A Simple Equation for Predicting Preferential Flow Solute Concentrations

The transport of pesticides and other chemicals through macropores has been widely observed and predicting it is a challenge. This article considers a simplified two-layer model, similar to overland flow models in which the processes of adsorption and desorption are separated. For the layer near the surface, or the mixing layer, the solute concentration in the layer is equal to that in the percolating water (including preferentially moving water). In the lower profile, the flow is partitioned between matrix and preferential flow. The solute concentration of the matrix flow is characterized by the soil condition near the outlet point, whereas the preferential flow is represented by the solute concentration in the mixing layer. The closed form equation, exhibiting exponentially decreasing macropore flow solute concentrations, is tested against solute breakthrough curves using three independent sets of experimental data. The predicted depths of mixing between 5 and 25 cm are physically realistic and the closed form is shown to reproduce the form of experimental data, particularly under conditions of significant macropore flow. Although highly simplified, the physically based model yields a framework for predicting solute concentration for preferentially moving water.