Evaluating landscape-level pesticide exposure from spray drift: A modeling study in an agriculture-dominated watershed using SWAT+ and DAD-Drift.

Pesticide transport from agricultural fields to surface waters is a major environmental concern, and reliable exposure assessments require models that capture the major transport pathways. While the Soil and Water Assessment Tool (SWAT+) is widely used for landscape-scale pesticide modeling, it lacks a process-based representation of spray drift, which can contribute substantially to surface water contamination. The newly developed Droplet and Atmospheric Dispersion drift (DAD-drift) model provides a mechanistic framework for estimating spray drift deposition under realistic field conditions. In this work, we present the first implementation of DAD-drift into a high-resolution SWAT + model of the upper Funne catchment in North Rhine-Westphalia, Germany. The application of two fungicides and five herbicides within a realistic 5-year crop rotation was simulated to assess the contribution of spray drift to landscape-scale exposure. The integrated modeling framework showed that spray drift markedly increases predicted pesticide concentrations in surface waters. Its effect was strongest for immobile substances with strong soil adsorption, which otherwise show little transport, while mobile substances were mainly governed by hydrological pathways but still exhibited additional concentration peaks from spray drift. Hydrological processes such as tile drainage, interflow, and surface runoff strongly shaped transport dynamics, controlling both timing and magnitude of pesticide export. Because these processes are sensitive to catchment parameterization, ensemble simulations were essential to capture variability and to robustly quantify the contribution of spray drift across scenarios. This study demonstrates that spray drift is a critical driver of pesticide exposure at the catchment scale and must be explicitly considered in exposure assessments. The combined SWAT+-DAD-drift framework provides a transferable tool for linking agricultural practices with water quality, and for evaluating the effectiveness of mitigation measures such as no-spray buffer strips or drift-reducing nozzles at landscape scale.