TEST OF A NON-UNIFORM MIXING MODEL FOR TRANSFER OF HERBICIDES TO SURFACE RUNOFF.

The applicability of a non-uniform mixing model to describe the transfer to runoff of four surface-applied herbicides (atrazine, alachlor, propachlor, and cyanazine) and three nutrients was tested using field plot data for four levels of residue cover. Starting from the time of runoff initiation, it is assumed that the degree of mixing between rainfall and soil solution, +63,decreases exponentially with soil depth. The adsorption-desorption process is represented by a proportional relationship, C//s equals alpha C, where C//s is the concentration of chemical in the adsorbed phase on soil particles (g/g), C is the concentration in soil solution (g/mL) and alpha is a constant. In all cases, model calculations compared well with measured chemical concentration-time curves. The structure of the model for the variable degree of mixing with depth, the partitioning equation for adsorbed and solution phases, and the downward displacement of the chemical by infiltration seems to reasonably represent reality.