title: Climate Change Impact Framework For Assessing Swiss Alpine Water Resources Using Transient Streamflow Scenarios
Creators: name: Wechsler, Tobias
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 910 Geography &amp; travel

description: In the Alpine region, global warming has led to a 2 °C increase in surface temperature compared to pre-industrial levels, which is approximately twice the global average. This warming impacts hydro-climatic variables and has further consequences for various sectors of water resources management (WRM). This thesis presents a climate change (CC) impact assessment framework applied to WRM sectors. It uses transient daily streamflow scenarios based on the EURO-CORDEX dataset and assesses changes over 30-year periods by comparing a reference period to future periods. The change framework comprehensively considers the CC impact on technical, legal, and ecological aspects and is applied to three critical Alpine WRM sectors: (i) Run-of-River (RoR) electricity production, (ii) environmental flow requirements, and (iii) lake level variability. RoR electricity production and the large prealpine lakes play a stabilising role due to their high turbine capacity and low volatility, while the lakes dampen inflows and can mitigate water level extremes. These two sectors are critical for alpine WRM and integral to national CC mitigation and adaptation strategies. However, However, the two sectors have received limited attention in past CC impact assessments.
RoR electricity production contributes to around half of Switzerland's hydropower production. The CC impact is assessed by using a Flow Duration Curve analysis and considering plant-specific characteristics. This enables CC impacts on RoR electricity production to be compared with loss due to environmental flow requirements and potential production increases through design discharge adjustments. The findings indicate a slight decrease in mean annual RoR electricity production (2 \% to 7 \%) by the end of the century, varying with catchment elevation. Seasonal projections suggest increased winter production (4 \% to 9 \%) when electricity demand is highest. However, the technical potential for production increase in winter (2.5 \%) is seven times smaller than in summer, and production loss due to environmental flow requirements is greater in winter (4.5 \%).
Environmental flow requirements, mandated for water-diverting power plants, are essential for ecosystem function. The change framework assesses how CC impacts the determination of the 347-day-streamflow value (Q₃₄₇, 95th percentile), i.e. the threshold used to derive environmental flows according to the Swiss Water Protection Act. CC-induced increases in Q₃₄₇ lead to a higher environmental flow, particularly for high-elevation catchments (> 2000 m a.s.l.). However, an increase in Q₃₄₇ results in a comparatively less pronounced increase in environmental flow. Taking this a step further, estimations of Alpine RoR electricity production alterations cannot be derived solely from changes in Q₃₄₇, as it necessitates considering the entire streamflow volume usable for HP production. The energy potential allocated to environmental flow requirements is estimated to be relatively small (1 \% to 7 \%) and plays a minor role compared to the overall energy potential. The dominant factors influencing changes in RoR electricity production are the CC-induced alterations in streamflow and the power plant's size of the design discharge.
Large perialpine lakes, vital for ecological, hydrologic, and socio-economic functions, are evaluated for the CC impact by combining hydrologic and hydrodynamic models. Annual mean lake levels indicate minor changes, but pronounced seasonal shifts. The extent of lake level management influences the magnitude of these changes: particularly summer lake levels are projected to decline by 0.04 m for the regulated lake and by 0.39 m for the unregulated lake (median, high-emission scenario). Such a shift could lead to more frequent and severe droughts in late summer, impacting the WRM of lakes.
The model-based change framework projects the CC impact on the three applied sectors of WRM. The CC impact assessments project changes already in the near future, with more pronounced effects expected over time and particularly in the absence of CC mitigation measures. Using 39 model chains over 30-year periods provides a robust foundation for assessing CC-induced mean changes, considering model uncertainty. The projected changes indicate shifts towards increased winter RoR electricity production and a higher prevalence of droughts in late summer. The energy potential share of environmental flow requirements in the overall energy potential remains relatively minor, but gains importance for aquatic life. Future research could explore interannual variability and the evolution of extremes, and consider more dynamic operational data, including water demand, to enhance the comprehensiveness of spatial coherence in WRM. Future climate services should support comprehensive decision-making processes related to CC mitigation and adaptation strategies, going beyond the sectoral perspective.