This study develops a spatially distributed, process-based model for a 196 km² mesoscale catchment in southwest Germany to better understand flash floods and runoff generation in headwater regions. The model highlights the importance of gradients, land use, and small-scale processes in shaping flood magnitudes, especially in poorly gauged areas. It also evaluates Nature-based Solutions at hillslope and headwater scales, showing their potential impact on downstream flood risk and informing improved flood hazard assessment.