Constant slope impedance factor model for predicting the solute diffusion coefficient in unsaturated soil

Solute diffusivity (ratio of diffusion coefficients in soil and free water, D-S/D-0) is markedly soil-type dependent, Soil texture and pore size distribution govern the threshold soil-water content (theta (th)) where D-S/D-0 approaches zero as a result of discontinuous diffusion pathways. In a recent study (Soil Science 161:633-645), we suggested that theta (th) can be predicted from the soil-water characteristic curve (SWC) based on the Campbell pore size distribution parameter, b. In this study, the theta (th)-b expression was recalibrated based on diffusivity data for three soils (Hiroshima sand, Foulum loamy sand, and Yolo loam) measured in this study plus 20 soils reported in the literature, obtaining theta (th)=0.020b. As the SWC is often not measured, a second theta (th) expression that requires only knowledge of soil texture and bulk density was calibrated from measured data. A third expression, including both soil texture, bulk density, and Campbell b, was also calibrated and gave the most accurate description of theta (th). The solute impedance factor (ratio of diffusivity by volumetric soil-water content), f(1) = D-S/(theta D-0), was shown to increase linearly with the water content available for diffusion, theta (a)=theta-theta (th). The slopes of the f(1)-theta (a) relations were similar for most soils and did not exhibit soil-type dependency. Based on this, a so-called constant slope impedance factor (CSIF) model to predict D-S(theta (a))/D-0 is presented. The model can be used in combination with any of the three suggested theta (th) expressions. Combined with the soil-texture/bulk-density dependent theta (th) expression, the model accurately predicted solute diffusivities for three independent soils for which the SWC were not known.