Alpine droughts under climate change: Assessing the relationship and impacts of combined summer drought-heat and winter drought events using a hydrometeorological model ensemble

Foreland and lowland regions in Europe are highly dependent on water stemming from the alpine water tower. Due to an increasing number of droughts, expected to exacerbate under progressing climate change, these regions experience severe economic and environmental impact. This affects e.g. hydropower production, ecosystem health, agriculture and drinking water supply. To take foreward-looking adaptation measures, there is the need to understand future climatic drought patterns and their impact on streamflow.

In order to evaluate and analyze past and future alpine droughts, a regional single-model initial-condition large ensemble (SMILE) with 50 members from 1991 - 2100, bias-corrected via MBCn and statistically downscaled, is used. This ensemble approach ensures the quantification of natural climate system variability, while improving the robustness of the results in future climate projections. To enhance interpretability and comparability, a global warming level approach is used. For each warming level (1.5°, 2°, 3° and 4°C), combined drought-heat events in summer (June to August) and snow-drought events in winter (December to February) are identified. To assess the impact of these drought events on discharge, a hydrological large ensemble for selected alpine catchments for the period 1991 – 2100 is created with the Water Balance Simulation Model (WaSiM), using the processed data of the SMILE as forcing.

The results of this study indicate, how future climate will change the water balance in selected alpine catchments based on the return frequency of severe drought events in summer and winter and their impact on runoff. Particularly, this study examines the cascading effects of winter snow-droughts on subsequent summer water availability, revealing how reduced snowpack accumulation under warmer conditions intensifies summer compound drought-heat events. By analyzing different global warming levels, the results provide scenario-independent insights, that are relevant for any emission pathways reaching these specific warming levels. This approach allows for direct comparison with policy goals, such as those specified in the Paris Agreement, and provide stakeholders with a concrete framework for assessing climate risks, regardless of the considered time frame.