COUPLING MIXING ZONE CONCEPT WITH CONVECTION-DIFFUSION EQUATION TO PREDICT CHEMICAL TRANSFER TO SURFACE RUNOFF

Modeling chemical transfer from soil solution to surface runoff is essential for developing a surface water quality model that can be used to assess pollution potentials of agricultural chemicals. Chemical transfer to runoff can be modeled as a two-rate process. A fast rate subprocess, which prevails at early stages of rainfall, causes an exponential depletion of chemicals from the mixing zone. A slow rate subprocess, which becomes significant under poor drainage conditions, transports chemicals into the mixing zone from the soil below. The two-rate process can be described by coupling the mixing zone concept with the convection-diffusion equation (CDE). We evaluated this coupling approach by comparing predicted results with measured bromide concentration data. A finite element scheme was developed to solve the CDE in conjunction with a near-surface boundary condition derived from a complete and uniform mixing theory. Overall results showed that without a calibration the coupling approach satisfactorily predicted bromide concentrations in both surface runoff and soil solution under the zero infiltration conditions. The proposed model adequately reproduced measured data for restricted infiltration conditions by introducing a mechanical dispersion coefficient (Dh). The fitted Dh is within the range reported in the literature for the repacked soil conditions. The coupling approach, while allowing for direct use of the mixing theory under free infiltration conditions, refines the theory for use under poorly drained conditions.