Prediction of Dispersivity for Undisturbed Soil Columns from Water Retention Parameters
2002
Perfect, E. | Sukop, M. C. | Haszler, G. R.
Dispersivity (α) is a required input parameter in solute-transport models based on the advection-dispersion equation (ADE). Normally α 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 α from independent measurements. In this study, short (6 cm long) undisturbed columns were employed to investigate the relationship between α and the water-retention curve as parameterized by the air-entry value (ψₐ) 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 α (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 α, and indicated that dispersion increased as ψₐ and b increased. Since both ψₐ and b increase with increasing clay content, α also increases moving from coarse- to fine-textured soils. Our regression equation can be used as a pedotransfer function to predict α 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.
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