Modeling solute transfer from soil to surface runoff: The concept of effective depth of transfer

A model for predicting the transfer of chemicals from soil to surface runoff water is described. The model applies a perfectly mixed reactor analog to an equivalent and homogeneous soil layer of uniform concentration near the soil surface, called the "Effective Depth of Transfer" (EDT). Solute concentrations in the EDT model are taken to be equal to the soil surface concentration as determined by an analytical solution of the one-dimensional diffusion equation for the limiting case of no infiltration. The model considers linear equilibrium sorption-exchange, and also accounts for rate-limited mass transfer through a laminar boundary layer at the soil surface-runoff water interface. The time-dependent EDT was found to vary as a linear function of the square root of time. The accuracy of the EDT approach is illustrated with one example, which compares the EDT solution for the soil surface concentration with the exact analytical solution of the diffusive mass transfer equation. The ability to change values of the physically based parameters adds considerable flexibility in application of the model to different locationswith site-specific runoff regimes, soil properties, and solute characteristics.