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DIVISION S-1-SOIL PHYSICS

A Time Domain Reflectometry Method to Measure Immobile Water Content and Mass Exchange Coefficient

^a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011 USA
^b USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011 USA

rhorton{at}iastate.edu

Physical nonequilibrium of water and solute transport in soil has been reported. One of the most common mechanistic models used to describe physical nonequilibrium transport phenomena is the mobile–immobile model (MIM). Two significant parameters in the MIM are immobile water content (_im) and mass exchange coefficient (). Previously, a method for determining _im and using sequential tracers (ST) has been used to characterize solute transport. In this work, we present and evaluate a method to estimate _im and using time domain reflectometry (TDR). The TDR method was tested in laboratory experiments using three 20 cm long by 12 cm diameter undisturbed saturated soil columns. The method used TDR with an application of CaCl₂ to obtain resident concentrations as a function of time. The data obtained from TDR were analyzed using a log-linear equation developed based on the ST method to estimate _im and . The _im and estimates from the TDR method were compared with the estimates from the ST method and from effluent data. A conventional inverse curve fitting method (CXTFIT) was used to estimate parameters from effluent data. The means of _im/ from the TDR method, ST method, and effluent data were 0.31, 0.30, and 0.26, respectively. The means of from the TDR method, ST method, and effluent data were 0.03, 0.03, and 0.04 h^-1, respectively. The values of _im/ and from the TDR method were very similar to the estimates from the ST method. In all three columns, the _im estimates from the TDR method were within the 95% confidence intervals (CI) of the estimates from the effluent data. In two of three columns, the estimates from the TDR method were within the 95% CI of the estimates from the effluent data. The TDR method is relatively simple, rapid, and had advantages over the ST method and conventional methods for measuring solute transport properties.

Abbreviations: BTC, breakthrough curve • CI, confidence interval • MIM, mobile–immobile model • PVC, polyvinyl chloride • ST, sequential tracers • TDR, time domain reflectometry • , mass exchange coefficient • _im, immobile water content

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