Coupling mixing zone concept with convection-diffusion equation to predictchemical transfer to surface runoff

Modeling chemical transfer from soil solution to surface runoff is essentia l for developing a surface water quality model that can be used to assess p ollution potentials of agricultural chemicals. Chemical transfer to runoff can be modeled as a two-rate process. A fast rate subprocess, which prevail s at early stages of rainfall, causes an exponential depletion of chemicals from the mixing zone. A slow rate subprocess, which becomes significant un der poor drainage conditions, transports chemicals into the mixing zone fro m the soil below. The two-rate process can be described by coupling the mix ing zone concept with the convection-diffusion equation (CDE). We evaluated this coupling approach by comparing predicted results with measured bromid e concentration data. A finite element scheme was developed to solve the CD E in conjunction with a near-surface boundary condition derived from a comp lete and uniform mixing theory. Overall results showed that without a calib ration the coupling approach satisfactorily predicted bromide concentration s in both surface runoff and soil solution under the zero infiltration cond itions. The proposed model adequately reproduced measured data for restrict ed infiltration conditions by introducing a mechanical dispersion coefficie nt (D-h) The fitted D-h is within the range reported in the literature for the repacked soil conditions. The coupling approach, while allowing for dir ect use of the mixing theory under free infiltration conditions, refines th e theory for use under poorly drained conditions.