Abstract
A mathematical model describing the exchange of solute from soil water to overland flow is presented. The model assumes that during infiltration a finite amount of chemical remains resident at the soil surface and is not convected with the infiltrating water. This solute is then transferred to the surface water via diffusion; driven by the concentration difference between the soil water and overland flow. A small scale field runoff trial was used to validate the model. Bromide was applied to the surface of three 18 m long by 1.5 m wide plots which were subsequently irrigated with a microjet sprinkler system. Runoff collected from the plots was analysed for bromide. The experimental data were in excellent agreement with model predictions.