Effects of soil moisture on the diurnal pattern of pesticide emission: Numerical simulation and sensitivity analysis

Accurate prediction of pesticide volatilization is important for the protection of human and environmental health. Due to the complexity of the volatilization process, sophisticated predictive models are needed, especially for dry soil conditions. A mathematical model was developed to allow simulation of the diurnal variation of pesticide volatilization as affected by soil-water content, the air–solid interface partition coefficient, soil-water retention function and soil surface resistance processes. The model formulation considered two possible water retention functions and two soil surface resistance functions. To test the model, simulations were performed for ten successive days of drying under typical semi-arid summer conditions following application of the pesticide diazinon to either a loam or sand soil. Results showed that the temporal variation and magnitude of diazinon emission were strongly affected by the air–solid interface partition coefficient, soil-water content and the surface resistance function. The model was capable of simulating complex diurnal patterns in the peak emission rates which are caused by changes in soil water content and air–solid partitioning. The water retention function formulation had only a minor effect on the simulated water content and volatilization rates, whereas the soil surface resistance function significantly influenced the volatilization rate. Neither the water retention function nor the soil surface resistance formulation had a significant effect on the simulated soil temperature.