Time domain reflectometry probe for simultaneous measurement of soil matric potential and water content
1999
Noborio, K. | Horton, R. | Tan, C.S.
Simultaneous measurement of matric potential, psi, and water content, theta, is demanded in many disciplines. Time domain reflectometry (TDR) has been routinely used to measure water content and electrical conductivity of soil. Previous efforts to combine TDR probes with porous materials functioning as tensiometers were successful, but these probes were still constrained by characteristics of tensiometers, such as the need to supply water and measuring ranges psi > -85 kPa. A new psi-theta TDR probe was developed to overcome short-comings of the previous work. A portion of the TDR rod was embedded in a dental plaster (gypsum), whose matric potential equilibrated with surrounding soil. The rest of the TDR rod was inserted into the soil. The TDR technique was used to determine dielectric constants, kappa, of both the gypsum and the soil. The new psi-theta TDR probes were tested in clay loam soil using a pressure-plate apparatus to produce kappa-psi relationships of the gypsum and the soil for -1000 < or = psi < or = 10 kPa. Changes in kappa of the gypsum corresponded well to applied pressures for -1000 < or = psi < or = -30 kPa, but kappa values did not noticeably change for psi > -30 kPa. Values of kappa of the soil corresponded well to the whole psi range tested. The new probes accurately measure theta and psi of soil when soil water content gradually decreases or increases. The newly developed psi-theta TDR probe requires no more maintenance than ordinary TDR probes and requires no additional instrumentation.
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