MODELING SOLUBLE CHEMICAL-TRANSFER TO RUNOFF WITH RAINFALL IMPACT AS A DIFFUSION PROCESS

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. 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. -from Author