CONSTANT SLOPE IMPEDANCE FACTOR MODEL FOR PREDICTING THE SOLUTE DIFFUSION COEFFICIENT IN UNSATURATED SOIL

Abstract

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 (θ 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 θ th can be predicted from the soil-water characteristic curve (SWC) based on the Campbell pore size distribution parameter, b. In this study, the θ 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 θ th =0.020b. As the SWC is often not measured, a second θ 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 θ th . The solute impedance factor (ratio of diffusivity by volumetric soil-water content), f 1 = D S /(θ Do), was shown to increase linearly with the water content available for diffusion, θ a =θ - θ th . The slopes of the f 1 -θ 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 (θ a )/D 0 is presented. The model can be used in combination with any of the three suggested θ th expressions. Combined with the soil-texture/bulk-density dependent θ th expression, the model accurately predicted solute diffusivities for three independent soils for which the SWC were not known.