Solute transport by surface runoff from low-angle slopes: theory and application

The removal of chemicals in solution by overland how from agricultural land has the potential to be a significant source of chemical loss where chemic als are applied to the soil surface, as in zero tillage and surface-mulched farming systems. Currently, we lack detailed understanding of the transfer mechanism between the soil solution and overland flow, particularly under field conditions. A model of solute transfer from soil solution to overland flow was developed. The model is based on the hypothesis that a solute is initially distributed uniformly throughout the soil pore space in a thin la yer at the soil surface. A fundamental assumption of the model is that at t he time runoff commences, any solute at the soil surface that could be tran sported into the soil with the infiltrating water will already have been co nvected away from the area of potential exchange. Solute remaining at the s oil surface is therefore not subject to further infiltration and may be app roximated as a layer of tracer on a plane impermeable surface. The model fi tted experimental data very well in all but one trial. The model in its pre sent form focuses on the exchange of solute between the soil solution and s urface water after the commencement of runoff. Future model development req uires the relationship between the mass transfer parameters of the model an d the time to runoff: to be defined. This would enable the model to be used for extrapolation beyond the specific experimental results of this study. The close agreement between experimental results and model simulations show s that the simple transfer equation proposed in this study has promise for estimating solute loss to surface runoff. Copyright (C) 2000 John Wiley & S ons, Ltd.