vacuum chamber Recalibration only recommended after long periods of inactivity.

Maintenance Gas Evolution –De-air water prior to filling and for subsequent water replacement Vacuum, autoclave, or boil –Check for bubbles frequently in simple designs –Add water regularly for advanced designs Maintain intimate contact between porous cup wall and soil matrix

Challenges I Choosing the correct tensiometer Hubbell and Sisson, 1998

Challenges II Weather Conditions Hansen and Pasian, 1999 Cloudy day Sunny day

Challenges III Low range: 3 to 6 kPa –Controlled and reliable Medium range: 9 to 12 kPa –Not stable or reliable High range: 15 to 18 kPa –Not stable or reliable Limited Matric Potential Ranges From a study by Hansen and Pasian, 1999 Low Range Medium RangeHigh Range

Future Tensiometers For long duration in situ use: –Cost effective advanced tensiometers For large-scale greenhouse use: –Micro-tensiometers with greater control of large ranges of moisture tension

Questions ?