: constants ■ Experimental data Regression Proposed constants  Olesen’s threshold water content model best predicted the tortuosity of three tested soils.  Lim’s conceptual model of series arrangement predicted the tortuosity at lower water contents well, and the combined model predicted the tortuosity at higher water contents well.  The prediction capabilities of water content based models, and water content- porosity/bulk density based models are highly relied on the accurate estimation of the model parameters. The proposed empirical constants did not predict the three tested soils well. Evaluation of Tortuosity Models for Predicting Solute Diffusion in Unsaturated Soils Hsinyi Chou and Laosheng Wu, Department of Environmental Sciences, University of California, Riverside, Riverside, CA Solute diffusion in soil - Fick ’ s Law The solute diffusion flux, J, is described as :diffusion coefficient of the solute in water :diffusion coefficient of the solute in soil :liquid phase tortuosity factor Use measured effective diffusion coefficients to evaluate tortuosity prediction models Objective Methods 0.01M KBr Air-dried, sieved soils were packed to the desired bulk densities into diffusion Tempe cells. Saturate the cell with 0.01M KBr solution. Place the cell into a pressure chamber to obtain various water contents by applying different pressures. No Flow Place an anion-exchange membrane in Cl - form on the top. Ensure the membrane is in good contact with soil surface. Remove membrane after 81 minutes of diffusion. Rinse the membrane with D.I. water. Extract Br - from the membrane by 50 mL 0.5M HNO 3. Use spectrophotometric method to measure the mass of Br - diffused to the membrane. Root Mean Square Error (RMSE) :the difference between predicted and measured values of tortuosity :the number of measured values Bias Results Conclusions Water content - bulk density - soil water characteristic based tortuosity models Olesen et al. (1996a ) : tortuosity at water saturation: soil texture parameters Olesen et al. (1996b ) CL, CS, FS,SF : gravimetric fraction (g/g) of clay, coarse sand, fine sand, and silt respectively BD :bulk density (g/cm 3 ) : threshold water content Water content – porosity/bulk density based tortuosity models So and Nye (1989) Millington and Quirk’s (1961) : porosity (cm 3 /cm 3 ) : bulk density (g/cm 3 ) Water content based tortuosity models Olsen and Kemper (1968) a, b, n : constants Papendick and Campbell (1980) Mullins and Sommers (1986) : volumetric water content (cm3/cm3) ■ Experimental data Regression Proposed constants Conceptual tortuosity models Khaleel and Saripalli, 2006 :the estimated immiscible fluid interfacial area for the corresponding idealized capillary bundles :the estimated immiscible fluid interfacial area in a real unsaturated medium :the matric potential (cm-H 2 O) : the saturation degree (%) Model Ⅰ.Parallel model Model Ⅱ. Series model Model Ⅲ. Combined parallel and series model Lim et al., 1998 Results Materials Soil Particle size distribution Bulk density (g/cm 3 ) Porosity (cm3 /cm3) sandsiltclay Sand Sandy clay loam Clay : a factor that includes both the entrance effect and the effects of increasing viscosity and ionic interaction along small pores :the residual degree of saturation :the effective saturation which is equal to :the ratio of the radius of the hydrated ion to the radius of the pore :the proportion of diffusion along the water-filled pores and the water films in the parallel model