Abstract

Modeling the coupled movement of salt and water in the soil profile is of great interest to soil scientists and agronomists when combating soil salinity and in maintaining the sustainable use of saline soils. The hydrodynamic dispersion coefficient is the most important parameter for simulating the salt movement process. In this study, an analytical solution to the 1-D convection-diffusion differential equation (CDE), formulated under appropriate initial and boundary conditions, was used to model the salt-water movement in the soil profile. Laboratory experiments, using a silt loam and a silty clay loam soil in short soil columns (10 cm), were conducted under the same conditions of leaching solution sodium absorption ratio (SAR) (2, 10, 20, 30) and concentration (80, 40, 20 10, 2, 0 meq/L) as those used in the analytical solution of the CDE. Breakthrough curves, as functions of pore volume and salt concentration in the discharged solution were determined. The appropriate analytical solution was fitted to the breakthrough curves using the Golden-Section Search method to determine the hydrodynamic dispersion coefficient for salt movement in the different soil profiles under the various experimental conditions. The computed results indicated that the 1-D CDE well represented the salt leaching processes as indicated by the experimental data, except for the cases that resulted in extremely low hydraulic conductivity causing incomplete breakthrough curves. For a given SAR, higher salt concentrations in the leaching solution gave better fits of the analytical solution to the breakthrough curve. For a given salt concentration, solutions with lower SAR values were better fitted. The hydrodynamic dispersion coefficient generally increased with increased pore flow velocity. The increases were exponentially related to pore flow velocity for the silt loam but the relationship was more linear for the silty clay loam.

Key words: Soil salinity, convection-diffusion equation, analytical solution, diffusivity coefficient, breakthrough curve.