Transport of salts in soils and subsoils

Factors affecting transport of salts in saturated-unsaturated dryland soils are reviewed. Simultaneous movement of water and salts occurring in homogeneous soils is discussed first. Governing equations describing combined diffusion-convection transient transport and miscible displacement of salts, as well as nonsteady water flow are given. Effects of salinity on soil water transmission rates are described. The governing equations and boundary conditions appropriate to three processes of the dryland hydrologic cycle: infiltration, redistribution, and evaporation, are formulated in a manner suitable for mathematical modeling. Numerical solutions are obtained. One-dimensional, vertical profiles of non-interactive salts are described for homogeneous, bare, fallow soil. Effects of physico-chemical interactions between solution and soil matrix on transport of interactive anions and cations are incorporated. Vegetation factors and their effects on salt dynamics and distribution are also considered. Consideration of salt transport in saturated-unsaturated heterogeneous fields concludes the paper. Statistics of field profile concentration and salt dynamics in field scales are given.