A physically based model for predicting solute transfer from soil solution to rainfall-induced runoff water

A model was developed to predict the field outlet concentration Hydrograph for chemicals released into overland runoff induced by continuous rainfall on a sloping soil. The model accounts for convective-dispersive solute transport in the soil and also considers rate-limited mass transfer through a laminar boundary layer at the soil surface/runoff water interface. Solutes are assumed to be subject to linear equilibrium sorption onto the solid phase of the soil. Concentration hydrographs for rainfall-induced runoff were derived by treating the runoff zone as a well-mixed reactor characterized by an appropriate residence time distribution. The model was used to predict experimental data obtained by Ahuja and Lehman (1983) for runoff from soil boxes with permeable bottoms. Good agreement was obtained between predicted and measured outflow concentrations for relatively low infiltration rates, provided relatively large dispersion coefficients were used in the calculations. Solute concentrations in the soil were also predicted well. Results for relatively high infiltration rates were less accurate, perhaps in part because of an incorrect assumption in the model that the infiltration rate remained constant in time and space during the runoff experiments.