Is drought protection possible without compromising flood protection? Estimating the maximum dual-use benefit of flood reservoirs in Southwest Germany

As climate change drives intensification and increased frequency of hydrological extremes, the need to balance drought resilience and flood protection becomes critical for proper water resources management. Recent extreme droughts in the last decade in Germany have caused significant damages to ecosystems and human society, prompting renewed interest in sustainable water resources management. At the same time, protection from floods such as the catastrophic 2021 event in the Ahr Valley remain heavy in the public conscience. In the state of Baden-Württemberg in Southwestern Germany alone, over 600 small (< 1 million m³) to medium-sized (1-10 million m³) reservoirs are currently operated for flood protection. In this study, we investigate optimal reservoir operating (storage and release) rules in a dual flood-drought protection scheme for selected modeled flood reservoirs in Baden-Württemberg. Daily target releases for drought protection are proposed based on modeled inflows from the calibrated hydrological model LARSIM. In a first step, the reservoir operation is optimized in a scenario of perfect knowledge of the future by using  meteorological observations as artificial weather forecasts in LARSM. The results of different operating rules are then evaluated based on their adherence to the target releases and flood protection performance. Rulesets that result in worsened flood protection relative to the current (flood-only) operation were eliminated as potential operation schemes. Based on provisional results, we will present and discuss the maximum potential benefit of adapting and retrofitting existing flood reservoirs for drought water storage.