Dual-Permeability Modelling of Preferential Flow in Structured Field Soil.

Accounting for preferential flow in quantitative simulation models of variably-saturated water flow and solute transport remains a challenge especially when attempting to relate soil structural properties to model parameters and to use experimental methods for model calibration and verification. Macroscopic model approaches often assume a partitioning of the structured soil into two separate but interacting porous domains. The empirical parameters of such conceptual two-domain models are representing sub- macroscopic-scale effects of the structured soil. Currently, structure-related parameters are mostly estimated or obtained by fitting rather than by direct measurements. The key macroscopic parameters of the dual-permeability water and solute mass transfer terms are describing the geometry (shape and size) and the hydraulic conductivity and diffusivity of the mass transfer between the preferential flow domain and the soil matrix. While the shapes and sizes may directly be obtained from soil observations, the hydraulic and diffusive resistances (e.g., due to coatings) at ‘internal’ surfaces of flow paths are hard to define or to measure. This contribution summarizes recent attempts to evaluate and parameterize sensitive soil structures and discusses how the local-scale parameters are affecting the larger scale results depending on the model and the dimensionality of the problem. Examples of model application describe 1D and 2D simulations of a tracer experiment at a subsurface drained field site.