Uncovering fine-scale surface flow dynamics with particle tracking velocimetry: A new benchmark for soil erosion modelling

High-resolution measurements of soil surface flow velocity are critical for advancing the calibration and validation of physically based runoff and erosion models, yet such data remain scarce, particularly under field conditions. This study demonstrates the application and feasibility of particle tracking velocimetry (PTV) for capturing spatially distributed flow velocities during artificial rainfall simulations on agricultural plots. Combined with structure from motion (SfM) for topographic change detection, PTV enables detailed, non-invasive measurements of surface flow patterns at millimetre to centimetre scales. Two process-based models were applied and compared against these flow velocity observations. We further analysed the influence of digital elevation model (DEM) resolution on flow simulations, revealing that while average velocities remained relatively stable, spatial flow patterns and rill formation were strongly dependent on resolution. Model comparisons showed that dynamic surface updates better reflected observed flow patterns compared to a static approach. Measured flow velocities from PTV show slight variation from model outputs, due to scale and the nature of measurement. Our results position PTV as a powerful tool for future soil erosion research, enabling spatially resolved flow velocity estimation, improved validation of hydrodynamic processes, and more physically meaningful model parameterisation. This study provides a proof of concept for in-field PTV during rainfall simulations on small agricultural scales and for integrating high-resolution optical measurements into process-based runoff and erosion modelling workflows.