Efficient calculation of solute transport in heterogeneous soils exhibiting non-Richards-type unsaturated flow

Soil heterogeneity hampers our ability to calculate field-scale solute leaching, Many of the existing field-scale modeling attempts are laborious, time consuming, and computationally demanding. Non-Richards-type flow (e.g., fingering) cannot be handled by methods based solely on the heterogeneity of the soil hydraulic properties. We developed a time- and cost-effective approach that starts with a plot-scale solute transport model that must incorporate any special flow features in the field of interest. The field was modeled as an ensemble of plot-scale columns with random properties. The computational demand was reduced by: (i) simplifying the plot-scale model; (ii) keeping only the most influential parameters stochastic; (iii) replacing a full Monte Carlo simulation by systematic parameter sampling; and (iv) transforming the model output for one column to that of another where possible. The method was demonstrated for a field exhibiting fingered flow in the topsoil. A recently developed model to calculate solute leaching front a single finger served as the column-scale model. Three parameters appeared to be key for solute leaching to the groundwater. For two of them, the breakthrough curve for one value of the parameter could be transformed to approximate the curve for all other values. Consequently, field-scale solute leaching could be computed with <20 model runs.