Estimating high resolution exposure of an agriculturally dominated catchment with DAD-drift model and SWAT+

Modeling environmental concentrations of plant protection products typically includes runoff, drainage, and leaching processes, which are well represented in recent landscape scale modeling approaches. However, the modeling of spray drift at the landscape scale is challenging and often simplified or neglected due to high computational efforts. For example, spray drift is often implemented by external calculation of drift curves with the pesticide load added directly to the channel network. Although this approach enables in general a basic landscape-level spray drift estimation, it lacks the spatio-temporal details such as the distribution of drift in relation to other landscape elements (e.g., water bodies, non-target areas). To address these limitations, we developed the Droplet and Atmospheric Dispersion drift (DAD-drift) model which integrates mechanistic droplet model, a micrometeorological model, and a three-dimensional Gaussian puff model designed for ground application. DAD-drift considers the physical principles of spray drift, the spatial relationship between application areas and to non-target areas, as well as local weather conditions at the landscape scale. Its modular design allows for easy integration with other models.

We combined, a high-resolution SWAT+ model of an agriculturally dominated catchment in Germany with DAD-drift to enhance our understanding of pesticide transport pathways and to assess the different exposure routes of plant protection products. Flow observation data are used for hard calibration, supported by additional soft calibration data (i.e., evaporation, surface runoff, subsurface drainage, groundwater recharge, total runoff). Agricultural practices, i.e. crop rotations with catch crops, tillage operations, and plant protection product application timing are adopted from a 5-year data set from 2019 to 2023. Results indicated that transport via spray drift is significant for exposure at the landscape scale, with the dominant transport pathway varying considerably based on individual substance properties and application timing.

The model setup can be used to identify critical source areas and to optimize the application of plant protection products. In addition, the effectiveness of risk mitigation practices, such as drift reduction nozzles and no-spray buffer strips, can be assessed. Furthermore, linking the exposure dynamics predicted by the SWAT+ model with effect modeling approaches is feasible.