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ORIGINAL RESEARCH

A New Standpipe Lysimeter Design for the Measurement of Soil Matric Suction

^a Earth and Environmental Sciences, Irving K. Barber School of Arts and Sciences, Univ. of British Columbia Okanagan, 3333 University Way, Kelowna, BC, Canada V1V 1V7
^b Saskatchewan Watershed Authority, Nipawin Regional Office, 201 1st Ave. E, Box 2133, Nipawin, SK, Canada S0E 1E0
^c Dep. of Civil and Geotechnical Engineering, Univ. of Saskatchewan, 57 Campus Dr., Saskatoon, SK, Canada S7N 5A9
^* Corresponding author (craig.nichol{at}ubc.ca).

Received 21 September 2007.

Long-term monitoring of matric suction and in situ soil water chemistry is critical to investigating vadose zone processes. This paper presents a new method of making these measurements, based on the concept of a sealed, high-walled zero-tension lysimeter referred to as a standpipe lysimeter (SPL). We present the design and operational principles of the SPL and provide proof of concept and operation from experiments and modeling. The SPL, designed to measure matric suction primarily in the range of 0 to 30 kPa, provides an alternate measurement strategy to in situ tensiometers or electronic sensors. It requires no calibration or maintenance, is suited to installation at any depth using a drilling rig, and is constructed of readily available, inexpensive, materials. A prototype design was tested in a laboratory column (0.56 m diam., 3.6 m height) over a range of infiltration fluxes encompassing two to three orders of magnitude. Laboratory testing indicated the preliminary SPL design underestimated matric suction by <4 to 12%. Further improvements to the design were developed using numerical modeling studies of unsaturated water flow. The modeling of the improved SPL design was used to assess its accuracy in matric suction measurements compared with the laboratory tested and modeled preliminary design. The modeling studies indicate improvement in measurement accuracy with the improved SPL design to a range of accuracy (<± 2.5% error) similar to the modeled accuracy achieved by installations of tensiometers or other matric suction sensors in typical borehole configurations. The SPL's response time may be days to months and is a function of construction materials, the properties of the surrounding soil, and the magnitude of infiltration fluxes. The SPL is therefore best suited to specific uses requiring long-term monitoring (years to decades) of deep vadose zones where response times are similar to those of the SPL.

Abbreviations: ECAR, equivalent collection area ratio • SPL, standpipe lysimeter • SWCC, soil water characteristic curve