Modeling Soluble Chemical Transfer to Runoff with Rainfall Impact as a Diffusion Process

Rainfall impact increases the transfer of chemicals from soil solution to surface runoff. This transfer is modeled here as an accelerated diffusion process. Raindrop impacts are assumed to increase the diffusion coefficient in the top 2‐cm layer of soil to constant or exponentially diminishing values with depth. Transient development of a thin, well‐stirred water layer on the soil surface is allowed. Process equations are solved by a finite‐difference numerical scheme. The model was evaluated on experimental data for Br in runoff with time from three soils with three infiltration rates. The model gave a generally good description of the data. At zero infiltration, a constant accelerated‐diffusion coefficient, D a , equal to 2.5 to 19 times the molecular diffusion coefficient, D a , was required. An exponentially decreasing D a , with surface values 3 to 54 times D a , gave better results in all cases. Given a suitable mechanical dispersion coefficient for two soils, the above D a values also gave good fit for two nonzero infiltration rates; for the third soil, a swollen clay, the very high D a values had to be decreased somewhat, apparently because of less surface perturbation by rainfall when infiltration was occurring. For a moderately soil‐adsorbed chemical, transfer to runoff is also increased significantly with accelerated diffusion, and the effect of infiltration is also very high. More of an adsorbed (vs. a mobile) chemical is lost to runoff; however, chemical transfer to runoff did not greatly change the shape and magnitude of a surface‐applied chemical pulse moving downward in soil.