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DIVISION S-1—SOIL PHYSICS

Prediction of Dispersivity for Undisturbed Soil Columns from Water Retention Parameters

^a Dep. of Geological Sciences, Univ. of Tennessee, Knoxville, TN 37996
^b Dep. of Plants, Soils and Biometeorology, Utah State Univ., Logan, UT 84322
^c Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546

* Corresponding author (eperfect{at}utk.edu)

Dispersivity () is a required input parameter in solute-transport models based on the advection-dispersion equation (ADE). Normally is obtained from miscible-displacement experiments. This dependency on inverse procedures imposes a severe limitation on our predictive capability. If solute breakthrough curves and soil hydraulic properties were measured simultaneously, pedotransfer functions could be developed to predict from independent measurements. In this study, short (6 cm long) undisturbed columns were employed to investigate the relationship between and the water-retention curve as parameterized by the air-entry value (_a) and Campbell exponent (b). We worked with 69 columns from six soil types ranging in texture from loamy sand to silty clay, conventional-till and no-till management practices, steady-state saturated flow conditions, and a step decrease in CaCl₂ concentration from 0.009 to 0.001 M. Breakthrough curves were measured by monitoring changes in effluent electrical conductivity using a computerized data acquisition system. Estimates of (calculated using the method of moments) ranged from 1 to 192 mm for the six soil types. Stepwise multiple-regression analysis explained 50% of the total variation in , and indicated that dispersion increased as _a and b increased. Since both _a and b increase with increasing clay content, also increases moving from coarse- to fine-textured soils. Our regression equation can be used as a pedotransfer function to predict from existing databases of soil hydraulic properties. Further research is needed to independently validate its predictive capability, and to develop strategies for upscaling the model predictions.

Abbreviations: ADE, advection-dispersion equation • b, Campbell exponent • D, dispersive coefficient • v, mean pore-water velocity • , dispersivity • , total porosity • _a, air-entry value • **, significant at the 0.01 probability level

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