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

Abstract. 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 for water supply downstream in a dual flood-drought protection scheme for 30 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. Modified operation rules are optimized in a scenario of perfect knowledge of the future by using meteorological observations as artificial weather forecasts in LARSIM. The results of different operating rules are then evaluated based on their adherence to the target releases and flood protection performance. Reservoirs were required to maintain the same level of flood protection under these modified rules. Optimized reservoirs were able to release up to 80 times their volume or improve up to 95 % of existing drought conditions (penalty and volume deficit) over a 24-year period, though never simultaneously—there seems to be a trade-off between relative water availability to the reservoir and ability to alleviate drought conditions. Certain reservoirs were near-optimal, others could be improved further, and still others were not very effective at reducing drought conditions. We find that relative water availability at the reservoir (expressed as the number of times the reservoir can be filled by the difference between the mean inflow and mean low flow) has a strong relation to the amount of water a reservoir can release for drought protection, but fails to summarily describe the reservoir’s potential impact on drought conditions downstream.