Prediction of Dispersivity for Undisturbed Soil Columns from Water Retention Parameters

Abstract

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 (ψ 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 CaCl 2 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 ψ a and b increased. Since both ψ a 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.