Modeling pesticides in global surface soils: Exploring relationships between continuous and discrete emission patterns

Continuous pesticide emission at constant rate does not occur in reality, but can be a useful and simple concept in modeling studies. To explore the relationship between continuous and discrete emission patterns, we introduced a simple equivalent approach based on a comparison of simulated surface soil pesticide concentrations. The simulated results indicate that, at high soil pesticide dissipation rates and low emission frequencies, the average concentrations under the continuous and discrete emission scenarios were very similar. We demonstrated that the continuous emission model that used the simple average method to calculate the emission rate always overestimated the simulated pesticide concentrations in the surface soil compared to the discrete emission model when using a one-year period based on agricultural practices. In addition, we incorporated the equivalent approach into the USEtox model (a screening-level tool), which can approximate the average pesticide concentrations in surface soil using the time-integrated fate factors at different emission frequencies. The results indicate that the continuous-emission simulations agree with the discrete emission for at least 90% of the selected pesticides based on annual or semi-annual emission patterns. Further studies into other topics, such as random emission patterns and simulation periods, are required to improve the model. Nevertheless, the equivalent approach presented in this study can aid in transforming discrete emission patterns into continuous-emission-based models and improve surface soil pesticide management.