Anthropogenic climate change is certain to affect regional and local hydrology (IPCC, 2013, 2014a, b). Major research efforts have enabled more precise and more reliable projections of regional and local temperature and precipitation changes (CH2018, 2018). The new Swiss Climate Change Scenarios (CH2018) are the result of a large modeling effort to downscale regional climate projections for Switzerland to local scales (CH2018, 2018). Warming in Switzerland is projected to be higher than the global mean warming. Without major mitigation efforts, mean temperatures are projected to increase by up to 6.8 ∘C by end of the 21st century in Switzerland (CH2018, 2018). This large increase in temperatures is accompanied by changes in many other hydrologically relevant variables such as precipitation amounts, precipitation type, and glacier volumes. The projected combination of increasing temperatures, changing precipitation patterns, retreating glaciers, and changes in snowpack will have a potentially strong impact on runoff regimes. Runoff regimes reflect the integral response in time and space of the hydrological conditions within a catchment and hence its water supply. Understanding and assessing changes in runoff regimes are crucial for many economic sectors, such as agriculture, fishery, hydropower generation, and tourism, and for the ecology within and around the rivers. Assessments of climate change impacts on river runoff are therefore particularly important for decision makers in adaptation planning and can also provide a basis for mitigation policies.

Several studies on the impacts of climate change on hydrology in Switzerland have been conducted in recent years. These have focused on changes in runoff regimes (Horton et al., 2006; Köplin et al., 2012, 2014; Addor et al., 2014; Milano et al., 2015), on changes in low flows (Jenicek al., 2018; Brunner et al., 2019a, b), on changes in high flows (Keller et al., 2018; Brunner et al., 2019a), and on the effects of retreating glaciers for rivers (Huss et al., 2008, 2014; Farinotti et al., 2012; Finger et al., 2012; Fatichi et al., 2015; Etter et al., 2017). Studies on climate change impacts on various aspects of the hydrology in the Alpine area have also been carried out in Austria (e.g., Weber et al., 2010; Blöschl et al., 2011; Prasch et al., 2011; Tecklenburg et al., 2012; Goler et al., 2016; Wijngaard et al., 2016; Hanzer et al., 2018), in Italy (e.g., Groppelli et al., 2011), in Germany (e.g., Hattermann et al., 2015; Nilson et al., 2014), and in France (e.g., Ruiz-Villanueva et al., 2015; Vidal et al., 2016). Most of these studies are case studies or focus on specific aspects of hydrology.

Previous studies have focused on changes in runoff regimes and included many catchments with diverse properties in Switzerland (Horton et al., 2006; Koeplin et al., 2012; Köplin et al., 2014; Addor et al., 2014). Köplin et al. (2012) found a shift in seasonality with increasing winter runoff, decreasing summer runoff, unchanging annual mean runoff for lower-lying catchments, and increasing annual mean runoff for high-Alpine catchments. However, these studies are based either on older climate model generations or on climate simulations downscaled with a delta change approach. The delta change method is based on climate simulations downscaled for 30-year periods both in the reference period and in future periods and does not provide continuous transient simulations for the whole 21st century. Therefore, this approach does not simulate daily to interannual variability (Bosshard et al., 2011). More sophisticated downscaling approaches have been developed, such as quantile mapping (Teutschbein and Seibert, 2012; Gudmundsson et al., 2012). They correct not only for the mean bias but also for the full distribution; they are applicable to long-term climate simulations and allow transient scenarios to be established. Using quantile mapping as a downscaling approach can intensify and increase the number of projected extremes (Rössler et al., 2019). Therefore, the present study uses the Hydro-CH2018-Runoff ensemble (dataset: Muelchi et al., 2020, 2021) run with the most up-to-date local climate change scenarios for Switzerland (CH2018). These scenarios used the quantile mapping approach to downscale the coarse climate model output. The Hydro-CH2018-Runoff ensemble includes transient daily simulations from 1981 to 2099 for 93 catchments in Switzerland and three Representative Concentration Pathways (RCPs): RCP2.6, RCP4.5, and RCP8.5. The new ensemble enables detailed quantification of changes in runoff regimes. In this study, we investigate changes in runoff regimes under a range of RCP scenarios. These 119-year continuous daily runoff simulations enable the estimation of the time of emergence (Giorgi and Bi, 2009; Leng et al., 2016) of those changes. The time of emergence is the time when the climate signal emerges significantly from natural variability. It is of particular importance to the question of how much time is left for adaptation planning. In this study, we analyze not only the time of emergence but also the evolution of time of emergence over time. Moreover, the ensemble allows for the quantification of changes at different global warming levels; these warming levels are particularly relevant for policymakers (e.g., James et al., 2017). The framing of the results as a function of global mean temperature change rather than time enables a direct link to the Paris Agreement target and indicates the changes to be expected in a +2 ∘C world and the consequences of missing this target. In this study, we assess changes for global warming levels of +1.5, +2, and +3 ∘C.

For this study, we use the Hydro-CH2018-Runoff ensemble of daily discharge simulations for 93 medium-sized (catchment size between 14–1700 km2) catchments in Switzerland (Fig. 1; Muelchi et al., 2020, 2021). The Precipitation-Runoff-Evapotranspiration HRU-related Model (PREVAH) hydrological modeling system (Viviroli et al., 2009) was used for simulating the hydrological response to the climate change scenarios. PREVAH is a semidistributed model based on hydrological response units and includes submodules to account for important hydrological processes related to snow, glacier, and soil moisture dynamics and evapotranspiration. PREVAH was calibrated and validated for each catchment (for more details, see Muelchi et al., 2021). The calibrated parameters were kept constant for the simulation of runoff under climate change, and land use was held unchanged for nonglaciated catchments. The minor impact of land use changes on changes in the runoff regime for Switzerland was assessed by Köplin et al. (2014). For glaciated catchments, glacier extents were updated every 5 years according to the glacier projections by Zekollari et al. (2019), which are based on the same climatic dataset. The land use of areas from which glaciers disappear was replaced by rock for areas above 3000 m a.s.l. and by bare soil for areas below 3000 m a.s.l.

The meteorological input used for the simulations comes from the latest Swiss Climate Change Scenarios CH2018 (CH2018, 2018). The CH2018 scenarios used EURO-CORDEX simulations (Jacob et al., 2014; Kotlarski et al., 2014) and applied statistical downscaling (quantile mapping; Teutschbein and Seibert, 2012; Gudmundsson et al., 2012) on chains of global circulation models and regional climate models (GCM–RCM chains). The results consist of gridded high-resolution (2km×2 km) daily temperature and precipitation data for Switzerland for the period 1981–2099. This dataset was then used to drive the hydrological model, resulting in the Hydro-CH2018-Runoff ensemble, which comprises daily runoff time series for 1981–2099 for each of the GCM–RCM chains. The Hydro-CH2018-Runoff ensemble contains simulations for three RCPs: 8 simulations under RCP2.6, 16 simulations under RCP4.5, and 20 simulations under RCP8.5. The GCM–RCM chains and their underlying emission scenarios used for this study are listed in Table 1. The Hydro-CH2018-Runoff ensemble comprises transient long-term daily runoff projections for Switzerland for the period 1981–2099. These transient simulations incorporate the daily to interannual climate variability. Changes in runoff constrained by the different warming levels were analyzed using the global mean temperatures of the driving GCMs (CMIP5; Taylor et al., 2012).

Winter runoff is projected to increase in Switzerland due to enhanced winter precipitation and increasing temperatures with climate change (CH2018, 2018). The higher temperatures result in more liquid precipitation and less solid precipitation in winter. This leads to less snow accumulation and thus to more direct runoff in winter. The relative changes in winter mean runoff in glaciated catchments are very high but still negligible as a contribution to the annual runoff volume. In nival and pluvial catchments, the contribution of winter runoff to the annual volume increases.

In spring, runoff in glaciated and nival catchments is projected to increase due to increased snowmelt, particularly in early spring. The combination of reduced snow accumulation in winter and earlier snowmelt shifts the peak in runoff regime to a month earlier. Lower-lying catchments show decreasing runoff in spring due to reduced snow accumulation in winter, which reduces the snow available for snowmelt.

The summer runoff in Switzerland generally decreases with climate change. The processes governing this decrease differ with location and elevation of the catchments. In lower-lying catchments, reduced summer precipitation and enhanced evapotranspiration result in decreasing runoff in Switzerland. In high-Alpine regions, where summer snowmelt and glacier melt dominate the runoff generation in the reference period, reduced snowpack and glacier retreat amplify the decrease in summer runoff. The large model spread in glaciated catchments in summer stems from the sensitivity of the runoff response to the glaciation of the catchment. In catchments where not all climate projections result in a complete disappearance of glaciers, the model spread increases, so future glacier retreat is a major source of uncertainty.

Autumn runoff is also projected to decrease due to increased evapotranspiration and slightly reduced precipitation in most catchments (CH2018, 2018). In a few of the eastern catchments, autumn precipitation slightly increases, but the effect of enhanced evapotranspiration and the reduced contribution from glacier melt and snowmelt are more dominant; autumn runoff also decreases in those catchments. Again, this decrease is amplified in catchments where glacier melt is important in early autumn under current climate conditions. However, the decrease in autumn runoff is most noticeable in early autumn, whereas in late autumn, at the end of October and November, the changes are less significant and can even change direction to positive values. This pattern is mainly found in very high Alpine catchments, where late autumn precipitation can fall more often as rain instead of snow and thus cannot be stored as snowpack. For the annual mean, a decrease in runoff was found in most catchments but with less robust signs among the climate models. This leads to the conclusion that, on the annual average, less water will be available in Swiss rivers. The shift in seasonality and thus a shift in the seasonal availability of water will impact many economic sectors. For example, increasing winter runoff may be beneficial for energy production, but decreasing summer runoff may lead to limitations in irrigation, particularly in lower-lying catchments where agricultural irrigation plays a crucial role.

In most catchments and seasons, the sign of change is the same for the high-emission scenario, RCP8.5, and the low-emission scenario, RCP2.6. Changes in seasonality or in the runoff regime are amplified by higher emissions and thus by increasing global mean temperatures. This amplification due to increased emissions and with intensified global warming shows the large benefits of mitigation. By mitigating climate change and following the RCP2.6 pathway, the magnitude of change can be reduced or even avoided, depending on the season and the runoff regime. However, some of the seasonal changes can no longer be avoided by mitigation actions, particularly in high-Alpine catchments with glacier influence. Responsible planners and policymakers need to adapt to this shift in the seasonal availability of water in our rivers.

Previous studies on climate change impacts on the runoff regime in Switzerland (e.g., Köplin et al., 2012, 2014; Horton et al., 2006) were driven by other emission scenarios, other and fewer climate model chains, different methods of postprocessing the climate model output, and/or different hydrological models and calibration. Despite these differences, the sign of change in those studies agrees in most seasons with the signs in this study. A pronounced dependence of runoff changes on elevation was also found by Köplin et al. (2012), with lower-lying catchments being less affected by climate change. The largest difference concerns the annual mean runoff. Köplin et al. (2012) found an increase (up to +50 %) in annual runoff for high-elevation catchments and no change for lower-lying catchments in the annual volume. In contrast, our study projects a decrease in the annual mean runoff not only for high-elevation catchments but also for most of the lower-lying catchments. However, not all catchments show a robust decrease among the climate model chains. This difference between our results and previous studies may arise from the use of the most recent generation of Swiss Climate Change Scenarios (CH2018), which project slightly different precipitation changes with less summer drying and wetter winters (CH2018, 2018) than the previous generation of scenarios, CH2011, which was used by Köplin et al. (2012). The different handling of glacier melt processes and the new projections of glacier extents (Zekollari et al., 2019) may also result in slightly different projections in glaciated catchments. The Hydro-CH2018-Runoff ensemble uses transient glacier projections that are updated every 5 years in the hydrological model. Köplin et al. (2012) used static glacier projections for 30 years. The largest uncertainties in glaciated catchments were also found by Addor et al. (2014) due to the different handling of glacier extents and resulting glacier melt. Furthermore, the difference in the input data (transient projections versus delta change projections with same baseline time series for the reference period) may add to the different signs.

Even though not all catchments show a time of emergence in the 21st century, significant changes in the distribution of seasonal means emerge particularly early in high-elevation catchments. This is due to the importance of snowmelt and glacier melt for Alpine runoff regimes. With climate change, the influence of snowmelt and glacier melt decreases due to higher temperatures and subsequent glacier retreat. Lower-lying catchments generally show a later time of emergence. This may arise from the large interannual variability in pluvial catchments in the reference period and in future periods. Köplin et al. (2014) also assessed a time of emergence for Swiss catchments but only based on significant changes for two scenario periods and 10 climate models. Since their climate models were postprocessed with a delta change approach, only the natural variability of the reference period is reflected in their simulations. Despite these differences in methodology and data, they also found earlier time of emergence in winter and summer for high-elevation catchments. Although the definition of time of emergence applied here is commonly used in other climate change studies (e.g., Mahlstein et al., 2011), this definition also has disadvantages. For example, if the rejection of the null hypothesis is unstable in its temporal evolution, the time of emergence may be determined too early. This has been shown for some of the catchments in the present study. However, in most catchments a persistent detection of time of emergence (p value < 0.05) in the distribution of the seasonal means was found shortly after the first detection of a time of emergence.

The findings of this study concern runoff regime changes for Switzerland. The pronounced changes in high-elevation catchments highlight the important influence of temperature changes on snowmelt- and glacier-melt-driven catchments; thus, they also indicate that similar runoff responses may be found in other snow- and glacier-dominated regions. The results for lower-lying catchments, which are mainly driven by evapotranspiration and precipitation changes, may not be directly transferable to other regions. The runoff response in such catchments depends on local precipitation patterns and their changes under climate change.

Various sources of uncertainty affect our results. A detailed discussion of the uncertainties for the simulations is provided in Muelchi et al. (2021). Uncertainties arise from all the steps in the modeling chain: the scenarios of greenhouse gas concentrations, the climate models and their boundary and initial conditions, the postprocessing method (Gutiérrez et al., 2019), the hydrological model (Addor et al., 2014) and its calibration, and the underlying glacier projections; all these need to be considered in adaptation planning (e.g., Wilby and Dessai, 2010). Working with three emission scenarios and an ensemble of climate models partly addresses some of the uncertainty issues. Muelchi (2021) compared the results for three catchments to simulations from three different versions of hydrological models, and the key findings are robust among the models and the catchments.

Changes in runoff regimes and their time of emergence were assessed with the new hydrological scenarios for 93 catchments in Switzerland. This study is based on the most recent generation of climate change scenarios, postprocessed with quantile mapping. Compared to previous studies on runoff regime changes, the results show similar signs of change for most seasons. The largest differences were found for the high-elevation catchments, which is likely due to the transient characteristic of the simulations and the implementation of transient glacier projections driven by the same climate model chains.

In general, winter runoff is projected to increase and summer runoff to decrease in Switzerland. The sign of change is robust across catchments, but the magnitude of change is more pronounced for high-elevation catchments. Particularly in summer, when snowmelt and glacier melt play an important role in runoff generation, glaciated catchments will face a strong decrease in runoff due to the retreating glaciers. In rainfall-dominated catchments, the changes are also often robust but at a lower magnitude. Whereas the higher-elevation catchments show increasing spring runoff due to earlier snowmelt, the pluvial catchments in the lowlands will face decreasing spring runoff. A decrease in runoff is also found for autumn and annual mean runoff in most catchments. These seasonal patterns amplify with global warming and with higher-emission scenarios. Furthermore, the model agreement among the climate models increases with the strength of the emission scenario and the distance in time. A time of emergence of seasonal mean runoff was mainly found in summer and winter and for only a few catchments in spring and autumn. Early times of emergence, before 2060 in winter and before 2065 in summer, were found for catchments above 1500 m a.s.l. Significant changes in catchments below 1500 m a.s.l. emerge later in the century. However, not all catchments show a time of emergence in all seasons.

The amplification of changes by enhanced global warming highlights the importance of climate change mitigation. By mitigating climate change and following the RCP2.6 pathway to keep global warming below 2 ∘C, the magnitude of change can be reduced substantially. The large decrease in summer runoff in glaciated catchments can be strongly dampened but not avoided entirely, because glacier retreat is projected even for the low-emission scenario. The present study can help to support adaptation planning in various sectors by presenting detailed information on changes in mean runoff.