title: Future changes of Swiss river runoff and extreme vertically integrated moisture transport
Creators: name: Mülchi, Regula Isabelle
Publisher: name: Universität Bern
Year: 2025
Subjects: subject: 550 Earth sciences &amp; geology
subject: 910 Geography &amp; travel

description: Climate change is likely to alter important hydrological variables due to increasing temperatures and changing precipitation patterns and extremes. Therefore, assessments of future changes in Swiss river runoff is crucial for adaptation and mitigation planing. In this thesis, changes in river runoff characteristics are analysed for runoff regimes and moderate low and high flows. In addition, changes in the atmospheric potential for floods are investigated.
In the first part of this thesis, new hydrological scenarios for Switzerland forced by the most up to date climate change scenarios (CH2018) are presented. The new dataset "Hydro-CH2018-Runoff'' consists of daily runoff simulations for 93 meso-scale catchments in Switzerland for the period 1981-2099. The hydrological modelling system PREVAH (Precipitation-Runoff-Evapotranspiration HRU-related Model) was thoroughly calibrated and validated for each catchment. The calibrated parameters show satisfactory performance with a median Nash-Sutcliffe efficiency of 0.82 and a median Kling-Gupta efficiency of 0.89 in the calibration and validation period. The calibrated parameters were then used to simulate runoff under climate change for the period 1981-2099 on a daily basis. These simulations were driven by the high-resolution CH2018 scenarios consisting of 68 model chains of global and regional climate models covering 3 different Representative Concentration Pathways (RCP): RCP2.6, RCP4.5, and RCP8.5. The Hydro-CH2018-Runoff ensemble is freely available for download and can be used for further impact studies.
Climate change impacts on runoff regimes in Switzerland are assessed with the Hydro-CH2018-Runoff ensemble. By end of the 21 st century, winter mean runoff is projected to increase while summer and autumn mean runoff are projected to decrease. In spring, runoff increases in high elevation catchments and decreases in lower lying catchments. The yearly mean runoff is projected to decrease in most catchments. The projected changes show a strong elevation dependence with high elevation catchments being more affected than low elevation catchments. Early significant changes emerge in winter and summer in catchments with mean altitudes above 1500 masl. Significant changes in catchments below 1500 masl emerge later in the century. However, not all catchments show a time of emergence in all seasons and in some catchments the detected significant changes are not persistent over time. The magnitude of change and the robustness in terms of climate model agreement on the sign of change (positive or negative) increase with increasing global mean temperatures or stronger emission scenarios. This amplification highlights the importance of climate change mitigation.
A strong elevation dependence is also found for projected changes in moderate low flows. In Alpine catchments (approx. >1500 masl), the magnitudes of moderate low flows are projected to increase and significant changes emerge early in the 21st century, while the frequency of low flows decreases. Seasonality of low flows shifts from winter and early spring to autumn except in very high elevation catchments where seasonality does not change. In lower lying catchments (approx. <1500 masl), low flow situations are projected to occur more often and the runoff during low flows is projected to decrease. The seasonality of low flows in lower lying catchments hardly changes. The elevation dependence as well as the climate model agreement on the sign of change in moderate high flows is less clear and often not significant. Most catchments show slightly increasing magnitudes and frequencies in high flows except very high Alpine catchments which show decreasing magnitudes and frequencies. The seasonality of moderate high flows changes only in Alpine catchments with a shift towards early summer.
The Hydro-CH2018-Runoff ensemble is based on a single hydrological model. A comparison of the simulations with simulations from different Swiss research institutions for a subset of the catchments was performed. Despite fundamental methodological differences between the different hydrological simulations, a good agreement on the sign and the magnitude of change in seasonal runoff is found. Largest differences between the simulations are found in winter and spring for Alpine catchments (snow processes) and in summer and autumn in lower lying catchments ( evapotranspiration processes).
The second part of this PhD thesis includes an analysis of future changes in the atmospheric potential for flood events in Switzerland. Extreme moisture transport (IVT) directed perpendicular to topography is strongly linked to major floods in Switzerland and serves as an important atmospheric flood precursor. In this thesis, future changes of IVT extremes directed towards orography are assessed for 9 climate models under a high emission scenario (RCP8.5). The results show that IVT extremes increase by 20-30% with climate change. The moisture component (TPW) of IVT is projected to increase by 25% while the wind component (SW) of IVT hardly changes. This suggests that the major driver of the increase in IVT is related to changes in the thermodynamics (Clausius-Clapeyron relationship) and not to changes in the dynamics. 100-year return levels of IVT are estimated using non-stationary Generalized Extreme Value (GEV) models with the covariates time, TPW and SW. Best performance is found for the covariates TPW and time. Since both covariates include a temporal trend, the estimated 100-year return levels increase by end of the century.