The influence of lakes and reservoirs on flood peaks at hourly vs. daily timescales in Switzerland

Abstract. Water bodies such as lakes and reservoirs can play a crucial role in reducing flood peaks both on daily and hourly timescales. While the effect of water bodies on flood peaks at different time resolutions has been demonstrated in the past, it remains unclear how they affect the ratio between daily and hourly peaks. Here, we analyse how water bodies attenuate flood peaks at daily and hourly time resolution and the relationship between flood peaks at these two time scales using two approaches: (1) four local case studies with gauges upstream and downstream of reservoirs, and (2) a large-sample hydrological dataset covering Switzerland. Our results show that hourly flows are dampened much more strongly than daily flows, which leads to similar daily and hourly flood peaks downstream of reservoirs. Specifically, our case study analysis highlights that (sub-)hourly flows are attenuated by up to 70 percent downstream of reservoirs during flood events with a 10-year return period. We also find that the attenuation effect is particularly pronounced in catchments that are heavily influenced by water bodies, i.e. those catchments where more than 60 percent of the area contributes to water body inflow. We conclude that considering water body influence on flood peaks is crucial to understand the similarity between daily and hourly flood peaks and that it should be considered in large-sample analyses using suitable metrics.