A quasi-analytical model for soil solute movement under plant water use

Solute accumulation in surface soils through capillary rise transport, driven by evaporation, is a serious management issue. In particular, for soils under a saline shallow water table, salt buildup can have a serious detrimental impact on agricultural productivity, For vegetated surfaces, evaporation is the sum of water loss directly from the surface and that taken up by plants for transpiration. We developed a procedure for the prediction of solute migration in soils under plant water use in a shallow water-table environment. For this situation, the advection-dispersion equation is shown to be linear with nonconstant coefficients, To solve this equation, the root zone is divided into a series of layers and for each layer the governing equation is approximated by a constant coefficient form with layer-averaged values for properties. We derived a solution to this equation in Laplace space, which is coupled to its neighbors by requiring the flux and concentration be constant across the layer boundaries, At each time level, a matrix system is posed for the equation coefficients and concentration resolved by numerical inversion from Laplace space. The good agreement between model predictions and solutions obtained from a finite-element analysis indicates that the procedure presented is of high accuracy and could offer computational savings over purely numerical procedures. The multilayer approach allows the representation of nonuniform plant water use functions, dispersivities, soil properties, and initial conditions.