Although both raindrop driven processes and diffusion play important roles in the transfer of chemicals from soil to surface runoff, current transport models either do not consider the two processes together, or use 'effective' parameters with uncertain physical definitions. We developed a physically based, solute transport model that couples both mechanisms and tested it with laboratory experiments. One unique aspect of this study is that all the parameters were either directly measured or previously published, that is, there was no model 'calibration' or 'fitting.' Our model assumes that chemicals near the surface of the soil are ejected into runoff by raindrop impact and chemicals deeper in the soil diffuse into a surface layer, or 'exchange layer,' via diffusion. The exchange layer depth and transfer processes are derived from the 'shield' concept in the Rose soil erosion model (e.g. Rose, 1985). The model's governing equations were solved numerically and the results agreed well with experimental data (R2>0.90). The model's sensitivity to various physical and chemical parameters illuminated the importance of both raindrop controlled processes and diffusion on chemical transport from soil to surface runoff.