Monday, November 13, 2006 - 1:45 PM

A Small Probe for TDR Tensiometry.

Jon Wraith1, Paolo Castiglione2, and Jill Flores2. (1) PO Box 173120, Montana State University, Montana State University, LRES Department, Bozeman, MT 59717-3120, (2) LRES Dept., Montana State Univ., PO Box 173120, Bozeman, MT 59717-3120

We designed a small soil matric potential sensor by embedding a miniature time-domain reflectometry (TDR) probe within a porous fritted clay pellet. The matric potential of the pellet equilibrates with that of surrounding soil, and the soil matric potential is estimated through measurement of the pellet’s water content, using the known water retention characteristic (WRC) of the pellet medium. The WRC for the fritted clay pellets was determined using a combination of hanging water column and pressure plates, and displays a relatively broad relationship between volume water content and matric potential for tensions to -1.5 MPa, making it well suited for tensiometric measurements. The reduced dimensions of the probe relative to alternative designs will facilitate rapid equilibration with the surrounding soil as well as provide opportunities for applications having smaller spatial scales. The embedded TDR probe is made from a standard BNC connector, creating a 4-mm length 4-rod transmission line. Measurements of the permittivity spectrum in the range 0.05 to 3 GHz were obtained using a Tektronix 1502 cable tester using time-domain spectroscopy. Analyses of the permittivity spectrum, combined with a calibration procedure, allowed us to correlate the pellet’s volume water content to the permittivity values displayed at high frequencies. The sensor was evaluated in soil samples having known WRC. The probe design offers a potential to obtain automated, in-situ measurements over a wide range of matric potential, and is able to function over the range -0.05 to -0.2 MPa, for which no single sensor design currently performs well.