VALIDATION OF A 2-DOMAIN-MODEL FOR THE SI MULATION OF SOLUTE TRANSFERIN SOILS BY LABORATORY AND FIELD EXPERIMENTS

The simulation of water and solute transfer in the unsaturated zone on the basis of the classical convective dispersive model gives dissatis factory results in many cases. Especially in the zone of evapotranspir ation the observed penetration depths of surface-applied chemicals oft en exceed calculated values. This has been attributed to preferential flow in domains with accelerated flow. The aim of the investigations p resented here was to extend the conventional theory of solute transfer in order to enable improved model predictions without introducing too much complexity to the model which might reduce its applicability. Th e mobile-immobile-concept of Coats and Smith (1964) was chosen to exte nd the model FLOTRA (Wagner et al., 1992), which is based on the conve ctive dispersive approach. The model parameters additionally needed by the model are the immobile water content theta(im) and the solute tra nsfer coefficient alpha. The applicability of the model was tested on the basis of several laboratory and field experiments. It is shown, th at with the mobile-immobile-approach modeling results of water and sol ute transfer in the top soil layers, which are most important in view of soil and groundwater protection, were improved compared to the calc ulations with the classical convective dispersive model.