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United States Department of Agriculture

Agricultural Research Service

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

Authors
item Lee, Jaehoon - IOWA STATE UNIVERSITY
item Horton, Robert - IOWA STATE UNIVERSITY
item Jaynes, Dan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 16, 1999
Publication Date: July 15, 2002
Citation: LEE, J., HORTON, R., JAYNES, D.B. A TIME DOMAIN REFLECTOMETRY METHOD TO MEASURE IMMOBILE WATER CONTENT AND MASS EXCHANGE COEFFICIENT. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL. 2002. V. 64. P. 1911-1917.

Interpretive Summary: Scientists have a wide array of computer models for predicting the fate and transport of agrochemicals applied to soil. Before these models can be accurately applied to specific situations, the parameters required for the model need to be measured or estimated for the soil of interest. One group of models, that is very useful for estimating rapid movement of water and chemicals through undisturbed natural soil, has been little used outside of the laboratory because of the difficulty in measuring the required model parameters. This research developed and validated a new methodology for measuring these parameters that is easy, quick, and adaptable for use in the field. We show that the new method gives model parameter estimates that are as reliable as the more traditional, but time consuming measurement methods. Now that the new method has been validated, scientists will be able to use the method with confidence in estimating parameter values for a wide range of soils and locations. The method will allow more accurate modeling of water and chemical transport through soils and better estimates of potential ground water contamination by agrochemicals.

Technical Abstract: 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 wim and mass exchange coefficient alpha. Previously, a method determining wim and alpha using sequential tracers (ST) has been used to characterize solute transport. In this work, we present and evaluate a method to estimate wim and alpha 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 colums. 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 wim and alpha. The wim and alpha 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 immobile water fraction from the TDR method, ST method, and effluent data were 0.31, 0.30, and 0.26 respectively. The means of alpha from the TDR method, ST method, and effluent data were 0.03, 0.03, and 0.04 h**-1, respectively. The values of the immobile water fraction and alpha from TDR methods were within 95% confidence intervals (CI) of the estimates from the effluent data. In two of three columns, the alpha 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.

Last Modified: 9/10/2014
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