The size, location and design of upstream detention basins govern their impact on water discharge at local and catchment scales

The flow chart shows the identification of spatially explicit detention basins, their incorporation in the distributed modelling and the received results at the local and catchment scale. Weather extremes in recent years, particularly floods, together with their projected intensification, underscores the need for sustainable approaches to vulnerability reduction through climate adaptation and flood mitigation. Nature-based solutions offer an alternative to conventional grey infrastructure, and upstream water retention is a well-established strategy for reducing downstream flood peaks. This study presents a theoretical assessment of the water-holding capacity and peak-flow-reduction potential of spatially explicit upstream detention basins (DBs) in Svartån, a medium-sized catchment (761 km2) in central Sweden. Potential DB locations were delineated using high-resolution spatial data and their hydrological functioning was evaluated through distributed modelling over a three-year period. Model performance in the baseline scenario was high (Kling–Gupta efficiency: 0.88). Without recalibration, a water-retention scenario introducing 37 DBs across the catchment resulted in a 2–3% reduction in 95th-percentile flows at the catchment scale. At local scales, the corresponding peak flows were reduced by up to nearly 70%, although attenuation efficiency varied depending on the interplay between upstream contributing area, DB-specific storage capacity, and the maximum allowable discharge. Distributed modelling thus provides a useful framework for exploring alternative DB configurations under site-specific constraints and for identifying designs that maximise flow attenuation or support shifts toward broader ecosystem-service provisioning.