Melting Alpine Water Towers Aggravate Downstream Low Flows: A Stress-Test Storyline Approach

Droughts can lead to extreme low flow situations in rivers, with resulting severe impacts. Upstream snow and ice melt in many of the world's mountain water towers can alleviate the hydrological consequences of drought, yet global warming threatens the cryosphere. To improve the understanding of melt water contributions during drought in the case of future glacier retreat, we developed stress-test storyline scenarios to model streamflow and tested them in the European river Rhine basin. Meteorological conditions of past drought and low flow years in Europe, 1976, 2003, and 2018, were repeated at three future moments in time, representing nowadays, near future and far future conditions. The latter two conditions were obtained by climate projections under the RCP8.5 scenario. Results show that the low flow situations caused by the meteorological drought situations aggravate in future conditions, more so for the far future and for the year 2003 because of the relatively large glacier ice melt contribution in the past. Summer (July-September) streamflow may decline by 5%-25% far downstream and 30%-70% upstream and the duration of extreme low flow situations may double compared to the selected past drought events. These results are relevant for the Rhine as a major European river but stand exemplary for many other river basins and highlight the importance of cryospheric changes for downstream low flow situations in a changing climate. The stress-test scenarios allow a glimpse into future extreme low flow events aiding adaptation planning, and might be adapted to include other important low flow drivers.

Automated summary

Droughts can cause extreme low flow in rivers, impacting negatively on their ecosystems. Melting snow and ice in mountain water towers can help alleviate the hydrological consequences of drought, but global warming threatens this cryosphere. A study on the European river Rhine basin shows that low flow situations worsen in future conditions due to increased meteorological drought and glacier ice melt contributions. These findings are important for adaptation planning and are relevant for other river basins.